P-51 6 x .50 ammo increase to 400 rpg complications?

[BarnOwlLover]

I'm asking about this because this seemed to be the standard load out for the F-82, as well as the Hellcat and Corsair, and the P-47 (originally) carried up to 8 .50s with up to 425 rpg. I'm asking this because the extra ammo will almost certainly add weight alone, let alone any possible structural changes. I'm also aware that staggering the guns like in other designs would've resulted in at least on gun barrel per wing protruding from the wing LE, which would likely contribute some drag.

In addition to what I've already mentioned (and I'm seeking further details on), are there any other possible issues? It should be noted that the NA-133 (navalized P-51H) would've had 300 rpg for all 6 .50s, instead of the 390/260/260 that the H usually ended up with. Though the 133 would've had a slightly larger wing than the H for about the same span (246 square ft vs 233 for the H).

Or could a scheme be used similar to this cutaway of the Boeing XF8B?

https://aviadejavu.ru/Images6/AE/AE94-9/28-1.jpg

 

[Shortround6]

I'm asking this because the extra ammo will almost certainly add weight alone, let alone any possible structural changes.

About 174lbs for the ammo/belts.
Space for different ammo bays/boxes and different doors to make loading easier?

A lot of the US ammo requirements were put into the contracts before they had much combat experience.
P-47s often carried under 300rpg in combat to help make up for the stuff hanging under the wings.
P-38s were supposed to carry 500rpg but often flew with 300rpg. (240lb weight saving)
260 rounds gives you 19.5 seconds of firing time.
400 rounds gives you 30 seconds of firing time.

Most Spitfires with 20mm cannon has 12 seconds of firing time.
A Tempest with Mk V cannon and 200rpg had 16 seconds of firing time.

I am not really seeing a problem.

The F8F-1 carried 300rpg.

 

[BarnOwlLover]

The NA-133 (USN-spec P-51H-based fighter) was due to carry 300 rpg for all 6 guns. I'm wondering if that would've been a better load out than having one set of MGs having 400/390 rpg and the rest having 270/260 rpg. Or why not do 270 (260 for the H) for all 6 to have the same firing time as long as it didn't upset weight balance?

Makes me wonder if the 400 rpg for the F-82 was just a max/overload, given the the XP-82 was stated to carry 300 rpg, though the XP-82 restoration project did managed to put 413 rpg for each of the 6 .50s (probably counting ammo in the feed chutes as well as in each ammo box).

The XP-72 (basically a P-47 re-engined to use the R-4360 corn cob) was armed with 6 .50s, but with typically 270 rpg. I guess that the USAAF learned that for most missions 400 rpg for 6 Ma Deuces was a bit overkill and just added weight (though also aerial marksmanship was improving with improved sights such as the K-14 series).

 

[Shortround6]

What does seem to be unknown is just which .50 cal gun they were planning on using in some of these end of the war aircraft.

The M2 was the fall-back position but the US had been trying very hard to build a faster firing gun and had just about reached it within a few months of the war ending.
There was gun that fired at around 950rpm that was adopted using a T number and 8-10,000 built or ordered(sources vary) but at almost the same time they decided to adopt the Gun that became the M3 gun that fired at 1200rpm.
Obviously they would shorten the firing times of the aircraft.
P-80s had 300rpg (?) as did the F-84 and the F-86 had slightly less using the newer guns.

We know what was done, we don't know why.

 

[33k in the air]

According to either the Flight Operating Instructions and the Erection & Maintenance manuals for the P-51D:

380-270-270 rpg (inboard-center-outboard) for 920 rounds per wing, 1,840 rounds total --- OR
500-270-270 rpg (inboard-center-outboard) for 1,040 rounds per wing, 2,080 rounds total (the inboard guns later reduced to 400 rpg for 940 per wing, 1,880 rounds total)

Alternative load out was:

490-xxx-490 (inboard-no center gun-outboard) for 980 rounds per wing, 1,960 rounds total


According to the Pilot's Handbook for Navy Models F8F-1/2:

F8F-1/1N = 325-300 rpg (inboard-outboard) for 625 rounds per wing, 1,250 rounds total


Figures vary by source; the weight of 100 rounds of .50-cal. belted ammunition was about 31 lbs.

 

[BarnOwlLover]

I wonder what the fascination was with having almost a full half minute of firing time with .50s? I know that the P-38 and P-47 were originally designed as bomber destroyers that ultimately saw mostly fighter vs fighter combat. Same thing ultimately happened with Hellcats and Corsairs. It seems that Shortround is sort of implying that 12-maybe 20 seconds of firing time is optimal for most fighters. And hence (given the reduced ammo that P-38s and P-47s often carried on fighter missions) carrying tons of ammo was overkill, or not really necessary for most missions.

