What the heck?

[guns-o-fun]

I recently rebarreled a Savage 111 to 280 Remington from 25-06.  The headspace passes the go-no go test with flying colors.  I fired about 8 rounds (factory, 140 grainers) and did the RCBS Precision Mic headspace measurement.  (the mic is off by about -0.005" compared to a go gauge, BTW).  On average, it looks like I have something like 0.001" to 0.002" (above SAMMI minimum) of headspace measured on fired cases.  I tried the bullet seating measurement.  Well, first of all, the land nut will not align at zero on the gauge body marks - it appears to be off by 0.030" (ie, I have to turn the land nut to 30 to line up with the marks on the body).  Anywho - the measurement tells me that I have NEGATIVE freebore!  What am I missing here?
BTW, the 280 kicks pretty darn hard.  I am a little worried that the bullets might be getting into the lands and increasing peak pressure.  Any help would be much appreciated.  Thanks.

[gunnut69]

I am not familiar with the RCBS guage in question but don't believe you do either as negative free bore isn't really possible. You can check for a lack of freebore by coloring the bullet with majic marker and chambering the round. Marks will appear on the bullets circumference when it is carefully extracted. Random scratches can be ignored (from chambering process) but several equadistant marks on the bullets ogive show a problem exits. I reccommend a call to RCBS customer service get help with the measuring devise. They have an outstanding customer service group and will help I'm sure..
 

[guns-o-fun]

I really should call RCBS.  I do realize the negative freebore is nonsense...I looked for rifling marks on chambered rounds - but not with magic marker.  Thanks for the suggestion  .

[gunnut69]

I am a strong RCBS supporter.. They have the best customer support that is possible!!! I suspect the guage is not registering correctly. If headspace was a problem it would have shown up on firing.. Since its a new barrel check to be certain the reamer didn't chatter and leave an uneven shoulder angle, which could mess with the guages measurement... If you chamber a barrel you can check your work by casting the chamber with cerrosafe. This'll give a look at the throat also.. Once had a friend chamber a 7mm STW and fired cases flattened primers and damaged cases. The cause was a reamer without an integral throater. Check the throat for bullet contact without firing...

[gunnut69]

Check the throat contact first and double ckeck the shoulder for uneveness caused by reamer chatter.. If theres doubt a chamber cast is a good way to look at whats in there..

[Catfish]

I am confussed about what the problem is. If you are right with your go and no go gages how can you be wrong?? If the factory ammo chamberes and goes bang when you pull the trigger and you not seeing presure signs on the cases you are good to go. I have chambered a couple of barrels and put them on Savage rifles. I have no go or no go gages, but I put together a dumby case with my dies and set everything to that case. When you fire a round in your chamber it is then head spaced to your chamber.    Are you looking at you measurements backwards? Could your measurement mean that you bullet is seated .030 off the lands?

[gunnut69]

Firstly headspace really has almost nothing to do with bullet seating, it deals with the positioning of the case in the chamber. If you use a cartridge to set the headspace the rifle is headspaced to the cartridge you used, not to a set of specs which define the cartridges calibre. For a rimless round the headspace is the measurement from the datum line (usually 1/2 the way down the shoulder of the chamber cut to the bolt face. If the chamber is too short the bolt will not close on a go guage. If it will not close on the 'no go' guage it is said to be in spec for the cartridge. If it closes on the 'no go' guage the headspace is too long. If it closes on a field guage the chamber is too long to a dangerous extent. None of the affore mentioned has anything to do with bullet seating and the relationship of the bullet to the lands/groves in the bore. The 'throat' is that part of a chamber in front of the case mouth that has the lands reamed away to groove depth.

[guns-o-fun]

Well...I finally found a response that RCBS sent to another precision mic user who had a similar problem.  Turns out you are not supposed to read the bullet seating nut and mic the same way as the headspace nut and mic.  You simply turn the the seating nut down until it just tightens on the bullet and then read the line on the nut that lines up with the line on the mic body.  According to this I have a throat somewhere in the neighborhood of 0.045".  Yes, the go and no go gauges give me a headspace indication somewhere between 0.001 and 0.003 inches.  The last Savage I rebarreled came out with somewhere around 0.0015 and 0.002 inch as measured with a fired case and plastigage.  The RCBS headspace gauge gave me a headspace measurement of around 0.002 after I calibrated it with the Forster go gauge (2.101").  The zero on the gauge itself is off by about -006" according to the go gauge measurement.  This whole thing just sort of gave me a nasty little brain twist, because I could not imagine how I could have negative throat length.  Their directions with regard to reading the bullet seating measurement are inadequate.  I will use an LNL case to measure where to seat bullets instead of the RCBS Precision Mic anyway because I do not use a micrometer bullet seater.