On mag height in well, pull the mag catch, strip a mag down to body only, lock the bolt home, shove the mag body into the well until the mag lips are tight against the bottom of the carrier, then look inside the left mag release channel to see how high the lower side mag catch bottom edge, is above the bottom of the receiver mag catch bottom of slot.  Should be only 1/8".

Also to point out, on non ejection side of receiver, bottom of mag catch slot to top of receiver, should be .9375" =/- .0035" . so if slot for catch is correct in lower receiver, then check the catch in play, since could be too small for the slot, with top of catch tab on it too low in the first place.

If more than this 1/8" that the mag can be pushed up to bottom out against bottom of carrier that the mag catch should be holding the mag in the well, then grab another upper to put on the lower receiver and do it again.

Hence is problem the lower receiver in play with multi uppers out of spec that it or the mag catch is holding the mag too low in the lower, or is the problem just the upper in play alone that its center of B/C channel is too high against top of receiver instead..

When reinstalling the mag catch, make sure that threaded end of mag catch is flush to outer face of button.
To add, when installing the threaded side of the match catch, should be semi snug in side of catch slot to being with, if a lot of slop between the catch to slot, need to know the height of the catch, as well the height of the receiver slot.

With this out of the way, can weed out if the problem is lower receiver out of spec, maybe mag catch out of spec, or upper receiver out of spec to being with to cause bolt over bolt over run strip problem, if the rifle is full stroking to get the bolt back past the mag to correctly strip a round in the first place.

From here, need to check for leakage problems, such as bottom of key leaking to top of carrier, gas block to barrel or gas tube leakage and gas block correctly indexed with barrel gas port, and need to make sure that gas tube has been indexed correctly to key as well.

If standard weight B/C, standard tension recoil spring for length of receiver extensiond depth in play, then normal weight buffer for depth of receiver in play (able to pull all the way back on charging handle, front of bolt stops 1/8"~ 1/4" in front of back edge of ejection window, and last 1" of CHH pull is same as middle of pull in tension), then rig should run fine.  

Add a oppressor in play, which does increase the bore back pressure/ port pressure that will cause the bolt to unlock sooner, then there is where a heavy buffer is needed, to bring the bolt unlock timing back to normal, so the residual bores pressure does drop back down to normal, so the spent case is not pressure welded to the chamber at unlock/which can cause the B/C to loose a lot of momentum at back stroke to cause short stroking.

As for the positive note, easy to tell when the  bolt is unlocking too soon/ rifle is over functioning, since with the spent case too pressure welded to the chamber, extractor is going to bend the hell out of the spent case rim backwards during the pull.  

So best way to tell if bolt unlocking timing is correct,  simple load a round in to the mag, insert mag/charge round, fire for effect and go police the spent case to check it.  Hence correct effect will be bolt locked back on catch (catch in front of bolt face, not just on the bottom of carrier), and spent case rim will not be bend back to hell from the extractor on chamber pull/spent case primer will not be completely flattened to sides of case primer pocket from ammo going over pressure.


And the lovely, the shorter the barrel, the more ammo sensitive the rig will be to start with.  
Hence this has to do with the over all working pressure that the ammo is generating to begin with, and the burn rate of the gun power is play, and the slower the burn  rate of the powder, the closer the powders max dwell spike will be to the gas port.  Hence ammo with faster burning powder, and ammo will slower burning powder can both be loaded to the same working pressure, but the slower the power burn rate, the higher the gas port pressure will be.  

So with commercial 223 ammo, its working pressure is lower that 5.56 Nato loaded ammo, and then your still back to the burn rate of the powder in play, that changes the amount of port pressure.
And to throw a curve ball, Nato 55gr ammo loaded to 62Kpsi working pressure, while say 223  69 grain ammo that is loaded to 55Kpsi working pressure, since the power used in the 69 gr ammo has a much slower burn rate, the 69 gr ammo can produce more gas port pressure at is lower over all working pressure, then higher working pressure nato ammo isntead.