Though, also as he said, it may not have been 100% clear what these planes would be doing prior to entering combat. Most German and Italian fighters did carry a lot of ammo, but not a lot of guns (maybe more ammo tried to make up for reduced armament/no. of weapons). Most Japanese fighters IMO did carry reasonable ammo as far as rounds per MG or cannon. Granted, the USAAF (and even the USN) didn't go gung-ho for cannons until after the war.

 

[PAT303]

I wonder what the fascination was with having almost a full half minute of firing time with .50s?

Same, give me four 20mm Hispano's with 15-18 seconds of ammo

 

[33k in the air]

I wonder what the fascination was with having almost a full half minute of firing time with .50s?

I'd say it comes down to the fact that most pilots were not expert shots, so more guns with more bullets equals a better chance of scoring a hit (the shotgun approach). Fewer guns and/or fewer rounds suited the crack shot (the sniper approach).

 

[gingerbob]

I suspect that the reason for more rounds on the inboard guns is "life insurance"- if you've shot your wad, you've still got a couple of guns if you need self-defense.

As for "30 seconds of firing", it is a judgement call between weight saving and "Gee, now that I've taken the trouble to get here, wish I had enough rounds to do what I could do!" How they arrived at approx 30 seconds? Probably some pseudo-technical round number based on limited experience, like a lot of the "requirements".

 

[BarnOwlLover]

Well, if we had Raufous Mk 211 .50 BMG rounds in World War II (or the 13.2x99mm Hotchkiss HMG rounds that the Belgian versions of the AN/M2 Browning allegedly used), 6 .50s with 270 rpg (or whatever that particular plane was loaded with), you'd have a hell of a loadout. But this was World War II, the .50 BMG as an anti-material round didn't advance to that point yet (and wouldn't for decades), and I do believe there was some debate that though a HE or HE/I and AP/I 13.2mm round existed in Belgium, there's some doubts at to how much more (if any) effective it was compared to equivalent .50 ammo.

 

[PAT303]

Or four hispano's, two loaded with SAPI every fifth round a AP-T, the other two HE-I. If you want to shoot up ground targets like trains add more AP-T.

 

[BarnOwlLover]

Problem was US made HS404s rarely worked as intended, though US makers never adopted the shortened chamber that solved most of the issues that the British initially had.

Of course, on secretprojects I've read that the USAAF wanted eventually to mount 4 20mm cannons on the P-51H and the P-82B/XP-82, but NAA said "nope". Of course, if there's any truth to that on either end, I can say that NAA did propose armament fits for the P-51 and A-36 that included 20mm and 37mm cannons or a mix of the two, per several posts made my drgondog when discussing Mustang armament fits. But each time NAA proposed anything other than .50s, the USAAF ignored them.

Thus if the USAAF wanted 20mm cannons on the P-51H or the P-82B (due to both aircraft's potential as interceptors with 5000+ fpm climb rates on normal max take off weight and WEP), and NAA told them no (story is that NAA spec'd out for 6 .50s on both planes already when the USAAF suggested it), North American had been burned before on doing such work expecting/hoping the USAAF would come around to the cannon armed P-51.

And yes, If you've read the chapter about the P-51H in Michael O'Leary's book "Building the P-51 Mustang", there were mock ups of the NA-117 (pre-production P-51H design, production models were the NA-126, with the P-51L being the NA-129), there's photos there (as well as probably in Boeing's archives, and maybe someone else's archives) of a P-51H mock up with 4 20mm cannons.

 

[GreenKnight121]

I wonder what the fascination was with having almost a full half minute of firing time with .50s? I know that the P-38 and P-47 were originally designed as bomber destroyers that ultimately saw mostly fighter vs fighter combat. Same thing ultimately happened with Hellcats and Corsairs. It seems that Shortround is sort of implying that 12-maybe 20 seconds of firing time is optimal for most fighters. And hence (given the reduced ammo that P-38s and P-47s often carried on fighter missions) carrying tons of ammo was overkill, or not really necessary for most missions.

I know that if I was flying a fighter tasked with escorting bombers deep into Germany and back that I would want every round I could stuff in my fighter. After all, there was the strong possibility of needing to engage in fighter-fighter combat multiple times going in, for several minutes over the target, and multiple times on the way out.

Bomber interception and fighter sweeps both result in far less combat per flight than did bomber escort potentially did.

 

[PAT303]

I know that if I was flying a fighter tasked with escorting bombers deep into Germany and back that I would want every round I could stuff in my fighter. After all, there was the strong possibility of needing to engage in fighter-fighter combat multiple times going in, for several minutes over the target, and multiple times on the way out.

Bomber interception and fighter sweeps both result in far less combat per flight than did bomber escort potentially did.

