The M14 Rifle, like all self loading firearms, requires a little looser headspace than a bolt action rifle, the usual SAAMI spec.  Here's the data:


M14 minimum headspace (GO):  1.634"  
M14 factory maximum (NO-GO):  1.638"
M14 field maximum:  (FIELD or REJECT):  1.6445"

M14 GO is the same spec as SAMMI NO-GO.  Don't let some gunsmith tell you your rifle is out of spec 'cuz it closes on a NO-GO gage.  The M14 should accept the SAAMI NO-GO gage to be in spec.

M14 NO-GO, the gage it should not close on before it leaves the factory (but afterwards it doesn't matter a bit) is the same as the SAAMI FIELD-REJECT gage!  No problems at all if you close on a SAAMI FIELD gage.  NO-GO gage is to provide factory accuracy and maximum life in service.  This is not a safety gage.

M14 FIELD-REJECT.  It's doubtful your rifle will ever wear to this point but swapping bolts around can result in this massive length.  Note again, this is not the same as the SAAMI field gage!

I received a call from a shop rebarrelling one of my rifles that the bolt would "almost close" on a SAAMI FIELD gage.  Of course they wanted to sell me a $100 bolt body in trade for mine!    My response was, of course, "Great, it meets specs!"

Your M14 will probably function on the range using the tighter SAAMI specs, but too tight a chamber will often result is operating problems.   The SAAMI minimum (GO) spec is a very short 1.630" and you will likely have problems that short.  Match M14 Rifles (an oxymoron the past few years) are usually speced at a tight 1.631" or 1.632".  You're on your own in this length.

GO = Minimum chamber size when new.
NO-GO = Maximum chamber size when new.  Only when new!
FIELD = Maximum chamber size in service.

-- Chuck