6BR primer cratering

[TackDriv3r]

I reloaded some ammo for the new custom built 6 BR, 107 Sierra MK, 27.0 - 30.0 grains of Varget, virgin Lapua brass, and CCI 450 primers. Doing 3 round groups from 27 to 30.1 grains in .3 increments. At 27 grains I was already getting primer cratering. When I got home, I measured the boltface firing pin hole and it measures .076" and I also measured a boltface from the .308 that uses a standard large rifle primer, it measures .074" and it seems both boltface firing pin holes are for large rifle primers. I did some googling and I see that the small rifle boltface firing pin hole is .060" and would this be the issue? Send the bolt off to be bushed and get a small firing pin in it?

Here are the pics of the cratered primers.

 

[CaptainMal]

It is not the size of the pin or the hole. It is the difference between the two.

Let me throw out some ideas and see if any stick. You say the gun is "new". Is the brass new also. If so, is it properly headspace/sized to the chamber?

A standard BR with heavy bullets like that often tops out just below or at 30 gr. of Varget. Variations in powder lot, case, chamber, bullet seating, bore size or carbon deposit and, of course, prime to boldface relationship all factor in.

Check as many of these factors you can. Chronograph something in the 2,700 fps range for your stock BR with that bullet. You are showing a flattening of the overall primer so there is what I think can be called...minor pressure signs.

 

[J F Johannes]

There appears to be some ejector swipes and primers seem a little flat.

Jim

 

[gambleone]

Greg Tannel of Gretan rifles does a great job bushing the firing pin holes.
https://www.gretanrifles.com

 

[jonbearman]

Had the same problem and sent my bolt to GreTan and I got it back in a week. No more problems like that.

 

[Texas10]

I see ejector swipes too. My load for that combination is 29.0 and I do get cratering. At 30.0g I get ejector swipes but that's in MY 6br with 26 in Shilen 8 twist. My X-Caliber was happy with 30.2.

It appears your bolt head could do with a bushing.

 

[243winxb]

You may have a fast lot of Varget?

Powder Lot Variations: A Case Study with H4831 – Hodgdon Extreme.https://discover.dtic.mil/results/?q= extreme temperature stability 4831

EXAMPLE-

Small arms propellant manufacturer Hodgdon claims that rifle powders in its Extreme line have small velocity
variations with both temperature changes and lot number. This paper reports on the variations in average velocity
for six different lots of Hodgdon Extreme H4831 tested in .25-06 and .300 Winchester Magnum loads. Compared to
the lot with the slowest average velocity, the other five lots of powder had higher average velocities ranging from
11.9 ft/s faster up to 111.9 ft/s faster in the .25-06 and from 13.6 ft/s faster to 111.1 ft/s in the .300 Win Mag. The
mean velocity differences between lots are highly correlated between the two cartridges with a correlation
coefficient of 0.96. This high correlation supports the idea that the experimental results reported here depend much
more strongly on differences in the lots of powder rather than other details of the experiment such as the choice of
primers, brass, bullets, and specifications of the rifle bore. The lot to lot variations in velocity seem higher than one
might expect from Hodgdon’s marketing claims.

EXAMPLE-

To consider the likely pressure variations, consider that QuickLoad V3.6 predicts a muzzle
velocity of 3174 ft/s and a peak pressure of 54371 psi for 79 grains of H4831 using the 155
grain AMAX in the .300 Win Mag. This is very close to the 3177.5 ft/s velocity that results from
averaging the measurements over all six lots of powder. However, compensating for possible
lot to lot velocity variations requires a range of powder charges spanning from 77.5 to 80.5
grains of powder and a range of pressures from 51063 psi to 57910 psi. It is possible that a
load showing no signs of pressure with one lot of powder might not be safe with another lot of
powder. At their web site, Hodgdon recommends working up new loads when the lot number
of any component is changed, and it is evident that this includes a new lot of Hodgdon

Interesting-
Cleaning brass in stainless tumbling media is important to consistency.
For example, we have noticed an increase in case capacity averaging close to 1 grain of
water after cleaning previously fired .300 Win Mag cases.3

 

[dgeesaman]

There appears to be some ejector swipes and primers seem a little flat.

Jim

Agreed. On my 6BRA I experimented more than necessary at high pressures due to influence of 'others' loads. I didn't push very hard, but I spent a lot of shots at the upper end of sensible. Cratered primers, ejector swipes, flat primers, and loosening primer pockets were my experiences. I did not get into clickers until still higher pressures.

Listen to the brass, this is the message. We shoot BR for the accuracy not the velocity.

David

 

[MislMan]

If these are the first hand-loads for this rifle, what seating depth are you using? Are you jamming into the lands or jumping?

