SHOULD the P39 have been able to handle the Zero? Was it training or performance?

[drgondog]

THE REASON WAS WEIGHT. WEIGHT per your photo says the P-400 gross weight was 7845#. The P-39C weighed 7075#, that extra 770# is the difference in the 355mph vs the 379mph. Everything about the P-39C and the P-400 was exactly the same (engine, propeller, aerodynamics) except the additional 770#. To put this in perspective, you know the performance benefits of dropping your external tank for combat, it is quite substantial or the pilots would not have dropped them. A 75 gallon external tank only weighed 500# FULL OF FUEL. We're talking about 770# here. This is the same reason that the Russian P-39s outperformed the Luftwaffe, they discarded the useless 30 caliber wing guns and one of the radios (didn't use their frequency) and got much better performance.

It was what it was. Weight is an integral part of the design philosophy of the company making the aircraft as well as the standards of the purchasing agency. The AAF required slightly higher Limit and Ultimate Load factors than Brits. The P-39 was designed to AAF standards for Structural considerations (8G Limit and 12G Ultimate) for original design combat gross weight. The YP-39 was designed at a much lower weight than ultimately included by the time combat mods were applied -As Required - it only had one path, namely to grow in Gross weight based on combat operational requirements. The P-39 had less range than a Spit and a 109, it peaked out on ROC at 7K, then petered out - making it useful close to front lines at low altitude. It was slaughtered in air to air combat versus A6M and Ki-43 and -61, it was hammered versus FW 190 A-3/A-5 and Bf 109F and G in North Africa. The Russians Claimed 'success' but not reflected in eyes of LW which considered it our (US made) 'poorest fighter'. By the time Bell learned from the mistakes and designed the P-63, it was useful for Russia but too late for US/Commonwealth operations by a year. The comparable period a/c for P-63 included Spit IX, Tempest, P-51B, P-47D, P-38H, Yak 3. Useful to Russians, but not necessary in contrast to Yak and Laag's coming on line.

The P-39C did not have self sealing tanks, oxygen equipment. The P-400 arrived in SWP with No oxygen equipment (VVS didn't need the O2).

The P-39D-2 had the more powerful engine and STILL couldn't climb past 20,000 feet (in less than 15 minutes) to intercept incoming Japanese bombers at Guadalcanal, even with O2 equipment - simply because that Allison peaked out between 12K and 14K

 

[Peter Gunn]

Okay, pick one. FW190 or Me109. Either one. Which version of each so we know we are comparing the correct version of the P-39.

I think drgondog has answered this more expertly than I could, my short answer would be "any marque of either v any marque P-39".

NAA promised a plane better than the P-40 and boy did they deliver.

Bell promises? Allow me to quote John Wayne... "I"ve been promised a posse, which I suspect will be Looong on promise and short on posse". I think you get my drift.

 

[P-39 Expert]

The P-39C did have armor, at least some and had BP glass, it didn't have self sealing tanks.
I would not use the Ray Wagner figures as gospel however. I used the photo to show the British aircraft only had 2 more RCMGs than the P-39C.

SOme of the disparity in figures come from different test conditions.
http://www.wwiiaircraftperformance.org/P-39/P-39C_40-2988.pdf
A P-39C that did do 379mph, however it was at 6689lbs, roughly 400lbs lighter than the figure Ray Wagner used. One wonders what was left out, ammo or fuel? A bit of both?
Please note the comments where under no circumstance was the oil cooling satisfactory to USAAF standards and in both high speed level flight and in climb both oil and prestone cooling did not meet USAAF standards.
Please note that Larry Bell & company were promising a LOT more than they could deliver.
Company performance chart.
http://www.wwiiaircraftperformance.org/P-39/P-39C_Chart-Bell_Aircraft-1400.jpg
Dated feb of 1940 which is well before the YP-39s flew.
I would note that the British had 675 Aircobras on order about 4 months before the first YP-39 flew let alone any P-39Cs

British test
http://www.wwiiaircraftperformance.org/P-39/AH573.pdf

I would note that while the US tested climb at 3000rpm for the first 5 minutes the British used 2600rpm for the entire climb but this was British procedure. ALL british climb figures are done at max continuous or a 30 minute rating and not at a 5 minute rating.

