Primer cratering issue - 6.5 Creedmoor

[rgitzlaff]

So I just built an AR in 6.5 Creedmoor. I'm using an Aero Precision M5E1 enhanced upper and lower, Aero Precision high pressure bolt and carrier as well. The load is a 123 grain sierra matchking on top of Varget and lit off with a CCI 400 small rifle primer. I started out breaking in with relatively low loads working my way up. Hodgdon says starting load for this combo is 36.0 grains (2,712 fps) with 39.8 gr (2,887 fps) being max. I started with 37 grains (2,710) off the bat and had cratering right away. It got worse at 37.7, and finally at 38.5 (2,785) it blew out the center. I figure either I need some of those military type primers or my working load really does need to be that low. I'm not getting any of the other signs of high pressure like ejector marks or anything. Any ideas or experience reloading this cartridge in an AR? And no, the bullets are NOT seated into the lands, they are loaded to mag length. Attached picture shows the primers and case heads. Closest group of 4 is 37 grains, next is 37.7, and last with blown primers is 38.5.

[hounddawg]

What brand primer? There was a thread here a while back on Winchester LRP's piercing

[rgitzlaff]

CCI 400. Apparently Varget must just be too fast a burner for the AR, I'll have to pick up some H4350 and start over and see where that gets me. That and maybe a primer with a harder cup.

[Rimfire5]

It is pretty difficult to find primers harder than CCI 400s or CCI BR-4s.
I can tell you from experience that Federal and Remington primers are not as hard. If you are cratering CCI 400s, you will really pit the Federal 205s and 205Ms or the Remington 6 1/2s and 7 1/2s.

Based on my experience, I would suggest that you probably do not have a primer pocket problem or a firing pin problem.
Precision barrels often are made with tight chambers.

I had a similar problem, but not as bad as you seem to be having, with a new Savage bolt action Long Range Precision rifle.
It shot incredibly accurately but it cratered primers - so much so that I couldn't get the brass into a press.
The cratered ridges on the primer wouldn't allow the base of the brass to slide into the shell holder so I had a devil of a time getting the brass deprimed.
I carefully documented the results with lots of close up photos of results at different loads from light to hot.
I contacted Savage and provided the data I gathered and they told me to send the rifle back.
It turned out that the chamber was too tight, which increased the chamber pressure and forced the primer to slide back in the primer pocket into the firing pin causing the cratering.
That movement of the primer also wore out primer pockets in 4 reloads or less.
Savage bored the chamber out a few thousandths and the problem went away.
The rifle, with an unmodified bolt and firing pin, now fires loads close to Pmax without primer cratering although there is no need to load that hot because the rifle shoots very accurately with loads that are in the middle of the load table.
Also I have just finished loading 100 brass on their 21st reload on the same brass without noticeable loosening of the primer pockets.

By the way, the CCI BR-4 primers and the CCI 400 primers, respectively, have produced the most accurate results in comparisons against other primers.

[Rimfire5]

rgitzlaff,

I suggest that after you fix what sounds like a tight chamber, you try fast and slower powders to find what your particular rifle likes. The AR recoil mechanism may have some preference for powder burn rate.

But to help you with your search, I thought I should add that I have two 6.5mm Creedmoors and both seem to like slower powders.

The Savage 12 Long Range Precision rifle with a 26 inch barrel likes IMR4451 Enduron but shoots IMR4350 almost as well. It does OK with RL-17 and N150.
It doesn't shoot IMR 4064, Varget, N140, or H4895 nearly as accurately.

The other 6.5 with a 24 inch barrel likes RL-17, IMR4350 and IMR4451 in that order.

[rgitzlaff]

From an article I just read, BR4, 450 and 41 all have cup thickness of .025 which is .005 over the 400 and are meant for higher pressure. However this could tie into another issue I'm having that I thought was unrelated. My extractor is leaving nice imprints in the case rims. I thought maybe the chamber was rough or had a burr. Now maybe I'm thinking it is just tight. Maybe the teamed used was worn or something. A loaded round chambers and extracts by hand easy enough. I don't know, but a different primer seems like the easy route to try right now, though I plan on sending back to vendor for not riding after break in anyway, so maybe have them inspect chamber at that time.

[Metal god]

I'm confused , I thought the Creedmoor takes large rifle primers and not small rifle primers ?? I know there are some 308 Palma brass that takes small rifle primers . Is this the case with the 6.5mm Creedmoor as well ?

[hounddawg]

I'm confused , I thought the Creedmoor takes large rifle primers and not small rifle primers ?? I know there are some 308 Palma brass that takes small rifle primers . Is this the case with the 6.5mm Creedmoor as well ?

you can get the several brands of 6.5 CM brass with either SRP or LRP

using 450's or BR4's might help

in order of cheapest to most expensive the other things I would check would be excessive shoulder bump, excessive firing pin protrusion, a weak mainspring or a loose fit between the pin and the bolt

[Unclenick]

Originally Posted by rgitzlaff

Hodgdon says starting load for this combo is 36.0 grains… I started with 37 grains…

I have three times now run into published load data for which the published starting load was already maximum for the gun or at least one of the cartridge components (like your primers). It doesn't usually happen, but when you go looking through published data and note the disagreement on what the maximum loads should be, you realize this is an approximating science and that all the published data is telling you for certain is that the described maximum load worked for the test gun employed using a cartridge loaded with the same bullet, same brass, same primer, and the same lot of powder the data developer used. Usually, that's not exactly what you have. So the recommended starting load is used to compensate for expected differences, which is why you want to start with it and not somewhere in the middle as a lot of people have suggested over the years and will assure you is just fine to do. Given that the starting load is occasionally already a maximum, these are just people who randomly never ran into one. That's the problem with anecdotal evidence. It doesn't normally involve a statistically adequate sample to assure you of anything.

