Can't make head nor tails of this thread
Solution to headspace problems in ANY rifle, but more so in break actions(T/C and NEF) or rear lug actions like the Rem 788.
FIRST...BUY a set of Hornady headspace gauges AND a good dial indicator and 1" mic...if you don't already have them.
SECOND...fire off at least 5 NEW rounds.
THIRD...MEASURE the shoulder to base length of the fired cases using the Hornady gauges...USE the longest one.
FOURTH...MEASURE the base OD of the fired cases at the pressure ring and on an unfired case at the same point...usually the pressure ring is very evident especially if the chamber is cut oversize(like MANY NEF's are prone to)
FIFTH...BUY a set of Redding Competition shell holders for the .378 case. Use these to setup your sizing die so you WON'T push the shoulder back...record ALL your numbers as each rifle will have different measuerments.
SIXTH...Setup your press by putting in the shell holder that came with the dies or the "original size" shell holder. Run the ram to the top of the stroke...screw the sizer die down to contact the top of the shell holder, pull back the ram and run the sizer down another 2 turns and LOCK IT IN...YOU DO NOT WANT TO CAM OVER AT THE TOP OF THE RAM STROKE...all you do is put extra pressure on the press linkage, frame, dies etc.
The object is to take out ALL the slop in the stroke and have the shell holder contact the base of the sizer die firmly.
SEVENTH...with the standard shell holder in the ram and the sizer die adjusted as I indicated, size one case and call it the "STANDARD"...KEEP IT SAFE.
EIGHTH...MEASURE the shoulder to base length with the same Hornady headspase gauge as you used measuring the other 3 cases.
This number will give you the amount of sizing THAT SPECIFIC SIZER DIE produces with the OEM shell holder in THAT SPECIFIC RIFLE...to be a "real reloader" you do this to EVERY ONE OF YOUR RIFLES.
NINTH... subtract the STANDARD from the cases fired in the rifle.
THIS NUMBER WILL BE THE AMOUNT OF HEADSPACE THAT SPECIFIC RIFLE HAS...it doesn't matter if it is an AR, boltgun, break action or locked breach universal receiver.
From now on use the closest Competition shell holder that will push the shoulder back at least 0.001". In some case you will need to order a shell holder from Redding to fit the actual headspace. I have rifles that have over 0.020" of "headspace" and shell holders for all 3 standard case sizes up to 0.025" over the standard 0.125" thickness.
I guarantee this will solve ANY headspace issue and will produce cases sized just enough to chamber easily, get the maximum amount of reloads and pretty much stop ANY incipient case head separation...unless the headspace is just totally way out of whack.
I have reloaded and fired ONE .223 case in a Ruger All Weather rifle over 50 times, not once but several times, using a set of Redding FL Bushing dies that happened to fit the chamber dimension perfectly and only work the base 0.001" and using a bushing that only sized 0.001". The case could have been reloaded more times, but I just got tired of doing it. It took me two days to do that test as it was.
The only NEF 223 I've had, had a chamber both oversize, cut too deep AND cut off center. I was going to rechamber it to 222 Mag but after I saw just how badly off center it was I thought about doing one of the several .473 size cases but the short barrel put the kybosh on that so I just cut off the barrel and turned it into a stub barrel and did the 17 FB and 6mm BR. There's enough barrel left to do a 22 cal on a XP-100 pistol at least.
Luck