I have a Bushmaster M4gery with a 14.5 barrel and a 4 position RRA telestock. The rifle will fire and cycle fine for a few rounds, and then it will eject the cartridge, but fail to push the bolt back far enough feed another one into the chamber. Sometimes it will do this on the 1st shot of the magazine, and it varries how many shots I can get off before this happens. I've chagned mags and ammo, but that doesn't solve the problem. I was thinking that it has something to do with the buffer spring or the buffer. Any suggestions? Possibly a lighter buffer spring?