Can we agree that at least some of the Ns had 120 gals internal and carried a 110gal drop tank? This is absolutely true.
You have figured out how to figure range/endurance from the pilots manual. 120 gal + 110 gal drop = 230gal less 20 gal reserve = 210 gal divided by 62gph = 3.4 hrs less 15 min combat and 20 min reserve for landing = 2.8 hr x 230mph = 644mi divided by 2 = 322mi radius after reserves. The 230mph is the bomber speed TAS and you are weaving to stay at their speed. Drop tank of up to 175 gal was available for the P-39.
Nope. The P-39N was out of deployment when 110 gallon tanks were a.) manufactured in-theater by Brits/Aussies and b.) the P-47 had priority because they WERE used for high altitude escort.
A drop tank FERRY tank, not self sealing, May have been used to Ferry from point A to Point B but not showing up in any Flight Testing.
Kindly point me to ANY document that states the Centerline rack Ever carried anything heavier that 500 pound bomb?
Last but not least, the P-39 Operating manual charts always express the airframe speeds in IAS - which works in your favor if the chart pointed to 230IAS at 20K - but is states Only without drop tank (75G) does recommended 2200 rpm @24"MP ----------> 200mph IAS ------> 298 TAS at 20K. The latter is a good escort speed but in both ETO, escorting 5K under the bombers is not very useful.
You don't seem to get your head around what you are reading.
Gross weigh at T.O. includes all the fuel, all the ammo, all the consumables, the fixed equipment, the pilot, the centerline rack, the guns, etc UNLESS otherwise stated in the report. The Report will further cite special preparations like taping over shell chutes, wing gun ports, the actual ammo/ballast carried if not max combat load, etc.
A well prepared Test report for speed runs and ROC tests will also state the fuel remaining at the altitudes recorded to give us techie/nerds insight to evaluate the actuals against the Analytics in Flight Performance Calcs . (BTW the P-39 nearly always fell short of Poppa Bell's engineering estimates)
America's One Hundred Thousand by Dean is very rigorous about presenting load outs for 'standard Fighter (less than full load out - but specificity in the actual load and weights) 'Overload fighter' (Full Internal Load), 'Fighter Bomber' (load out when carrying bombs or external fuel tanks').
In my own research in which I have the airframe specs from factory for Empty, Basic (w/GFE and other related weights such as residual fuel/oil remaining after mission that is not easily recoverable post flight for 'empty tanks, guns, racks') and the above mentioned load outs which are useful load items - the weights are accurately reproduced.
Weight and Drag increments are Essential reported items when Range, Climb and top speeds for given altitudes and throttle settings - and often insert Horsepower ratings per the manufacturer at those settings and altitude. A Professional report will further reduce the speed data as a function of Mach Number, Standard Temp and Pressure vs altitude.
ROC is inversely proportional to Gross Weight and proportional to the difference between Power Available less Power Required. Excess Power = Thrust x Velocity - Drag x Velocity ----------> strip weight and climb increases, add drag and climb decreases, run out of excess power and climb ceases (so to constant altitude turns.