 

[JFrank]

I reloaded some ammo for the new custom built 6 BR, 107 Sierra MK, 27.0 - 30.0 grains of Varget, virgin Lapua brass, and CCI 450 primers. Doing 3 round groups from 27 to 30.1 grains in .3 increments. At 27 grains I was already getting primer cratering. When I got home, I measured the boltface firing pin hole and it measures .076" and I also measured a boltface from the .308 that uses a standard large rifle primer, it measures .074" and it seems both boltface firing pin holes are for large rifle primers. I did some googling and I see that the small rifle boltface firing pin hole is .060" and would this be the issue? Send the bolt off to be bushed and get a small firing pin in it?

Here are the pics of the cratered primers.

Only question - How’d it shoot??
J

 

[TackDriv3r]

I saw ejector swipes from 28 grains to 30.1 grains and velocity was 2625 average at 28 grains and roughly 2800 fps at 30.1 grains. On my post, I posted " 27 grains to 30.1 grains" and its incorrect, used those numbers off the top of my head and checked my notes and its "28 to 30.1 grains." I seated the Sierra 107's at .005" off the lands. Those virgin Lapua brass are those old brass that came in those cardboard boxes. Here are some pictures of the target and the velocities. As for the ejector swipes, I don't know what to look for except try again later and bump the shoulders back .001 - .002" and retest it after I get that bolt bushed,,,, I am sending that bolt off today to be bushed. During the 100 yard 3 shot group test, wind was from 10 o clock around 12-15 mph.

 

[dgeesaman]

I have ejector swipes on my brass with ten firings on them. For my 6BR brass it’s the first pressure sign. Then I see flattened primers, loosening primer pockets, and cratered primers and sticky bolt lift.

If light ejector swipes are your only pressure sign I would personally live with it. If the others are present I’d back off. Your 28.6-28.9 groups look promising.

 

[243winxb]

Do Rem 7 1/2 primers flow. The CCI 450 may be a soft lot.

 

[bozo699]

TackDriv3r,
Just food for thought have you measured the length of fired brass and the new unfired? I wondering how much head space you have?
Wayne

 

[Tommie]

About 15 years ago I was working up loads for a new build. I used 4 different powders. Varget gave me extreme pressure with moderate loads. I’ve heard similar stories from others. I suspect there is a QC issue with Varget. Scared me away from that powder.

 

[Webster]

It is not the size of the pin or the hole. It is the difference between the two.

Let me throw out some ideas and see if any stick. You say the gun is "new". Is the brass new also. If so, is it properly headspace/sized to the chamber?

A standard BR with heavy bullets like that often tops out just below or at 30 gr. of Varget. Variations in powder lot, case, chamber, bullet seating, bore size or carbon deposit and, of course, prime to boldface relationship all factor in.

Check as many of these factors you can. Chronograph something in the 2,700 fps range for your stock BR with that bullet. You are showing a flattening of the overall primer so there is what I think can be called...minor pressure signs.

I have a 6BR on a Rem 700 action. All your cases look like bolt face swipe marks, also several cases have what looks like a hint of ejector hole swipes. Ejector hole marks are not primer related. Everyone calls it cratering , it's a raised area. I would call a depression a crater. Anyhow over the years I have learned to interpret the raised area around the firing pin depression as a pressure indicator. Also hard bolt lift and ejector hole swipes. If I sleeved the firing pin hole I would lose the ability to use this as a pressure indicator.

To get these case head swipe marks other than ejector hole swipe marks the case head has to be held tight agianst the bolt face as the bolt is rotated. Why do you have cratering at 27.0 grains???

30.0 gr. of Varget 105 gr. bullet is commonly used with Varget. Look at the AccurateShooter 6BR load data.

 

[Dusty Stevens]

The newest varget is way hotter- like what used to be a 31.5gr load is now 30. Send that bolt to carlsbad and get it fixed cheap and fast.

It is not the size of the pin or the hole. It is the difference between the two.

Let me throw out some ideas and see if any stick. You say the gun is "new". Is the brass new also. If so, is it properly headspace/sized to the chamber?

A standard BR with heavy bullets like that often tops out just below or at 30 gr. of Varget. Variations in powder lot, case, chamber, bullet seating, bore size or carbon deposit and, of course, prime to boldface relationship all factor in.

Check as many of these factors you can. Chronograph something in the 2,700 fps range for your stock BR with that bullet. You are showing a flattening of the overall primer so there is what I think can be called...minor pressure signs.

The surface area of the pin is what makes the difference. You can blank a properly fitted .075 dia then properly fit it to low .060’s range and it wont

 

[Bill K]

Looks more like you are playing awful close to too much powder and high pressure beginning to show.

 

[JFrank]

Now you guys got me looking at mine and I don’t know they look that much different.

 

[Old Navy]

This is what a really bad Remington pin/hole does to primers even with the lightest loads. Sorry about the focus. Gretan fixed the problem.