This morning's brain twister

S

stagpanther

New member
I built a 25 PRC blackjack a few years ago based on a savage 110 long action but have had inconsistent results (and then the collapse of the blackjack altogether taking my unfulfilled and paid bullet order with it). The barrel was custom made and chamber reamed by a prominent company that shall go nameless.

I did what many others did at the time since there were no dies being made for it--simply took a redding 6.5 PRC bushing die and got a set of .257 bushings to dial in the appropriate neck tension and neck diameter and then sized factory 6.5 brass. No turning was necessary and the process was pretty straight-forward. First firing was generally OK and I was getting 5 shot groups in the .75 MOA range in the 150 to 200 yd range--nice, but nothing to write home about in contrast to its sensational reputation (yeah, the latest boy-wonder phenomenon).

I never did much shooting with it after other interests dragged my attention, but did notice after a few firings of the sized brass that pierced primers started appearing even with low-powered loads. I took the rifle apart and redid the headspacing with PTG's go no/go guages (sans ejector and extractor in the bolt head) and got a solid stop with no go after setting the go and retightened the barrel nut and put the rifle back together. Firing the cartridges I'm still getting pierced primers on low pressure loads; so now I'm think I might have a long headspace issue resulting in the brass slamming back into the bolt face upon firing. I check with the no-go guage and it gets a solid stop as before--but measuring fired vs unfired factory 6.5 brass I'm getting about .007 elongation in the case as measured on the shoulder. Furthermore, as you can see from the photo below (fired resized 25 PRC on the left, unfired factory 6.5 PRC on the right), I'm getting noticeable flow/expansion from the case head which probably is weakening the brass and inviting a blow-out sooner or later.

This is a stumper that I haven't encountered before on my savage builds--but just out of curiosity I took a factory fresh 6.5 PRC cartridge (I don't have any stand-alone factory brass), pulled the bullet and inserted it into my rifle--it chambered just fine with no resistance. There's only a .002 OD of the neck difference between the unfired 6.5 brass and that of my 25 sized brass.

Looking at the neck/shoulder junction which has a ring and obvious gas leak signs it looks to me like the neck is expanding up and out upon firing and possibly the case is recoiling back into the bolt face while at the same time elongating out case/shoulder length.

What do you think? (yes, I figure the next step is to pull the barrel and do a cast, but is my theory way off base?).

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Per your suggestion Jim-- I did the "old days" check of applying and trimming scotch tape to virgin brass rim face, the bolt would close on 2 layers but got a solid stop with 3 .
 
I see similar soot pattern on my brass neck quite often. Mostly I just ignore it.

I read that the probable cause is the neck doesn't have enough time to expand and seal. Seating bullet deeper and crimping will help. On my swedish Mauser, I convert the brass from .30-06. I tried leaving the neck wall thicker (0.015" vs 0.012"). It seemed to help. The idea is to better fill the oversized chamber neck.

-TL

Sent from my SM-N960U using Tapatalk
 
yeah i see soot on the neck of a lot of my brass, and that mark at the bottom looks like when i full resize. it's where the die stops touching the brass (in my case)

but that may not be what you are seeing.

have you mic'd the case just above and just below the mark?
 
while the soot is a little more pronounced than what i normally see, the shoulder is sealing the gas and the neck is springing back in after the bullet leaves,
or that is what i think is happening there.

you did say you are not turning these case necks ?
 
what brand of primers ?

i have china white box large rifle primers that blow holes in the firing pin dent. even below minimum starting loads.

but they are inconsistent about it. one out of five or so...

i stopped using them and counted that as learning curve and went back to searching for "good" primers.
 
Sorry--stepped away for a while because I took the rifle apart and redid the headspacing based on an unfired 6.5 PRC case made by Norma. There's absolutely no doubt that the headspacing was too long IMO; the flow and measurements confirmed that in my mind.

So I decided to take the unfired 6.5 PRC case and put just two layers of magic tape on as a "no-go" and put a bushing in the die that is .001 narrower in diameter for a total of .002 tighter and still not shave the bullet on seating.

