I will try to sum up (brief history) the US .50 cal gun, and it's ammo.
For the gun pretty much unchanged from the early 20s to the late 30s/1940. Some details but function and rate of fire didn't change much.
These guns fired at 600rpm at BEST. They were often tested with short belts and longer belts or high lifts of the belt from the ammo storage to the gun feed way slowed them down.
These 1920s and 30s guns were known as the M1921 although a few other years might be thrown in. By 1837 or so the gun had been redesigned to allow different versions to be built off the same receiver (aircraft gun, ground (surface) gun with air cooled barrel or water cooled and feed could be switched from left side to right side. This was called the M2 but the rate of fire stayed the same as the older guns. Please note that the "ground" gun with air cooled barrel fired at a lower rate.
By 1940 they had figured out how to increase the rate of fire and older guns could be converted to new standards with a parts kit. the firing rate is given variously as 750-850rpm but this covers not only the variation from gun to gun but different installations. Either as part of the M2 or separately ( I am going by memory here and not looking things up) the roughly doubled the belt pull (force the gun exerted on the belt to move it or lift it) by changing the cam tracks. How quickly the M2 was gotten into service by the US is subject to question but since the US was not in a shooting war for 1940 and most of 1941 it really doesn't have a lot of bearing. The British got some of the M2s during this period.
The M2 carried on through the war essentially unchanged (as far as rate of fire goes) until the end of the war. Around 8,000 (?) of the faster firing (1200rpm) gun (later called the M3) wer built in very late 1944 and 1945 under a "T" number but actual details of where they went seem a bit sketchy. The M3 had to be built as such and while many parts were interchangeable an M2 could not be modified into an M3. Now please note there were a number of projects starting in 1942 to build this 1200rpm gun (at least three companies and some companies had more than one project going at the same time) so the planners might have expected the fast firing guns to show up before they did. Some of the projects were pretty much failures from the start and others just missed the reliability goal (number of broken parts per 5000 rounds) and the programs dragged out over two years.
Please note the increased belt pull helped sort out some of the early feed problems (but not all).
Please also note that the US ordnance dept had some rather strict requirements for the number of jams or broken parts allowed per 1000 rounds (or 5000 rounds) fired and, barrels aside which were considered expendable, a rather high expectation of gun life, like tens of thousands of rounds given replacement parts.
Ammo, as has been mentioned, went through a lot changes. While during the 20s and 30s you had the Ball and AP rounds, and tracer. There was no HE ammo or incendiary ammo in service. The Ball and AP rounds had a velocity of about 2500fps. This rating was for all guns at least until the M2 HB ground gun came along with it's 45 in barrel.
In the late 30s the improved smokeless powder/s that had been developed in the 1920s and 30s allowed for higher velocities. There is some confusion here as some books from the 30s or early WW II list MV of 2400fps for the old ammo in range tables (to account for a worn gun instead of factory fresh) and the new ammo is listed as 2700fps but with a lighter bullet this soon went to the more well known 2880fps. I don't know how much was due to the ighter bullet, how much was due to a different powder and how much was due to accepting higher pressure or different barrel life, the new powders may have been a bit "cooler" and allowed things to be pushed a bit more. However the .50 was always a barrel burner and needed short bursts and cool down periods if there was not to be a huge consumption of spare barrels.
the M1 incendiary was not adopted (or at least issued) until after the M2 AP and M2 ball ammo was issued and these are the rounds that have 2880 fps muzzle velocity. The AP and ball ammo are issued in 1940. It is not until very late 1941 or 1942 that the M1 incendiary is issued in numbers (basicly and enlarged British Dixon bullet.) and it may have taken a while before it's use was wide spread. This was followed by the M8API which carried roughly the same amount of incendiary material as the .303 incendiary bullet of about 1/4 it's weight.
So we have 2 if not 3 different capability guns (not counting the M3) different installations (and the synchronized guns could be cycling at 400-500rpm)over the years and several different sets of ammo. the low velocity no incendiary ammunition group. the high velocity but still no incendiary ammunition group, the high velocity with incendiary ammunition group and the M8API ammo years, this leaves out Korea.
Trying to assess or evaluate the Buffalo, F4F, P-36, P-38,P-39s and early P-40s (though the E) needs to take into account the rapidly changing guns and ammo and what was known or planned for the future near future.