Hi,
Even if you use chromium/borium compounds you will always run into the heat problems associated with higher rates of fire which will always become unsustainable.
Agreed, but I don't think that the capability for sustained fire would actually be necessary.
Here's a video on USAAF machine gun burst lengths and practical rate of fires, showing that with the aircraft version of the M2, a burst of 50 rounds would damage the barrel and reduce both muzzle velocity and accuracy (from the channel "WW II US Bombers", timestamped to 116 seconds):
View: https://youtu.be/VtiB7z94Uro?t=116
Later in the video, it's also pointed out that the sustained rate of fire in the case of the B-29 was limited to 30 rounds per minute, with 40 rounds per minute being permissible in emergency cases.
The video also shows a table with continuous firing limits which considers "cook-offs" (rounds being inadvertently fired due to the primer self-ignition caused by heat transfer from the overheated barrel), and for most burst lengths, the limit is 30 rpm or lower.
What I'd speculate is that the heavy barrel would at least allow a longer initial burst (or a greater number of short initial bursts) due to its higher mass absorbing more heat before reaching a dangerous temperature. I think you're probably right that it wouldn't make much of a difference once the heavy barrel has reached the critical temperature, if that's what you'd consider the "sustainable" case.
There might be a good reason the heavy barrel didn't make it into aircraft, but I've never found this mentioned anywhere.
From the "USN Report of Joint Fighter Conference NAS Patuxent River", quoted via Tony Williams:
"In addition to that you have one more great advantage - that is you can have longer and more frequent bursts without damage to the gun with the 20 than you can have from the .50 cal. That is important for the strafing airplane, because they are burning up their barrels and ruining their guns on one flight. Sometimes it is long before that one flight is over. They will come down with screaming barrels and get trigger happy, and then all the barrels are gone in one flight. It should not happen in a 20mm. Of course, you have disadvantages. You have a heavier installation, one-half as much ammunition for the same weight. Our standard ammunition in the Navy is 400 rounds in one gun. The Fleet has set up 30 seconds of fire as a minimum requirement for the .50 cal gun. We can't do that with the 20, so we give them 200 rounds. The 20 is lethal enough to get far more results out of that 200 rounds than the .50 ever will out of 400 rounds."
That sounds as if the Navy was ready to change something about their fighter armament. I find it somewhat surprising that they didn't try the heavy barrel on the M2 .50 cal, but again, maybe they had a good reason I am simply unaware of!
Regards,
Henning (HoHun)