Headspace: Mix and Match Bolts or Return Barrel?

[MaineMan2]

Before you read this, I am fairly certain the answer is “return the barrel that fails NO GO gauge test.”  However, I can swap bolts around so that I have 2 barrels that both pass headspace tests. 

I’m assembling a second, lighter upper for my first 308 AR build. In This thread I asked if I should have a dedicated bolt for each barrel and the answer appears to be “yes”.

My first barrel is a Fulton Armory “medium weight” 18” Criterion barrel (2.9 lbs.) Initially my Aero Precision bolt would not close on a GO gauge.  Neither would the bolt from a Windham Weaponry 308 AR.  Fulton Armory fixed it so that it closes on a GO gauge with virtually no slop. They specifically call it a “match chamber”.

My second barrel from Faxon is a 16” Big Gunner profile.  My new Aero Precision bolt barely closes on a NO GO gauge in the Faxon barrel. However, the first AP bolt will not close on a NO GO gauge in the Faxon barrel. Both bolts have what seems like a lot of leeway closing on the GO gauge in the Faxon barrel. 

So I could assign the first bolt to the second Faxon barrel and pass the NO GO gauge test, then assign the second bolt to the Fulton armory barrel. Is that an acceptable solution or should I get a replacement Faxon barrel?

[shooterrex]

If you have a bolt for each barrel that closes on the go and doesn't close on the no go you are good.

These are hunting guns right? Not long range bench guns.

[MaineMan2]

58 minutes ago, shooterrex said:

If you have a bolt for each barrel that closes on the go and doesn't close on the no go you are good.

These are hunting guns right? Not long range bench guns.

Actually the Fulton Armory upper will be more of a bench rest rifle.  The Faxon barrel upper definitely is hunting. 

[DustBuster]

I have two Faxon barrels, both need their own unique sized bolts to pass the go/no-go tests.  One bolt fits perfectly in one barrel and will fail in the opposite barrel, and visa versa.  I just got lucky and had a bolt that worked for each.  The spent brass from  each I assume will end up being close to identical some where between 1.634 and 1.630.  

If you have a bolt that fits each barrel and passes both the tests, I’d say keep those bolts forever with those barrels, no prob.  I still don’t know how I lucked out with my first barrel (a match chamber) and the first bolt I purchased.  They fit perfectly.  Then the second a Faxon Pencil barrel did not function with that first bolt, but a Sharps bolt fit that pencil barrel perfectly.  Luck of the draw which cancelled out chamber differences from the same company.

[MaineMan2]

Conclusion:  everyone building one of these rifles needs headspace gauges because the variances in chambers and bolts are too significant.

Without going into too much detail, there actually are 3 barrels and 3 bolts here now.  The tight chambered Fulton Armory barrel plus 2 Faxon 16" Big Gunner barrels.  First, the original Aero Precision nitrided bolt that was so tight in Fulton Armory barrel I had to have them ream the chamber out a bit.  I also have a second nitrided AP bolt plus a nickel boron AP bolt that I found on sale.  As explained above, one of the Faxon barrels has an OK chamber while the second has what appeared to be a loose chamber - only the original, tight AP bolt passed the NO GO test in that barrel. 

So the third Aero Precision BCG arrived today and I immediately stripped the bolt, got out the gauges and went to work.  This new bolt will not close on the GO gauge in the tighter of the 2 Faxon barrels.  It barely closes on GO gauges in the looser Faxon barrel (loosest chamber of the 3 barrels).   I don't really need to test it in the Fulton Armory barrel.  Perfect! 

The first bolt stays with the Fulton Armory barrel.  The second bolt stays with the tighter Faxon Barrel (passed GO and NO GO tests), and the nickel boron AP bolt will be dedicated to the looser Faxon barrel.

Remember that the 308 Winchester GO gauge is 1.630" and NO GO is 1.634".  Obviously I have barrels and bolts that have variances of substantially more than 0.004".  One of my bolts failed the NO GO test in the looser Faxon chamber, but the newest bolt just closes on the GO gauge in that same barrel.  While I haven't tested it yet, it seems safe to assume that the new bolt would fail the GO test in both of the other barrels.

I was NOT expecting this much variance in chamber/bolt dimensions.

Edited September 30, 2022 by MaineMan2

[DustBuster]

You sorted it out good, but now have enough parts to warrant 3 guns, haha.

I actually have had three Faxon barrels now in my experience with Lr308.

The first two Match series Fluted 18” were exactly the same and passed the tests with my first ever bolt.

The last barrel now is a Supposedly looser spec Duty Series and Lo and Behold, it is the tightest chamber and required a looser bolt.  You also have discovered this revelation of parts and pieces.  I hope I don’t ever come across the Third size variant like you have, because it would take luck like winning the Lotto to find a match like you happened to have found.  Good Job