Primers - Again
Date: Oct 01 2008
My first lesson in primers came long ago. I was trying to get a .257 Roberts to shoot groups smaller than a snuff can and wasn’t having any success. My luck came in the form of bad luck when I ran out of primers. At the time I was still working as a cop and my loading and shooting was regulated to my days off. I was aggravated I had to waste time driving to the gun shop and even more so when I found they were out of Remington 91⁄2 primers. I purchased a couple boxes of Federal 210 primers and drove home, annoyed I’d wasted two hours of daylight. Any real handloader knows you cannot just go in a gun shop, buy primers and walk out. There’s always a certain amount of gawking and conversation that must take place.
Switching primers necessitated backing off the powder charge and working up to the maximum velocity listed for IMR 4831 and a Hornady 100 grain Spire Point. Through the process I noticed all the shots were touching even though the powder charge was increasing. When I reached my target velocity I switched to another target and fired a three shot group that measured .25. The second three shot group measured .28 and the third, .25 again. (I saved those targets.) Just by switching primers I had turned a two inch rifle into a tack driver!
Another primer lesson came while working with a .35 caliber wildcat cartridge I put together on the Winchester WSSM case. In the end the project was scrapped for a number of reasons but during the process of trying to develop an accurate load I stupidly substituted a magnum primer for a standard primer. I was working with a load of around 58,000 psi and knew this because it had been pressure tested. The result was a bolt that had to be beaten open with a rubber mallet and an extractor that broke in the process.
I use a lot of Ramshot ball powders because they meter very consistently and that makes my life easier. When using these powders in cartridge cases with a capacity similar to a .308 Winchester, I’ve found best accuracy sometimes comes with magnum primers. If a maximum load with a standard primer does not produce the accuracy I expect I will typically back off to the starting load, switch to a magnum primer and slowly work back up to the target velocity. This does not work every time but often enough it’s worth trying. With the .35 WSSM I did not reduce the powder charge and the resulting pressure was obviously excessive.
The most dramatic display of primer inconsistency I have witnessed was about five years ago during the pressure testing of a custom .358 Winchester rifle with builder Charlie Sisk. The load was 48 grains of Ramshot TAC behind a 225 grain Nosler Partition, loaded in new Winchester brass with CCI Bench Rest Primers. The inconsistency of the primers was not the dramatic thing; it was how consistent they were inconsistent.
The rifle had been shooting everything we put through it pretty well and after a fouling shot, the first two shots with the above load were almost touching. When I fired the third shot I thought I might have pulled it because it landed two inches from the first two. I fired another shot and it landed right beside the last shot. I told Charlie where the last two bullets landed and he said that was odd because the pressure on the last two shots was markedly lower than the first two.
We essentially had two separate groups. Had we not had pressure data I would have suspected something might have gone amiss with the scope or bedding. I fired three more shots. Shots five and seven landed right beside shots one and two and shot six grouped with shots three and four, actually going in the same hole as shot number three. (A target backer confirmed this.) All shots were fired from cartridges loaded with brass, bullets, powder and primers from the same lot and under the exact same conditions on an indoor, 100 yard range.
Examining the target and data from the chronograph and Pressure Trace system we found it to be grouped just like the bullet holes. Group A had an average velocity 38 fps faster than the shots that formed group B. Most astounding was that the average pressure for Group A was 4173 psi higher than the aver-age of the pressure for the shots in group B. It’s been my experience that a velocity variation this low will generally not adversely influence point of impact so the culprit must have been pressure or a bi-polar gremlin living in the rifle.