I had this problem... here is what happened:
Symptoms same as yours, and excessive recoil. Now, I am a big guy, but this thing would bruise my shoulder after a day at the range.
What was happening, the b/c was coming back too fast, not giving time for the extracted brass to eject.
Cause was probably that a batch of 16" barrels that hit the market about that time (late 80's) had oversized gas ports.
Cure, the easy low cost one, replace the buffer spring with a Wolff Extra Power Action Spring.
www.gunsprings.comwww.gunsprings.com/RifleShotgun/Colt_RsNF.html#ARStock No. 16501 - CAR 15 and M4....................$ 14.99 ea
Years ago, some guys had this problem, and thinking it was not getting enough gas, drilled the gas ports larger. This just made the problem worse.
Wolff has a standard power, reduced power, and extra power springs for the CAR-15/M4 rifles. You want the extra power spring.
Also, make sure you have a metal buffer, not one of those crappy black plastic shot filled buffer.