Glad you are sourcing wwiiaircraftperformance, that is the source of most of my radical theories on the P-39. Please indulge me, print the P-39K performance chart. Then take a pencil and chart the performance figures of the A6M2, SpitV, Me109G1 and maybe the Wildcat over the P-39K lines. Then chart the P-39C figures in your report above. This will clearly show the performance of the P-39C as compared to the other planes. Couple of observations, the 6689# figure is with "mean fuel" which is roughly half the tank capacity. Add back half the fuel 360# and you get approximately your 7075# gross weight. Virtually all the American fighter tests used this "half fuel" figure for gross weight, since you couldn't fight your enemy full of fuel since you would still be on the runway and you couldn't fight with no fuel for obvious reasons so for calculations they used mean fuel.
Now when you graph the P-39C climb take one thing into account: see that little curved line starting at 12500' lowering the climb figure? The engine was at that time limited to 5 minutes at maximum power. So these tests had the pilot climb at full power (3000rpm) for the 5 minutes and then reduce rpm to 2600 (maximum continuous or max cruise) for the remainder of the climb. Check the P-38, P-40 and P-47 tests before August 1942 and they all reduced power from max power to their respective maximum cruise power in climb at the 5 minute mark per regs. The Army changed the 5 minute maximum to 15 minutes maximum RETROACTIVE during mid year 1942 for virtually all their fighter combat engines (V-1710, R-2800, etc.). Now all these planes could climb at full power (3000rpm for the V-1710) for the full 15 minutes which greatly increased their climb rates. Give the P-39C the same benefit of 3000rpm for 15 minutes. Take the rate of climb at 5 minutes (3700fpm) and from that point draw your climb rate line up diagonally to a ceiling of about 37000' and you have the climb rate for the P-39C at 15 minute full power. Full power (3000) for 15 minutes in climb makes a huge difference over full power for 5 minutes then reducing power to max continuous/max cruise (2600rpm). Once you graph all these lines on the same graph it is easy to see the performance of each plane at every altitude.

 

[Shortround6]

The US use of the Aircrobra was also handicapped by climate. They usually try to adjust or correct performance figures to a "standard" day of 59 degrees F or 15 degrees C. and standard air pressure. Operating at temperatures much higher reduce performance due to less lift from the wing, less power form the engine (even at the same pressure) and less 'bite' from the propeller.
For some weird reason the US sometimes gave pilots charts that used a base line of 32 degrees F (0 degrees C) and the P-39 is one of those planes that use the point of freezing as a base line with notes to increase the take off distance and the time to hight by 10% for every 20 degrees F (or 10 degrees C) above the baseline. SO on a 92 Degree F day on a pacific Island or in North Africa the P-39 will need 30% longer runways and take 30% more time to make it to 15,000ft than the charts say. Other planes also suffered a similar loss of performance but if they started with a 59 degree base then they might only be around 15% below book figures. You can have hot days in Russia but a lot of their use would have been on days without such a large difference from the book figures.
P-39 had one of the higher wing loadings of the early fighters and one of the worst power loadings. (Not counting WEP) and may be affected even more by extreme heat.

 

[Elmas]

"Omnia in mensura et numero et pondere disposuisti."

Thou hast ordered all things in measure, and number, and weight.

Book of Wisdom, 11:21.

I had my M.of Sc. in Structural Engineering in 1979, so numbers have been, and still are, a very important part of my life. But, IMHO of an humble technician, sheer numbers are not at all the best way to evaluate the overall performance of an airplane, let alone to compare two or more.

In an airplane there are many other parameters to evaluate, not all susceptible to numerical evaluation: the design of an airplane is the result of many compromises, and the best airplane it is not the one that has the best numbers, but the one wich is a better compromise. And often this compromise depends on the circumstances: best compromise for PTO can be totally different for ETO.

At the end of the '60s, in Tobruk, Lybia, I had long conversations with a Gentleman who had served as Navigator in Lancasters and was ending his career in the RAF commanding the Rescue Service at El Adem airport. At my question wich one was better, if Spitfire or Me 109 ( I was very young, then…) he replied "Oh, they were pretty equal. One had some slight advantages, Spitfire turned better, 109 was a little bit faster… the difference was made by a better training of the average pilot of the RAF than the average pilot of the Luftwaffe…" So his words.

Btw, P-39, as already said in many other posts, was hated by Italian Pilots who received this aeroplane in the Cobelligerent Air Force. It was assigned to the 4° Stormo, the spearhead of Regia Aeronautica, with Pilots seasoned with years of war. They accepted to fly it just for discipline and accidents were numerous.

