[4V50 Gary]

I think you got the point on magazines and timing.

There's a great video at Springfield Armory National Historic site which shows a firearm firing in the full auto mode. You can watch the firing, extracting, ejecting, feeding. It does this several times until jam! An action can only feed as fast as the feeding device. Timing the two is tricky and this is one problem the U.S. Army faced when it tried to convert the full auto versions of the M-1 to take the BAR magazine. The light bolt of the M-1 was simply too fast and the BAR magazine spring could not keep up with it.

Garand proposed two modifications. Either lengthen the M-1 receiver 1/4" to allow more distance for the bolt to travel (thereby giving the magazine more time to push up a fresh cartridge) or modify the BAR magazine to be faster (in which case it would not longer be compatible with the BAR - thereby defeating the original concept of making the two weapon systems compatible). Remington finally came up with the best solution when their engineers put a buffer into the recoil guide spring. Said buffer would slow down the slide and correspondingly, the bolt. No modification to the receiver or the BAR magazine was required.