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?

 

[jonbearman]

Did you try to chamber one and not shoot it and inspect to see if the lands are touching? I would do a chamber cast and when you get the tools start measuring to see what is really going on.It may have a very short freebore which is not the norm. Do you reload?

 

[LHSmith]

Did you mop out the chamber after the initial cleaning?

 

[michaelnel]

Yes I do reload. I have CCI BR4 primers, Reloder 15, Berger 105gr VLD Target and 105 Hybrid bullets to load, and I have a set of Redding Competition dies. I was just going to use the Lapua factory stuff for break-din and to create some fireformed brass to reload.

I also have a 30" Criterion bull barrel on order (also in 6mm BR) so was intending to delay my load development until I get that installed. As it is though, I don't feel I should shoot any more of this ammo until I know why it's doing this. If they are a little long, I can just seat them a little deeper and they not be worried about shooting them.

I will try blackening the bullet on one of the rounds with a Sharpie and chamber it to see if the lands are marking the ogive. Good idea, I should have thought of that.

And yes, I sure did mop out the chamber after cleaning it initially, but I admit I did NOT do that out at the range after each of the bore cleanings. I probably should have, as I keep my patches in a jar very saturated with Shooter's Choice solvent so they are ready to use, but I suppose even using a bore guide for my rod there could be excess solvent in the chamber. Another good thought!

 

[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?

 

[Erik Cortina]

Only one land? I think what you are seeing is due to the bullet rubbing against chamber as you extract round and ejector pushes round off to one side.

I would blame it on chamber not being clean and dry.

 

[dmoran]

I agree with Eric

Besides, seating into the rifling is not a big pressure rise (many seat into the rifling/lands).
If your bolt closes fine on a loaded round, it wouldn't be in the rifling hard anyway.
Plus, the Lapua factory ammo should be loaded to SAAMI, as should be your factory chamber.

 

[michaelnel]

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, so the liquid in the chamber theory seems the most reasonable. Thanks for all the great help guys!

 

[gman88188]

I have had a similar problem with my Savage 12 F-Class. There is at least another post where the COAL is shorter 2.385 comes to mind. I haven't been able to get the hornady modified shell but I finally found one available. I measured yesterday using the old tried and may be true method and got a reading of 2.358 touching the lands with 107 Match Kings. By the way the lot I just weighed is under 102 grains each. I was experiencing hard jams into the lands at the SAAMI standard. But this may not be correct due to the action of the bolt and the cleaning rod during removal. The hard jam affected the bolt closing so it was just that, jammed hard into the lands.

I am going to confirm actual length as soon as the modified shell arrives. My comparator indicates 2.804 for the match kings and 2.778 for Berger 105 Hybrids. The Hybrids touched the lands at 2.383. I set the COAL to 2.369 which produced great 1/4 MOA groups at 200 yards.

I think the Savage Barrels may not be correct, so I am going to cast it next to see what's up. It shoots great with a shorter length COALs, so my plan is to replace the barrel when I wear it out with a custom barrel.

 

[lrgoodger]

Since you were shooting 105s I presume the barrel is an 8 twist. What are your plans for the takeoff barrel?

I shoot Varmint for Score matches in factory class and I will be needing a new factory Savage barrel for my model 12 LRPV soon, since the original barrel now has over 3000 rounds on it.

 

[johnmyers]

How about neck diameter?

 

[michaelnel]

Since you were shooting 105s I presume the barrel is an 8 twist. What are your plans for the takeoff barrel?

It is an 8 twist. I have no plans for it other than to take it off.

 

[nosualc]

Wet chamber is my bet.

-nosualc

 

[michaelnel]

I shot about 40 rounds of the same ammo yesterday with no problems at all, other than catching the free recoiling rifle's scope on the bridge of my nose. :-[

 

[GarethMarijt]

Michael,

I had exactly the same problem (even down to using the Lapua 105 gr ammo). I ended up sending the rifle back to the shop where I purchased it. They only had lighter bullets to test with and thought that it was just a tight throat which would improve with time and use.

I started loading to the length obtained by using the OAL guage when it arrived, as loading to the length suggested in the Vit manual led to indentations on the back of the case

Gareth

 

[Jeff Conover]

I run into this problem with the 6 Dasher. Controlling the carbon fouling is paramount. During barrel break in sometimes I will have the 17 or 18th shot show pressure like a cratered primer, or even blanked primers. My bore scope inspection showed a carbon ring at the front of the chamber, but before the throat, right before the step at the end of the case mouth. Take a brass brush with solvent and insert to this area twist it 20-25 times. Using a bore guide and the cleaning method can miss this area. The 6 BR case family and 8 twist barrels I have found to collect carbon which is more of a problem than copper fouling. This is a critical area for accuracy and carbon increasing pressure here can lead you down the wrong path chasing accuracy issue and velocity standard deviation.

 

[cocopuff]

From what I read,it is a new rifle..There should be NO carbon build up .I found the same issue with my rifle and I found a small base body die curred the problem..

 

[michaelnel]

Guys, there is nothing wrong with the rifle or the ammo.

The day the pressure signs happened was break in day, and I was cleaning the bore frequently, but neglecting to swab out the chamber after each of those cleanings. Solvent built up in the chamber and caused the overpressure.

Since then I have shot about 50 rounds more of the Lapua factory ammo and about 40 of my handloads, without any repeat of the pressure problem.