 

[Shortround6]

The Army changed the 5 minute maximum to 15 minutes maximum RETROACTIVE during mid year 1942 for virtually all their fighter combat engines (V-1710, R-2800, etc.). Now all these planes could climb at full power (3000rpm for the V-1710) for the full 15 minutes which greatly increased their climb rates. Give the P-39C the same benefit of 3000rpm for 15 minutes.

That might lead to a lot wrecked engines. I would double check your source on that.

The V-1710 went through 3 different crankshafts by Early 1942. The 3rd crankshaft stayed unchanged until the very late war 12 counterbalance cranks.
The first crankshafts were plain steel, by that I mean no special surface finish. the Next crankshaft was shot peened and had a much improved resistance to stress. It would operate practically forever at a level that would destroy the plain crankshaft in fairly short order. This was followed by a crankshaft that was both shot peened and nitrided. This crank would operate forever at stress levels that would destroy the shot peened crankshaft in short order.

Allowing full combat power for 15 minutes at a time was going to significantly shorten the life of the older crankshafts. The new crankshafts could be dropped into an old engine at overhaul time. Without some regard for the older engines this would be a very poor blanket policy. I don't know when the shot peened cranks were introduced but obviously the P-39C and the majority of the Ds and P-400s did not have the nitrided crankshafts.

You may also have a cooling problem, use of military power was always dependent on coolant and oil temperatures. If the temperature limit/s were exceed the engine had to be throttled back regardless of the number of minutes that been spent at that power level. If the P-39 was exhibiting marginal cooling in 5 minutes or under (although it can get pretty hot in Ohio in the summer) then pushing the engine for 15 minutes may not be a good idea.

I have no idea what, if anything, they did to address the cooling issues of the P-39C. One hopes they did something because if you are having cooling issues at 1150hp then with 1325hp engines or operating at WEP settings things are going into the red zone real quick.

 

[drgondog]

Glad you are sourcing wwiiaircraftperformance, that is the source of most of my radical theories on the P-39. Please indulge me, print the P-39K performance chart. Then take a pencil and chart the performance figures of the A6M2, SpitV, Me109G1 and maybe the Wildcat over the P-39K lines. Then chart the P-39C figures in your report above. This will clearly show the performance of the P-39C as compared to the other planes. Couple of observations, the 6689# figure is with "mean fuel" which is roughly half the tank capacity. Add back half the fuel 360# and you get approximately your 7075# gross weight. Virtually all the American fighter tests used this "half fuel" figure for gross weight, since you couldn't fight your enemy full of fuel since you would still be on the runway and you couldn't fight with no fuel for obvious reasons so for calculations they used mean fuel.
Now when you graph the P-39C climb take one thing into account: see that little curved line starting at 12500' lowering the climb figure? The engine was at that time limited to 5 minutes at maximum power. So these tests had the pilot climb at full power (3000rpm) for the 5 minutes and then reduce rpm to 2600 (maximum continuous or max cruise) for the remainder of the climb. Check the P-38, P-40 and P-47 tests before August 1942 and they all reduced power from max power to their respective maximum cruise power in climb at the 5 minute mark per regs. The Army changed the 5 minute maximum to 15 minutes maximum RETROACTIVE during mid year 1942 for virtually all their fighter combat engines (V-1710, R-2800, etc.). Now all these planes could climb at full power (3000rpm for the V-1710) for the full 15 minutes which greatly increased their climb rates. Give the P-39C the same benefit of 3000rpm for 15 minutes. Take the rate of climb at 5 minutes (3700fpm) and from that point draw your climb rate line up diagonally to a ceiling of about 37000' and you have the climb rate for the P-39C at 15 minute full power. Full power (3000) for 15 minutes in climb makes a huge difference over full power for 5 minutes then reducing power to max continuous/max cruise (2600rpm). Once you graph all these lines on the same graph it is easy to see the performance of each plane at every altitude.

Got a better idea for you. Compare 'Period P-39 vs Period 'whatever' at the rated Hp and Boost and combat weight for each?

The P-38C was in same period as the Bf 109F-1 and Spit Vb, delivered in January, 1941 with 20th completed in March 1941, None of the three P-39C's were close to meeting Bell claims for performance, nor were the production P-39D-1 and -2 delivered at the same time as the FW 190A-1. It was a hazard to life and limb without self sealing tanks and O2.

