I understand that it was possible to boost the RoF of the M2 by replacing the recoil buffer with a solid metal piece. However, this was decidedly unofficial as it put extra stresses on the gun, harming reliability. The official RoF remained 750-850 rpm.
The M3 looked the same as the M2, but many of the parts were changed to enable it to withstand the added firing stresses, and these were not interchangeable with the M2.
Tony Williams: Military gun and ammunition
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