6.5 Creedmoor: Which Primer for Accuracy?

[Swifty Morgan]

Today I went to Bass Pro and picked up the thousand large rifle primers I had ordered. I figured I was all ready to start making rifle ammo for the first time. I want to make some accurate stuff. I bought 140-grain Nosler Accubond bullets. I thought maybe I could produce hunting cartridges that were also pretty good for target shooting.

I was pretty pleased with myself until I went to the Starline site. I had been planning to use old Sellier & Bellot cases, but I thought I should check out Starline just to see what they had. This is when I learned about small-primer 6.5mm. The site says:

Many target shooters prefer the Small Primer for the increased consistency and accuracy. While other small pocket manufacturers use a smaller than standard flash hole, Starline uses our standard .080" flash hole, as we feel that it gives better ignition with some of the slower powders and in cooler conditions* (and you won't be breaking off decapping pins either!)

What?! Did I buy the wrong primers? Did the whole 6.5 Creedmoor picture change while I was waiting for my large primers to arrive?

I wonder what the small primer market is like right now. Let's check Ammoseek. Yeah, basically I can learn how to make small rifle primers myself or do without until the riots are over.

 

[jmr40]

If I understand correctly Starline, and maybe others offer both versions. The vast majority of 6.5 CM brass is made for large rifle primers.

IME the S&B brass works great and uses large rifle primers. Which Starline brass did you buy?

 

[Unclenick]

Primers contain small amounts of explosive that is hard to control the exact quantity of. As a result, pressure wave standard deviations of sometimes 10% or more result, and for large primers that much variation has a bigger effect on starting pressure than for small ones. That said, you sometimes see loads that are more consistent and accurate with large magnum primers than with standard ones, so there is an interplay with the powder's ignition characteristics that makes this inconsistent, depending on the components. If you can afford to try both approaches, I would do that.

 

[cdoc42]

If I'm not mistaken, I believe experiments have shown there is no advantage to anything other than large rifle primers in the 6.5 Creedmoor.

 

[Swifty Morgan]

Since posting, I read that several industry people are saying small primers are actually dangerous for this caliber.

 

[Rimfire5]

I started shooting my 6.5mm CMs several years ago with Large Rifle primers simply because that was the only brass that was available. I bought Norma and Hornady brass. I always use Federal 210M large rifle match primers with my target loads because I have found them to provide the most consistent results, at least for me.

I prefer to shoot Lapua brass because it gives me the longest life for reloads and that longer case life results in lower cost, even though Lapua brass isn't cheap.

When Lapua finally became available, it was only available in small rifle primer versions.

Needless to say, I compared the results with large primer brass and small primer brass over a pretty large sample so I could make a statistically valid comparison.

I shot 147 groups with Federal 210M large rifle primers before I changed to small rifle primers.
And after I got the Lapua brass, I shot several different small rifle primers and got better accuracy.
CCI BR-4 small rifle primers were the most accurate and were 19.8% more accurate than the 210M primers by 19.8% over a 911 group sample.
CCI 400 small rifle primers were second best and were 9.5& more accurate than the 210 M LR primers over a 314 group sample.
Federal 205M small rifle primers were worst and were 25% less accurate than the 210M LR primers over a 23 group sample. I didn't bother shooting more groups with a primer that didn't measure up.

If I had shot only Federal primers, I would have concluded that the Large rifle primers shot more accurately.
Since I shot three different primers, I now conclude that small rifle primers can provide better accuracy as long as you pick the right primer(s).

 

[Unclenick]

Good experiment. I will point out that in both Charles Petty's 2006 Handloader experiment with about a dozen different small rifle primers behind a fixed load of RL10X and a 55-grain Hornady V-max bullet to see what velocity differences were produced, and in this study of measured blast waves from primers, the Federal 205 and 205M were the mildest. That suggests to me that for a given powder and case fill and case capacity, there is going to be a minimum ignition gas volume and heat requirement due to the amount of empty space in the case.

Incidentally, in the linked study, the Courtneys found a strong correlation between primer weight to the nearest milligram and the standard deviation in their output, cutting the SD in half when sorted by weight to that resolution. Oh fine! Just when you thought you were doing everything imaginable...