The P-39Q, introduced in Spring 1943 was first to achieve reliable 3200 fpm in Combat Power at S/L but reducing to ~ 3000 fm at 12000 and dropping to about 2000fpm at 20K. . Performance tests flown at design Combat load of 7570 #, but only included 87 gallons of fuel. With 33 gallon fuselage kit, the fuel load jumped to 120 gallons and 200 more pounds. It reached US combat zone with 332nd FG in February 1944 but were quickly swapped for P-40, then P-51B by June 1944.

So the P-39Q period to period has to compete with P-51B/C, P-38J-15, Spit XIV, FW 190A-7 and A-8, Bf 109G-6 - all of which climb faster at rated Combat Power (available with 150 grade fuel for example for the P-51B), are faster in level flight - at all altitudes- and roll with or better than the P-39Q. The Q would normally outturn all the above fighters except the Spitfire and the P-38J with boosted ailerons and skilled use of throttles. It has the shortest combat radius, is a sad sack above 20,000 feet and can't find a fighter to fight on even terms except for the occasional LW fighter flying low and medium fast where the P-39Q performs well... The ETO banished them to Bomb Squadrons in the 12th AF for close air support, the 9th/15th and 8th AF rejected them from operating in THEIR airspace, the SWP relegated them to CAS and then got rid of them as fast as P-40/P-38/P-47 and P-51s arrived.

 

[P-39 Expert]

Here you go boys, the vaunted FW190A6 vs. P-39N is attached. Looks like the little Cobra is faster below 5km, and the same combat speed at 8km (26000'). A little slower than the FW190's emergency maximum good for one minute with liquid injectant. US war emergency was good for 5 minutes and the engine had to be torn down and inspected after every use. Bet that German emergency maximum was an engine destroyer. Second page is rate of climb. The P-39N climbs significantly faster at all altitudes, period. A whole hell of a lot faster below 6km (20000') and about 65% faster at 8km (26000'). Personally, give me the best climb rate.

 

[Hansie Bloeckmann]

Three comments. First the mixed armament thing, trying to deal with two different trajectories when deflection shooting. That is why I suggested an all 50 cal armament package. Second, history tells us the most US versions of the 20mm Hispano cannon had almost as much trouble with jams/un-reliability early in the war as the 37mm. Another point for staying with just Browning 50 cal guns. Finally, as far as I know, during WW2 'wet wings' implied unprotected fuel tanks. I am suggesting adding self-sealing rubber bag tanks as per other American fighters.

I'm with the school of thought that endorses all MG's- whether wing, cowl or through the prop hub be the Browning M-2 in .50 cal. Much more reliable than the 37 mm cannon, and as General Patton once remarked: "In combat, a gun that won't fire reliably is as useful as a pecker on a Pope" I like the prop hub mounting, when feasible, as then, where the nose is pointed, the rounds go--Hansie

 

[pbehn]

Here you go boys, the vaunted FW190A6 vs. P-39N is attached. Looks like the little Cobra is faster below 5km, and the same combat speed at 8km (26000'). A little slower than the FW190's emergency maximum good for one minute with liquid injectant. US war emergency was good for 5 minutes and the engine had to be torn down and inspected after every use. Bet that German emergency maximum was an engine destroyer. Second page is rate of climb. The P-39N climbs significantly faster at all altitudes, period. A whole hell of a lot faster below 6km (20000') and about 65% faster at 8km (26000'). Personally, give me the best climb rate.

Is this a wind up? Are you saying that if the RAF and US air forces had been given the correct graph and data then the P-39 would have been a winner in service? 601 squadron changed from Hurricane Mk IIs to P39s flew one mission with it then changed to Spitfire Mk Vb and went to Malta. The P 39 would have been no use at all in Malta. In the UK the FW 190 was superior to the Spitfire MkV forcing the Typhoon into service early. When the P51A arrived it was used because at low / medium altitudes it was a top performer, liked by all who flew it. As soon as the P 51 arrived people started figuring out how to put a Merlin in it because its only problem was altitude performance. Merlins were also put in P-40s for the same reason. Did anyone ever suggest putting a Merlin in a P-39?

 

[buffnut453]

I'm with the school of thought that endorses all MG's- whether wing, cowl or through the prop hub be the Browning M-2 in .50 cal. Much more reliable than the 37 mm cannon, and as General Patton once remarked: "In combat, a gun that won't fire reliably is as useful as a pecker on a Pope" I like the prop hub mounting, when feasible, as then, where the nose is pointed, the rounds go--Hansie

Depends on the timeframe you're talking about. The 50cal only became an effective weapon in wing installations in 1942 which wouldn't be much use to the RAF in the Battle of Britain.

