FtEj

FtEj is often a sign of a short stroke; check that the ejector fully telescopes under the bolt face. The ejector spring should be VERY stiff.  The ejector spring is, should be, the same spring as the one used under the selector detent, consider it a spare.

FtEx

Usually a weak extractor spring, can also be caused by over function, tight HS, rough chamber, or too small of a chamber. Check the extractor groove for signs that the extractor is firmly grabbing the case. If there is a mark from the extractor on the extraction groove then you probably have an over function or tight HS.

FtF

Often a sign of short stroke, perform short stroke check as outlined in the Troubleshooting Checklist. Can also be caused by a bad magazine, retest with a known good magazine. Can also be caused by mix and match of parts with M4 ramps. Uppers with M4 ramps require barrels with M4 ramps or the nose of the round will catch under the barrel extension. FtF may also happen if the barrel sits too far forward (>0.012" between the front of the magazine well and the barrel extension) in the upper.

M4 ramps  
CLEARING UP “M4 FEED RAMP” MISINFORMATION  


Headspace
Chris J. Pikus' HS article
Headspace, Torque Values, and Barrel Break In

In general, longer HS leads to greater reliability, shorter HS leads to increased accuracy. The military favors reliability, some companies prefer accuracy.

HS gage dimensions as follows:
Military
FIELD 1.4730-0.0002"
NOGO 1.4706"
GO 1.4646+0.0002"

Civilian
FIELD 1.470"
NOGO 1.467"
GO 1.464"
Colt's FIELD II 1.4736"

New assemblies must be checked with a GO gage before test firing, use of a loaded round is not enough as loaded rounds can be shorter than the GO dimension. HS that is too tight can lead to FtEx and FtEj due to increased chamber pressure. HS that is too long can cause case head separations. Short HS is of greater concern as it affects reliability. Case head separations are not as common with military ammunition as it has a thicker case.

Chromed bores cannot easily have their HS deepened. Non-chromed bores can be reamed deeper with a standard chamber reamer. Swapping bolts (assuming that the bolts are good) rarely changes the HS more than a few thousandths of an inch.

Dimpled primers

It's normal for chambered but unfired rounds to have a slight dimple after chambering. The dimple is left by the floating firing pin.

Wear inside upper at cam pin clearance cut

It is normal that a slight ramp is worn into the inside of the upper receiver at the rear edge of the cam pin clearance cut.