Pressure Signs, Lapua Factory Ammo in Brand New Savage 12 Benchrest

[michaelnel]

I picked up my new Savage 12 Benchrest yesterday from the gun store. It's chambered in 6mm BR. I took it home, swabbed out the filthy bore with several wet patches (Shooter's Choice bore cleaner) and several clean dry flannel patches. Mounted the scope rail and the scope and took it out to break it in.

I had purchased two 50 round boxes of Lapua 105grain loaded ammo, and we shot that, and that only. I experienced several instances of very hard bolt lift accompanied by extractor flow impressions and cratered primers. Not all rounds did this, only some. The rifle shot well (under 1/2 MOA for every group we shot with a best of .3 MOA).

We cleaned the bore after each round for the first ten rounds, then after every group of 3 for the rest. We ended up shooting about 30 rounds total, and the rifle performed great, except for these bigtime pressure signs from this Lapua factory ammo.

My theory is perhaps that the bullets were jamming in the lands, although I would sure be surprised if they were with a stock rifle and factory loaded ammo. I have a Hornady COAL gauge and the 6mmBR modified case, but the bullet comparator insert for my calipers won't arrive until next week. When it arrives I will make measurements on the chamber vs the COAL and ogive dimensions on these Lapua rounds.

Anyone else have any theories as to what would cause these high pressure signs?

[wbm]

You don't have to wait for some fancy measuring device to determine if the bullets are contacting the lands....just cycle a few Lapua rounds through the rifle and look for the land marks on the bullet. If there are none then your problem lies in another area. Could be that the Lapua ammunition is just too hot for that particular rifle.

[michaelnel]

I took one of the new Lapua rounds that had never been chambered and blackened the bullet with a Sharpie. The chamber was clean, swabbed out and dry.

I put the rifle on SAFE2 position where you cannot pull the trigger but you can cycle the bolt and carefully chambered the round and rotated the bolt closed.

Then I opened the action and ejected the round, and caught it in my hand.

Sure enough there is an impression of one of the lands on the ogive of the bullet. Seems I have a very tight chamber, unless Lapua intended for these to jam, which I doubt.

I guess I can go though the remaining rounds and seat them all a little deeper, maybe 0.020" ? Does this sound like a reasonable plan?

This barrel is not going to be on there for the long term, I expect to be installing the new barrel within a couple weeks. Maybe I should just not shoot it any more until then?

[wbm]

I guess I can go though the remaining rounds and seat them all a little deeper, maybe 0.020" ? Does this sound like a reasonable plan?

Sure does if .020" is enough. Seat them deeper and check again. If there are no land marks give them a try. Sounds like your barrel is a shooter so you might want to give it a go for a while before you invest in a new one....more expensive barrels do not necessarily equate to better accuracy. "New don't always mean better."

[michaelnel]

Turns out my Sharpie test was bogus. The stuff likely got scraped off during the chambering / ejection cycle.

My buddy brought over his 6mm comparator insert and we pulled one of the Lapua rounds apart to use the bullet for the chamber measurement tests.

We used the Hornady OAL gauge to measure the chamber. 1.811", 1.811", 1.810".

We used the comparator on 10 of the Lapua rounds that had not been chambered, and they varied from 1.783" to 1.790" (somewhat more than I thought they would be, but it could be operator error on my part doing the measurements).

Anyway, it is clear they aren't jamming, on average it's a 0.023 jump.

I think what was happening was that during this break in shoot I was using a couple sopping wet patches after every three rounds and although I was drying the bore with several dry patches, I was NOT swabbing out the chamber. There was likely a buildup of solvent in there for the first rounds shot after the cleaning, and that may have increased the pressure.

[michaelnel]

Sounds like your barrel is a shooter so you might want to give it a go for a while before you invest in a new one....more expensive barrels do not necessarily equate to better accuracy. "New don't always mean better."

The barrel has been on order for a couple weeks already. I agree the stock one looks like it shoots good, but I had the same experience on my 10FCP .308. That stock barrel shot good too, but switching to a 28" Criterion Bull barrel improved that rifle, and it is so much easier to clean.