 

[tomo pauk]

Depends on the timeframe you're talking about. The 50cal only became an effective weapon in wing installations in 1942 which wouldn't be much use to the RAF in the Battle of Britain.

Buy the heavy machine guns in Belgium or Italy in the second half of 1930s if the US types were not up the task with 600 rpm.

I'm with the school of thought that endorses all MG's- whether wing, cowl or through the prop hub be the Browning M-2 in .50 cal. Much more reliable than the 37 mm cannon, and as General Patton once remarked: "In combat, a gun that won't fire reliably is as useful as a pecker on a Pope" I like the prop hub mounting, when feasible, as then, where the nose is pointed, the rounds go--Hansie

Having 3 synchroised .50s under the cowl of the P-39 and 4th firing through the prop, with wing guns deleted, would've made the P-39 lighter and would've also shaved some drag. I'd also remove one of radio sets, and replace the rear-most armor with duraluminium deflection plate.

 

[Glider]

Taking the points one at a time.

Just which Luftwaffe plane(s) were so superior to the P-39?

The P39 arrived in Aug 1941 roughly when the Fw 190 entered service with the Luftwaffe and the Me109F was the standard fighter both of which had clear advantages over the P39.

Regarding lend lease, how could the British take the P-39 under lend lease when they had so publicly derided them as inferior under the hard money contract? "Oh these P-39s suck, but we'll take them for free"? They no longer needed them after winning the BoB and their internal fighter production was then adequate. They didn't need them and couldn't pay for them. Bell had the last laugh though. They got a $2million cash advance from the British in the original order. Bell was a relatively new company struggling under depression era economics and although $2mil doesn't sound like much today, in 1940 dollars it was a fortune and put them in the black for the rest of the war. And the Army took over the British order and had P-39s available from 1941 on.

The RAF didn't need the P39 because it was so far behind the performance of contemporary fighters and your right, production was more than sufficient. The point behind the comment on lend lease was if and I repeat if, the RAF had needed them then they could have been supplied via lend lease.
Your observation on Bell being saved by the UK financially also applied to Curtis. They had huge orders from Europe for the P40 and almost none from the USA (or UK). They spent a lot of money expanding the productions lines but when Europe fell there was no money coming in and the UK taking up the cost of all those orders saved them. Had we not done so its an interesting what if, what would the USAAF look like in late 1941 without Bell or Curtis as politically its unlikely that the US authorities would have spent the money.

 

[Glider]

Apologies to one and all as I hadn't seen this posting when I replied to the other response.

Cost was the big issue, especially for the British. They had stood alone against Germany since the fall of France and they were dead broke. They had to order those planes before France fell not knowing how long the war would run. After they were safely past the Battle of Britain and a German invasion was no longer possible, they moved to cut their losses and weasel out of not only the P-39 contract but a similar contract for P-38s that were ordered without turbochargers. Those were hard money contracts and payment was demanded. The British knew that if the Battle of Britain had not gone their way and they needed the P-39s that they could have quickly stripped the excess weight (.30 cal MGs, heater, unnecessary radio equipment) from those planes to make them competitive.

I am sorry but this is total rubbish. The P39's were ordered by the UK in September 1940 when it was already clear that the BOB had been won and of course France had fallen some time before. As a result it wasn't a case of cutting losses, or weaseling out of a contract (not a fan of the UK are you) it was because the P39 wasn't a match for the latest fighters and wasn't combat ready for Ground attack, a role it could have been very useful in. We did what we did with later Hurricanes, send them to Russia or somewhere else anywhere apart from Europe.
Just a thought did the UK pay for the P400's used by the USAAF?

 

[pbehn]

The British were desperate for fighters in 1941 as Russia was desperate for the British to put pressure on Germany. However those fighters had to be superior in every respect to the Spitfire MkV, some fighter sweeps over France with Mk Vs resulted in no aircraft returning home. Being equal to the MkV was of no use at all because that was in no way good enough. To be any use at all the P39 had to be the equal of the P51A and Spitfire MkIX and it wasn't.

 

[Hansie Bloeckmann]

Depends on the timeframe you're talking about. The 50cal only became an effective weapon in wing installations in 1942 which wouldn't be much use to the RAF in the Battle of Britain.

