12 F-Class 6BR headspace questions

[obiwannabe]

Background first. This year The Boss bought me a 12 F-Class in 6BR in lieu of a Christmas bonus.:thumb: (Yeah, he actually did and no, we're not still hiring :p)

I topped it with a Sightron 8-32x56 that I had on hand and ordered the components to start loading. Everybody said to buy Lapua brass but I couldn't find any--out of stock everywhere. Norma brass was actually cheaper ($93/100 at Midway) and available so I bought 200 pieces.

I loaded some ladders with RL-15, 8208, and Varget with 105gr Hornady BTHP's and CCI450 primers starting 0.015" into the lands as measured with a Hornady gauge and a piece of new brass that I drilled and tapped myself. Shot the RL-15's first starting at 27gr. That's a fairly light load. Quickload predicted ~2680fps at 47,500psi using a start pressure of 7200. Actual velocity was only 2580 but it must have randomly landed on a low node as it was wicked accurate. After sighting in my first 3-shot group went 0.159".

Nothing I have shot in 4 sessions with this gun has grouped larger than 0.43". The best was with 28gr of 8208xbr--2826fps and a 5-shot group measuring 0.136". Really could have been one round hole but I was giggling on the last shot and it screwed up my breathing sending one a bit high. So, out-of-the-box this rifle is abso-friggin-lutely brilliant. Except...

It craters the crap out of primers on every load. And I mean BIG craters. Didn't actually pierce one in 100 rounds, but they're just a bug's breath shy of piercing. About 20% of the craters were so pronounced that I had to grind them off to get the fired case to fit in a Redding shellholder. None of the primers had backed out and none were flattened at all with any load.

So I researched. Common problem with Savages. Firing pin hole, firing pin tip profile, headspace--got it. So, have the firing pin hole bushed, re-profile the pin tip or try a PTG bolt head. Well, I had a PTG bolt head on hand for another project. Very reluctant to change anything on this gun but I just wasn't comfortable with those craters.

Now the PTG bolt head is considerably shorter than the factory bolt head when measured from the back of locking lug to the bolt face--about 0.0125". I ordered a PTG go gauge so I could re-set the headspace.

Then I took some measurements with a Hornady comparator and a 0.375 headspace bushing. Brand new Norma brass is 1.138"-1.140". The PTG go gauge is 1.141". My fired cases all measure 1.145".

I had a hard time finding any official headspace specs from CIP. I did find a reamer print from Kiff labeled " 6mm BR Norma (CIP) European Standard" that shows 1.1453" minimum to a 0.375 datum line. In fact, all of Kiff's 6BR reamer prints show this same value.

It looks like the factory headspace was correct based on the fired cases, but new cases are a bit short resulting in high-normal to slightly excessive headspace on first firing. Can't explain why the go-gauge is so short.


So what's the best thing to do here?
Reset the headspace with the go-gauge and bump the shoulders on my fired brass? Reset the headspace on a fired case, let the brass stretch and hope the new bolt head solves the crating issue? Any other suggestions?

[rmkey]

Sorry I am too ignorant to help but couldn’t the firing pin length and profile be reduced. If there are no signs of pressure it must be the firing pin. I would be terrified to change anything on a rifle that accurate!


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[obiwannabe]

If there are no signs of pressure it must be the firing pin.

That's the other option. Go back to the factory bolt head and try changing the firing pin tip profile. I don't have an extra firing pin handy if I dork it up though.

[RustyShackle]

I would swap bolt heads or bush your current, and buy a handful of firing pins. Then measure and select the best fit for your bolt head.

[Zero333]

The Hornady headspace comparator measurement can be a bit different than in the cartridge print. Especially once you use that comparator bushing thousands of times it tends to wear in and open up a little since it's made of aluminum.

Change the bolt head, add 2 pieces of painters/masking tape to the base of the go-gauge and if it chambers then rifle has too much headspace and you'll need to adjust it.

Ideally you want the bolt not to close on 1 piece of tape on the go-gauge. but 2 pieces is still acceptable, in my opinion. Each piece of tape should measure 0.004". Measure the COAL of the go-gauge before and after the tape to verify the thickness of the tape.

Remove ejector, preferably.

[Mozella]

My rule of thumb is to reset the head space when I change the bolt head. It's easy. If you don't have the tools, perhaps you can borrow them from a fellow Savage shooter.

I use a go-gauge first and then apply a piece of clear packing tape to the gauge and use that as a no-go.

You should check your firing pin protrusion. I'm not going to look up the specs for you, you can do that easily enough yourself. You might need to reduce the protrusion if your primers look bad when there is no chance of over pressure. It's easy enough to adjust the primer protrusion stop once you take the bolt apart. Google will provide you with videos.

Hint. Put an allen wrench in the assembly bolt in such a way that when you put the bolt down on the floor, you can step on the rear part of the bolt and have the bolt handle and allen wrench work in such a way to unscrew the bolt. Use a cloth or piece of carpet to protect from scratches. Savage typically puts the assembly bolt on with a LOT of torque and it's hard to get it loose the first time. Again, there are YouTube instructions for this if what I wrote doesn't make sense.

[OLEJOE]

When headspacing, be sure the bolt head is not bottoming out on the barrel before the go gauge bottoms out in the chamber. I just went through this with a bolt head change. The breech face of the bolt was deeper than the factory head.

[Texas10]

It appears that you have an unusually good shooting, right out of the box rifle, so if were me, I would do as little work as possible to remedy the cratering issue. I have fixed this very issue before on a new Savage who's firing pin measured .066 (and was cratering and blanking everything I shot in it) by buying half a dozen firing pins and hand selecting one for better fit.
I will offer this suggestion as a starting point. Measure your firing pin using a good micrometer. It should measure around .0692 to .0698. You'll also find it's probably out of round by .0005 or so, therefore get the max dimension.
The bolt head hole is a bit more difficult to measure. A good drill index will work if you don't have access to a pin gage set. You're looking for about .002 clearance max.

You'll need to sign a release of liability to buy firing pins from Savage however, they will work with you and have the cheapest prices around, not to mention availability.

Good luck and let us know how what your final solution was.

[obiwannabe]

I pulled the barrel and installed the PTG bolt head. No issues with bottoming out on the breech face. I lapped the lugs and re-set the headspace with a loose-ish feel on the go-gauge.

Took it to the range Sunday afternoon. Didn't shoot in the teens like before, but then conditions were different and, frankly, I'm not always that good either. Same load shot 0.46. Lots of factors in play, though. Fired cases measure the same as before, 1.145".

Still getting very small craters on about half the primers. No where near what it was and looks much safer to me. Probably not done fiddling, but I'll start by re-working the load and see what happens.

Last time I called Savage, 3-4 months ago, they refuse to sell parts directly to me. No waiver. Girl on the phone said "we don't sell parts anymore" and directed me to Numrich or Brownell's.

[obiwannabe]

After much testing, I think the PTG bolt head is poo. At least in this rifle.

Never could get accuracy matching what it was from the factory. Working through the load ladders again, groups were typically 2-3X larger then what they where with the factory bolt head.

Switched back to the factory bolt head and everything started humming again just like that. Since everything shot so well before I just settled on a lower charge where the cratering was not concerning. If I have to choose I'd rather have accuracy than speed.