Once a fighter was in contact the tanks were dropped and the engine boosted on full rich mixture, your not going to engage in multiple fights because you don't have the fuel, also by the time your finished the first engagement you would be not only below bur far behind the bomber group you are defending so burning more fuel you don't have trying the catch up is not an option. Having less ammunition for cannons is not that great a disadvantage because no fighter that flew in WW2 would be in a flying let alone fighting condition after a 2 second burst of SAPI HE/I from four Hispano's hits home,

 

[BarnOwlLover]

And the issue with wing mounted HMGs or cannons is that if you got anywhere near where the gunfire was set to converge, you weren't getting out of there in good shape, if at all. I think the reason why the USAAF especially stuck with .50s for most of World War II was reliability (it already took a bit of time to get the AN/M2 to perform as hoped, and there was the M2 TXX (higher rate or fire) and M3 (much higher rate of fire) coming around as well, though that was late/post war), and weight (1 .50 AN/M2 or M3 was 55-65 lbs, a Hispano Mk 2 was over 100 lbs for the gun, and the Mk V was just under 85 lbs).

Even in the USN, pilots tended to favor the .50s for that reason for air-to-air work, though they liked the cannons for ground strafing.

 

[GregP]

50-caliber ammunition in a belt weighs 20.3 to 22.3 pounds per 100 rounds. The difference comes from changing the ammunition type among tracer, ball, AP, etc. So, call it 21.3 pounds per 100 rounds. That's 9.66 kg per 100 rounds.

If the stock setup called for 380-270-270 (inboard-center-outboard), that's a total of 1,840 rounds. If it went to 400-400-400, that's 2,400 rounds.

1,840 rounds would weigh 373 to 410 pounds with the average being 392 pounds or 178 kg.
2,400 rounds would weigh 487 to 535 pounds with the average being 511 pounds or 232 kg.

So, the total effect would be to add an additional 119 pounds (54 kg) very close to the CG. Since the aircraft generally flew at between 8,500 and 10,000 pounds, the added 119 pounds would not be much noticed, assuming there is room for the added ammunition in the wings. I'm not too sure there IS room, but maybe. I've seen the ammunition boxes up close and personal MANY times, but never when loaded with ammunition. Usually, when I seen them open, they have some small luggage in them so the pilot can stay overnight at some airshow. The ammo boxes aren't that big but, if they COULD fit in 400 rpg, I don't see it as making much of a difference in flying characteristics. On the other hand, it might make the pilot happier since he would have more ammo.

 

[BarnOwlLover]

I'm thinking if they wanted to do that (they being NAA), they'd have to stagger the guns like on the F-82 or say a P-47 or Hellcat. Which, one, is the room in the wing for that, and two, on the F-82, that wing center section was big enough to completely house the gun barrels without them protruding. I'm not sure that if you stagger the guns that you'd get away with that on a normal P-51 wing.

 

[bobbysocks]

From what I remember everything on the 51s was API and tracer. Tracers ran every 5th (?) round until you got within so many rounds of the end of the belt. Then it was every round or every other round was a tracer to warn you that you were running out of ammo. A lot of guys didn't want to use up those last rounds in case they got jumped coming home. It wasn't uncommon to get separated from the group during a dogfight and having to find your way home alone. Many times they would find a straggler from another group or a lone bomber and join up with them.

 

[Shortround6]

50-caliber ammunition in a belt weighs 20.3 to 22.3 pounds per 100 rounds

That is way lower than anything else I have seen.
Most sources say about 30lbs per hundred.
Pre war M1 ammo was about 1846 grains per round using a 753 grain bullet.
Times 100 gives 184600 and dividing by 7000 (grains per pound) gives 26.37lbs per hundred, no links.

The 7000 grains per pound gives a little bit of perspective. If you lighten the bullets by 70 Grains (683 grains) you only lighten the belt by one pound.
A lot of the US .50 cal M2 and M8 ammunition used bullets that were around 710 grains. Some of the tracer bullets were around 675-680 grains.
an old post on this site says about 267grains per link or 3.8lbs per 100 rounds of links.
Same post says 67.2 gains per link for .30 cal.

I am pretty comfortable with 30lbs per hundred for WW II ammo and links.
They are working on (or have already made) lighter ammo/links in the last 1/2 dozen years but the problem for WW II is that you need modern tech to do it. Like replace the brass case with plated aluminum and prayer.
Unprimed case is supposed to weigh 865 grains with a tolerance of up to -50 grains.

 

[Greyman]

British manual says; 'Weight of belt, per 100 rounds: 30 lb. 4 oz., approx.' (A.P. 1641 L, Vol. I - Browning .50 in. gun, No.1, Mk.II)