Yes, and didn't Browning chamber the .30 cal. 1919 MG for the British .303 cal. round?? Not sure, but seems likely to me. Thanks

 

[Hansie Bloeckmann]

Buy the heavy machine guns in Belgium or Italy in the second half of 1930s if the US types were not up the task with 600 rpm.



Having 3 synchroised .50s under the cowl of the P-39 and 4th firing through the prop, with wing guns deleted, would've made the P-39 lighter and would've also shaved some drag. I'd also remove one of radio sets, and replace the rear-most armor with duraluminium deflection plate.

Agree, the weight of 4 .50 Brownings in the wings, fully loaded and mounted properly would most assuredly have increased the drag..

 

[pbehn]

Apologies to one and all as I hadn't seen this posting when I replied to the other response.

I am sorry but this is total rubbish. The P39's were ordered by the UK in September 1940 when it was already clear that the BOB had been won and of course France had fallen some time before. As a result it wasn't a case of cutting losses, or weaseling out of a contract (not a fan of the UK are you) it was because the P39 wasn't a match for the latest fighters and wasn't combat ready for Ground attack, a role it could have been very useful in. We did what we did with later Hurricanes, send them to Russia or somewhere else anywhere apart from Europe.
Just a thought did the UK pay for the P400's used by the USAAF?

Not only did the British order 675 P39s named as "Caribou" in 1940, there were also another 150 ordered under lend lease in 1941. When they arrived they were tested and tried out, found to be not good enough so all were sent or diverted to Russia.

 

[buffnut453]

Buy the heavy machine guns in Belgium or Italy in the second half of 1930s if the US types were not up the task with 600 rpm.

The problem there is that you're applying the retrospectroscope. In the mid-1930s, pretty much every fighter in the world was armed with 2 or 4 rifle-calbre machine guns. Even the early P-40s carried a mix of 30cal and 50cal weapons as late as 1941. I think it's asking a bit much for the crystal ball to foresee that 50cal weapons would be the preferable option in time to impact the Battle of Britain...and that's before we consider the other logistical issues.

Additionally, the main problem with the US 50cal in wing mounts seems to have been with the guns jamming due to mis-feeds. Now that can be blamed on poor/non-representative testing, but the simple fact remains that every US fighter with wing-mounted 50cals suffered stoppages right through until the latter half of 1942: P-40, F4F, F2A and the P-51.

 

[Shortround6]

Buy the heavy machine guns in Belgium or Italy in the second half of 1930s if the US types were not up the task with 600 rpm.

The Breda 12.7 machine gun was just about as heavy as the US Browning.50. and was rated at 700rpm instead of 600. perhaps the fact that the bolt only had to move 79% as far due to the shorter ammo has something to do with the 16-17% higher rate of fire

I also have serious doubts about the Belgian guns rate of fire. Like how long they could keep it up. I have no idea how close in contact the FN factory was with Colt although they seemed to co-operate on other things but it took the US 3 1/2 to 4 years to get the .50 cal up to the rate of fire of the Belgian guns with an acceptable degree of reliability. That is an acceptable number of jams and broken parts per 1000 rounds fired. Perhaps the US standards were too high?

Having 3 synchroised .50s under the cowl of the P-39 and 4th firing through the prop, with wing guns deleted, would've made the P-39 lighter and would've also shaved some drag. I'd also remove one of radio sets, and replace the rear-most armor with duraluminium deflection plate.

Other things in the nose of the P-39 include an oil tank for the reduction gear box, a glycol tank (not to be confused with the Prestone tank). the aircraft battery,
any oxygen tanks for the pilot. P-39s carried 200rpg for the .50 cal
I would note that 30 rounds of 37mm ammo weighs about the same as 200 rounds of .50 cal ammo. Yanking one 238lb gun and 60 lbs of ammo (298lbs) and replacing it with two guns weighing 138lbs and using 120lbs of ammo (258lbs) isn't really going to turn the P-39 into rocket plane.
Just use skinny pilots

This is only half jest. Early weight tables called for 160lb allowance for the pilot. later tables increased the weight to the more standard 200lbs although the 200lbs includes parachute. Not sure if the 160lb figure does.

Some units may have flown the P-39 with only 300rpg for the wing .30 cal but this saves about 165lbs.
30 rounds of 37 is less than 15 seconds firing time.
200 rounds of .50 cal is around 20 seconds assuming you can get the synchronized guns up to 600rpm.
300 rounds of .30 is about 15 seconds for the wing guns. 1000 rounds is ridiculous.
Under wing .50s had 300rpg and that is good for around 23 seconds at 780rpm.