From: Near Portland, OR
The 37mm of the Ju 87G was a modified AA Gun, it's penetrative power was likely less than the AT gun due to smaller or shorter cartridge.
Both guns had 12 rounds.
The P-40 had a short production run with Merlins as P-40F and L albeit not with two-stage supercharged engines.
The weight of the projectiles between the German BK 37 and the US M4 were in the same range. Different round type had different weights, but they were between 380g and 685g for the German BK 37 with the AT round being the heaviest and the APCR round the lightest. The HE round for the P-39 was 608g and the AP round was 750g. The M4 auto cannon was also developed from an AA gun. The ammo clip for the M4 was 30 rounds.
The Packard Merlins were initially built to supply Hurricanes built in Canada (the Mk X-XII). Some were allocated for the P-40F in an attempt to give it better performance, but the P-40 was really not in the same league as German fighters. A skilled pilot could win an air battle in an inferior aircraft against a less skilled pilot, but with equal pilots an ME-109 or Fw-190 would usually beat a P-40.
Packard Merlins also made it into the Lancaster BIII and Mosquitoes built in Canada and Australia as well as the Spitfire Mk XVI. The Spitfire XVI was a Mk IX with the Packard engine. It got a different designation because the tool kit needed to work on the engine was different from the Rolls engine.
Once the British showed that performance of the Mustang could be greatly improved with the Merlin, the bulk of Packard output went to Mustangs Once the Mustang was sucking up engines, Curtiss was low on the priority list to get them.
Packard built about 55,000 Merlins during the war. Rolls built close to 150,000. Total Allison V-1710 production was around 70,000. At low altitudes the Allison and Merlin were pretty much equal, but the Merlin had much better high altitude performance. Allison worked on a two stage supercharged V-1710 for most of the war, but it wasn't introduced until 1946 where it replaced the Merlin on the P-82. The Allison supercharged V-1710 pretty much matched Merlin performance, but it was too little too late. Few were built because by the time it was introduced the jet age was making piston engines obsolete.
Packard started making Merlins in 1940. The War Planning board was working on plans to fully mobilize US production then. Instead of taking the attitude that American engines were superior in the face of contradictory evidence they should have sucked it up and realized that the British built a better liquid cooled engine and switched Allison's production line over. They could still be proud of American radials. US radials were definitely world class, some of the best fighters of the war were powered by P&W 2800s and the radials in bombers were clearly excellent too. Not only were the power to weight ratios very good, but US made radials were extremely reliable, even in harsh conditions.
If the switch to the Packard Merlin had been made early enough, it would have had a minimal disturbance on production and the P-38 would have definitely been a better fighter. The P-39 may still have ended up in the lend-lease bargain bin because of its range, but it would have been a better fighter. The P-40 was only marginally better with the Merlin, it was an antiquated airframe that was essentially a generation behind the curve. It was a contemporary of the Hurricane (it was the same core plane as the P-36 with an inline engine). Because of it's "old school" design, it did lend itself well to the harsh conditions on poor front line airstrips. I have argued for quite some time that the service rating of the P-40 should be 1 in the game. It was unusually reliable for an inline engine plane in part because of the relatively simple radiator system right under the engine. The rest was due to it's heavier than normal construction that was more typical of late 30s designs.
WitP AE - Test team lead, programmer