Blowing primers but with a twist

[MidwestPX 172]

I've been doing some load development with .308 and ran through some test charges in .4 grain increments. I'm working with two different headstamps and two different primers because the S&B brass I have won't accept CCI primers (undersized primer pocket and #34 primers have thicker walls than normal making them a little tougher to squeeze into the pocket). At 45.2 grains, I blow a primer so I back down the charge weight and load a batch of five to double check. Since I have two case/primer combinations, I decide to load five of each in case there's any variation resulting in significantly different pressures. At this point, I figure this is all academic since I can't imagine a case and primer change making an appreciable difference. What I loaded:

 

44.5gr H4895 (dialed down from 45.2 since that blew a primer and my powder measure is +/- .2gr)

155gr AMAX

2.800" COAL

 

Brass used:

 

5 - PMC with CCI #34 primers

5 - S&B with Winchester LR primers

 

The five PMC rounds all fired without issue. Firing pin strikes were slightly cratered, otherwise no signs of pressure. The five S&B were another story. Two blew primers. The remaining three had completely flattened primers. I don't have any velocity data because I managed to shoot my chronograph (left my Magnetospeed at home like an idiot...) on the first round.

 

I find it hard to believe that a case and primer switch caused such a drastic change in pressure but I'm always learning. Can anyone shed some light on just what the hell happened?

[302w 83]

I'm far less experienced with loading than you are, but it sounds like you are pushing .308 to the limit and the Military primers are more forgiving to pressure than your commercial stuff. I'd be willing to bet you would blow primers with your PMC brass and commercial LR primers. 

 

Edit:

 

Also, I'm sure you know that 7.62 and 308 brass is different. It is possible that one of your cases is thicker.

(I don't load 308, but IIRC NATO brass is thicker)

[MidwestPX 172]

I'm far less experienced with loading than you are, but it sounds like you are pushing .308 to the limit and the Military primers are more forgiving to pressure than your commercial stuff. I'd be willing to bet you would blow primers with your PMC brass and commercial LR primers. 

 

Edit:

 

Also, I'm sure you know that 7.62 and 308 brass is different. It is possible that one of your cases is thicker.

(I don't load 308, but IIRC NATO brass is thicker)

All my brass is .308 Win, not 7.62 NATO so that shouldn't be an issue. You do propose something that's caught my interest though. I'm going to load some PMC brass with Winchester primers. I loaded 45.2, 45.6 and 46.0 charges with PMC/CCI because the 45.2 charge that blew a primer was S&B/Winchester and I'm curious if I can push higher with the PMC/CCI combo. However, I'm also going to load some PMC/Winchester to see if you're on to something with the mil vs. commercial primers.

 

Edit: Scratch that...I don't have any unprimed PMC to play with. Nuts. The S&B just may get relegated to plinking use.

[magnawing 5]

Check your crimp. You may be getting slightly more crimp on the S&B causing increased pressure. Different manufacturers' brass can have slightly different physical properties causing one to crimp a little tighter than the other.

 

 

This signature is AWESOME!!!

[Vlad G 345]

Weight your brass. Assuming the PMC and S&B are sized and trimmed the same, see if the S&B is heavier. You might have a bit less case volume in the S&B

[Old School 611]

Weight your brass. Assuming the PMC and S&B are sized and trimmed the same, see if the S&B is heavier. You might have a bit less case volume in the S&B

 

I'm with Vlad !!   Probably a case volumn issue.

 

Why are you riding the pressure edge?

[T Bill 649]

Case volume could be part of the reason. Needs to be checked by measuring the weight of a liquid used to fill the case. Just weighting cases will not give you a true volume capacity. Other things to consider is the temperature range of the primers and the size of the flash hole. I know of a BR shooter who swears flash hole size needs to be checked. How much this is an issue in standard loads I don't know.  Primer temperature will change burn rate, that i do know. Winnie LRP is known as the 'hottest' in il's class, and 4895 the easiest powder to ignite

.

[MidwestPX 172]

Check your crimp. You may be getting slightly more crimp on the S&B causing increased pressure. Different manufacturers' brass can have slightly different physical properties causing one to crimp a little tighter than the other.

 

 

This signature is AWESOME!!!

I may try loading a few with an intentionally tighter crimp to try to quantify the impact of the crimp. But man, this is a big pressure difference.

 

Weight your brass. Assuming the PMC and S&B are sized and trimmed the same, see if the S&B is heavier. You might have a bit less case volume in the S&B

IIRC, the PMC was a couple points heavier on average than the S&B. I remember it was pretty close so I wrote it off as inconsequential.

 

I'm with Vlad !!   Probably a case volumn issue.

 

Why are you riding the pressure edge?

Equal parts morbid curiosity and practicality. I'd like to know how hard I can push them first so I don't unnecessarily leave velocity on the table. I'm over Hornady book max of 44.0gr at 44.5gr but well under Hodgdon max of 46.0gr.

 

Case volume could be part of the reason. Needs to be checked by measuring the weight of a liquid used to fill the case. Just weighting cases will not give you a true volume capacity. Other things to consider is the temperature range of the primers and the size of the flash hole. I know of a BR shooter who swears flash hole size needs to be checked. How much this is an issue in standard loads I don't know.  Primer temperature will change burn rate, that i do know. Winnie LRP is known as the 'hottest' in il's class, and 4895 the easiest powder to ignite

.

Now THIS, I did not think of. I didn't know Winchester LRPs are hottest. This could be the issue. I may just unprime a few PMC cases and reprime with Winchester to suss this out a little more. I just have to remember to not be a moron and a) leave my good chronograph at home and b) not shoot the backup that I keep at the range.

[T Bill 649]

Good info on primers.

 

http://ingunowners.com/forums/ammunition-reloading/174704-choosing-right-primer.html