Here are the two test cases I just fired with the same loads as the dirty ones above--overall case length growth was .004 and no apparent flow and mouth/neck seal looks much better (and neck diameter is about .004 fired diameter less than the previous stuff above. Now this could vey well be simply the case on one firing is able to "take the punishment" without showing signs of excessive space between it and the chamber--so I'm going to just repeatedly fire these two cases for at least another 3 firings to be sure the spacing problem truly is licked.

I still find it odd that a 6.5 case chambers no problem in a .257 chamber.

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I don't know how well all the shifts and adaptations are doing in the loading, I hope that fixes the case stretching.

mm(6.5) = 0.25590551 inch
Even firearms live in an alt-reality!
Click to expand...

Of course the portal to that reality is defined by the confusion and conflation of bore, groove, and bullet diameters.

A lot of calibers are described by bore diameter, others by groove diameter, which is often but not always bullet diameter. Then the internet screws it all up by calling the groove diameter the "bore."

The British termed the 6.5mm Mannlicher their .256 and Charles Newton had his own .256, both their .264" bullets notwithstanding.

Ol' Charlie struck again at Savage with the .250-3000, having a .250" bore to shoot .257-.258" bullets. Roberts and Ackley followed suit, then Remington went groove to distinguish the commercial .257 Remington Roberts from the .25 Roberts wildcats.
 
Ol' Charlie struck again at Savage with the .250-3000, having a .250" bore to shoot .257-.258" bullets. Roberts and Ackley followed suit, then Remington went groove to distinguish the commercial .257 Remington Roberts from the .25 Roberts wildcats.
Click to expand...
That's not even taking into consideration of calling the 378 weatherby a 378 cause it fits a bigger case than the 375.:) I'll get some follow-up shots soon on the 25-6.5 PRC. :)
 
Try seating a 6.5 bullet deep in the case and see if it chambers. It is odd that the 6.5 case chambers without being necked down.
 
Try seating a 6.5 bullet deep in the case and see if it chambers. It is odd that the 6.5 case chambers without being necked down.
Click to expand...
I actually tried that with the High BC ELDX type bullets, but the cartridge is meant to seat those long so I'm sure the base of ogive is stopping the bullet in the bore itself. I might still rustle up a shorter small base type and see what happens. In retrospect, I screwed up and should have done a cast while I had the barrel off yesterday.
 
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I seated a .264 partition as far in as I could and the bolt stopped about 3/16 short of engaging the receiver lugs, so I'm not worried about mistakenly firing a 6.5 PRC in the 25 bore. I did, however, chamber a 6.5 PRC case without a bullet or powder and fire the primer with no issues. I took a look at the freebore where it meets the leade and it does look like there is possibly a pronounced ledge, so the question in my mind is was a 6.5 PRC reamer possibly used instead of the 25 blackjack one to ream the chamber. I'm not sure what the consequences might be, if any, if that's the case though I suspect the neck brass will get overworked fairly quickly. Since blackjack bullets lies at the bottom of the ocean with the Titanic I can no longer have access to their schematics, so I'll see if I can get something from the barrel manufacturer.
 
It sure seems like a 6.5 reamer was used. Can you color the neck and bullet on the 6.5 case and see if the bullet stopped the bolt from closing or the neck stopped it.
 
Well I'll be danged!

I took a fresh 6.5 PRC Norma cartridge right out of the box, colored as you can see and attempted to chamber it--I thought for sure the bullet would be stopped by the bore--but if you look closely at the picture you can see a faint outline on the case mouth where the neck was stopped. It doesn't look like very much and I wonder if it wouldn't be possible to cam the mouth into battery with enough force on the bolt.:eek:

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DON'T! Not even if you can. It may just be a scuff from the case mouth ramping up the shoulder and into the neck recess in the chamber, but you need to check that first. If that case mouth actually wedges into the start of the freebore, it can raise pressure substantially by preventing the release of the bullet from occurring at its normal point in the powder burn curve. I would definitely do a chamber cast at this point. If the chamber's neck recess is longer than the necks of your cases, then that is just a chambering scuff. If it is equal to or shorter than your necks, you need to trim your cases back 0.02" or get a properly dimensioned reamer run in there. It's not unheard of for newer cartridge designs to result in some initial reamer dimensioning discrepancies between reamer suppliers.
 
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