Small primers vs Large Rifle and why

[DaveB]

Mr. Big suggested a 6.5 modification to improve accuracy:
"
Swap those BR2`s for some 450`s and get a box of Lapua brass with the small primer pockets and flash holes
"

The math behind the calculations that support the use of SR primers is really impressive.

Plus, there is a lot of experience to digest because today's bench rifle rifles are so repeatable the change of a primer can be observed in downrange results.

So What should you use? And why?

 

[Hunter 257W]

I've dealt with this issue with my 22CHeetah wildcat. While the cases can be formed from 243's, part of the reason for creating the CHeetah was to use and hopefully benefit from the supposed accuracy improvements that benchrest shooters were claiming when using SR primers. Jim Carmichael, who created the cartridge reasoned that since the SR primer ignites a smaller total % of the powder, any variation in flame between individual primers would correlate with less total variation in powder ignited by the primer. It sounds good. The only problem with testing it is that you need cases that are otherwise identical with the only difference being primer pocket size. I don't have that for the 22 CHeetah. Most of my cases are the very high quality Remington Bench Rest Basic cases which are 308 dimensions with small pockets. I run these through 3 form dies, a neck reamer then load them with standard loading dies. Case necks are turned and reamed, primer pockets uniformed. I have tried forming CHeetah cases with WW 243 brass, also Reaming and Turning necks and uniforming primer pockets. Accuracy is significantly better with the small primer brass but as I stated already, this is more uniform brass to begin with so it's impossible to know how much of the gain is due to the primer size.

One other significant factor should be mentioned. I've used mostly IMR 4320 in the amount of around 40 to 41 grains and the Remington 7 1/2 primer. I've had delayed ignition at temperatures as warm as low to mid 50's. I sure wouldn't dare try to deer hunt with such ammo due to the cold weather in deer season - not that I'd even think of using a 22 CHeetah to deer hunt anyway.

 

[vonb]

No SR primers for me in a hunting rifle. If you need SR primers to hold pressure, you're definitely over SAAMI specs.