A test ammo gauge will always tell you if it was ammo, or the rifle instead (could be either shot stroking, someone riding the charging handle back down, or some other problem like holding onto the mag as a hold point when firing the rifle).




As for back to ammo, coupe of problem that an ammo test gauge will determine.
The first is that you are not fully sizing the case all the way  back down, hence have a gap between the bottom of the die and the shell holder under actual ramming tension.
The second, is that you are using way too much bullet crimp pressure, this is causing the cause shoulders to slightly bulge, and again will rear it ugly head when the ammo is run through a test gauge.

Also, could just be the load of the reloads, and although they may cycle fine through your rig, could be too much pressure through his rig instead (tighter chamber).

Yep, and most of the time on a single stage press, the size is not adjusted downward to kiss the top of the shell holder under actual ramming tension.




On a progressive press like a Dillon with a Dillon die set, the bottom of the sizer is not set up this way, but instead adjusted with a Case gauge to get the case correctly sized to begin with.  Hell, most of the time when you are using a Hornady die (meant to kiss the die holder at cam over) set with the Dillon, you end up facing the bottom of the sizer to give the correct releif so the sizer does not have to be cranked down to acheive the correct sizing of the case/ bind the hell out of the shell plate.


Lastly, unless you are using something like a Lee FCD for 223 that works off the shell holder/shell plate, but instead a bullet crimping dies that is dependent off the OAL of the cases, you have to be dam sure that the entire lot of cases are all trimmed the same lenght.  The reason for this, you can not put a heavy bullet crimp on a 223 case, since it ends with the shoulder of the cases bulging/ the case not longer correctly sized.  So all it takes is one too long of case to sneak in the works with a standard bullet crimper to over crimp the case/bulge the shoulder, and that finished round will not longer case gauge correctly; even through it was correctly full sized to begin with.


As for the Lee FCD for bottle neck cases, before you put it into play, pull it apart to clean and polish the internals.



On the right is the FCD and you want to polish both the top of the sleeve and the inside of the die where the top of the sleeve is going to be pressed in via the shell holder/shell plate to create the crimp.  Also, just a touch of grease at the contact area does wonders as well.

And again, notice that the FCD is not dependent of the OAL of the cases, since it creates the bullet crimp inward via the crimping sleeve being pushed upwards via the shell holder/shell plate isntead.