As to the primers, the first thing I would do is look at the firing pin tunnel in your bolt. The cratering is pronounced, so if there is a chamfer there (as some Remington bolts have had) or if the tunnel diameter is too loose around the firing pin, it can explain the craters and should be corrected.

If the firing pin tunnel is fine, the next question is, what is your barrel length and how do your chronographed velocities compare to the published velocities for these loads? If your velocities are greater than the published velocities even after allowing for any difference in barrel length there may be, then you have higher pressure than the developer of the published loads got. There can be several reasons for this:

• Your bullet has less jump to the lands than the test gun did. This can raise pressure as much as 20% or so in rifles. It may be due to:
  • Your chamber has a shorter freebore than the data chamber did.
  • You are seating the bullet out further than the publisher did.
  • You are using a different bullet than the publisher and that has an ogive that is further forward than the developer's bullet did.
• You have a warmer primer than the publisher used. In small rifle primers, I've seen data showing this can affect pressure by as much as about 10% in the extreme case.
  
• You have a case with less water overflow capacity than the one the data publisher used. In some cases, it can also cause about 20% pressure difference (300 Win Mag is notable for this extreme; most other cartridges don't have so such a great range of case capacities between makers, but it's something to keep an eye out for).
  
• You have a different lot of powder than the data developer used. This can cause between 3% and 10% pressure difference in some extremes.

So there is still a fair amount left for you to investigate here. If it turns out the primers are flimsy, I suggest moving to the Federal 205MAR primer, which is a standard strength primer with the cup thickened to achieve military sensitivity specs. The CCI #41 primer made for military sensitivity is a magnum primer, so you may have to cut powder charge as much as 5% for that, but also their cups are no thicker than those used for their magnum rifle primers. CCI achieves the lower military sensitivity by altering the geometry of the primer anvil rather than thickening the cup as Federal has done. So, if you need a thicker cup, Federal is the way to go.

[Metal god]

you can get the several brands of 6.5 CM brass with either SRP or LRP

I kinda figured but wanted to ask . I'm assuming there is a benefit to using small primers instead of large . What may that be ?

[tangolima]

Primer catering is a sign of over pressure. I would not have kept upping the charge when I see one. It goes against the principle of working up a load.

-TL

Sent from my SM-G930T using Tapatalk

[hounddawg]

Lower ES in muzzle velocity and the ability to use the thicker cups in SRPs

not to derail but here is a good read on them

http://www.targetshooter.co.uk/?p=2613

BTW I have had 2 rifles in the last few years that started cratering using mild to medium loads. One was due to mis adjusted firing pin protrusion, the other was cured by replacing the firing pin spring on a brand new rifle so there are many things that can cause cratering.

[hooligan1]

Metalgod, they believe it takes longer to loosen primer pocket with the small primer, Lapua makes both I believe and now Starline makes both.
So theory is you can load a tad hotter and not worry about primer pocket, this is what I've read on the subject.

[rgitzlaff]

This particular problem is fixed. I tried a bunch of loads with CCI 450's and the cratering problem is gone, so magnum primers fixed that. I'm going to try some BR4's next and see if that differs at all in velocity.

Now another problem that I have that still hasn't gone away are the extractor marks on the inside of the rim. I'm talking about the part of the rim that the extractor claw grips to pull it out of the chamber. It isn't pulling hard enough to bend the rim or anything but it is putting a pretty decent indentation in the brass. It should not be over-gassed because I can adjust the gas block down to where it won't fully cycle and it still gets the same marks. Gas system is also 2" longer than rifle length, so the dwell time is longer than standard to give gas pressures time to drop before extracting. Brass is Peterson.

[Metal god]

Now another problem that I have that still hasn't gone away are the extractor marks on the inside of the rim.

That is likely an over gassed issue with your BCG trying to extract the case before the pressure has dropped enough allowing to case to let go of the chamber walls . Adjustable gas block is one way to fix the issue . Heavier buffer , stiffer buffer spring or both to slow everything down a bit during the cycling process is another .

There's other things that can fix the issue but the ones I mentioned are where to start .

[BumbleBug]

I had primer cratering on lighter loads in my 6.8mm SPC (it's not an AR). I switched to thicker cup primers & the problem went away for both LgR & SmR loads. I use CCI 450's exclusively now in small primer brass. Looks like CCI450, CCIBR4,Rem 7-1/2 should be GTG.

{Edit: Please read the board rules on posting copyrighted materials. You can do something like link to someone else hosting the information, though.}

[Metal god]

What's your gas system length compared to your barrel length ? I looking to know what your dwell time is ( distance between your gas port and muzzle )