Does primer size matter? 6.5 Creedmoor small v. large rifle primer brass comparisons

Does primer size matter? 6.5 Creedmoor small v. large rifle primer brass comparisons

Most rifle shooters, myself included, tend to view the use of a small rifle primer in a larger center-fire cartridge as a good thing. Years ago when I watched my first benchrest match and talked to one of the shooters, he pointed to the 6PPC’s small primer as the reason for its success. Take a look at another round that hammers: the 6BR, and again, you see a small primer.

When the 6.5s started making it big, the 6.5×47 Lapua was often viewed as superior to the 6.5 Creedmoor largely because of the use of a small rifle primer. There was concern the small rifle primer wouldn’t work reliably in cold weather. This led to some pretty interesting tests by cold climate shooters that seemed to indicate this wasn’t as much of a concern as shooters had initially thought.

Some brass makers, like Lapua, began producing a 308 Winchester load that used a small rifle primer specifically targeted to the long range shooter. When 6.5 Creedmoor brass became more commonly available, some brass makers offered it with a small rifle primer (instead of the specified large rifle primer).

It seems like a small rifle primer is the way to go when you order your 6.5 Creedmoor brass? Or is it? I decided to take a look at how 6.5 Creedmoor loads behaved when they were both shot through the same gun using different primers. To view SAAMI specifications of large and small rifle primers, click here and scroll to page 36.

This wouldn’t normally be possible since most brass makers either produce 6.5 Creedmoor brass in large or small rifle primer size. Starline brass offers 6.5 Creedmoor brass in both large and small primer sizes. This means we can make a direct comparison of the two different types of brass. Some manufactures offer a .059″ (1.5mm) flash hole with their small rifle brass, Starline uses a .080″ (2mm) flash hole in both large and small rifle primer versions of their 6.5 Creedmoor brass. You’ll note flash hole size isn’t specified in the SAAMI document referenced above.

Comparing hand load data is always interesting, there are so many variables to control for it is hard to get reliable data. Let’s say for instance, in the case of this brass case test, I ran the exact same load in both cartridges. One could argue that the load was optimized for a large or small rifle primer and that it doesn’t necessarily show the benefits of one over the other. To address this I decided that I would test 5 different charges of the same powder, H4350, with the same bullet (142 SMK). I loaded 10 rounds of each load in each type of brass and fired two five shots groups at 100 yards. I used a MagnetoSpeed V3 Barrel Mounted Ballistic Chronograph to gather data for each ten shot series. The loads that were shot with new brass are referred to as “new” in the tables and charts below. This allowed for a larger sample size than simply 3-5 rounds, and a wider variety of loads. After I conducted the first test, I reloaded the same cases and repeated the test, this time with fire formed cases. This second set of data is referred to as “1XF” in the tables and charts below.

Before we get shooting, let’s take a look at the disclaimer below:

WARNING: The loads shown are for informational purposes only. They are only safe in the rifle shown and may not be safe in yours. Consult appropriate load manuals prior to developing your own handloads. Rifleshooter.com and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.

For a test rifle I went to my favorite 6.5 Creedmoor rifle.

I built it with parts from Brownells, including:

All the parts; the barrel, chassis, scope and trigger, work well together for a nice shooting rifle.

It started in an AI AX chassis over to an MDT ESS. I’ve been shooting the ESS a lot lately and have grown quite fond of it! If you’ve been following my blog, you’ll notice it is now sporting a carbon fiber fore end and folding stock, I love this rifle! (Click here to learn more about the ESS)

I’ll be posting a separate review of Starline brass later, however, let me say that I am fairly impressed with this stuff, especially when you consider the price- at the time I’m writing this Brownells is selling it for about half of what Lapua costs. For the first series of rounds, I used the brass straight from the box, no prep. For the second “1XF” series, I ran the brass through a Redding full length bushing die and chamfered the mouth prior to loading.

What primers did I use and why?

Easy, the ones I had and normally use. Wolf large rifle and CCI 450 small magnum rifle primers. Why those? Anecdotally I’ll tell you I’ve always gotten the lowest SDs out of the Russian primers. I used to use Tula before they stopped importing them. When those dried up I switched to Wolf. While the CCI 450 is listed as a magnum small rifle primer it is my go to primer for my 6BR, 6×47 Lapua and 6.5×47 Lapua, I’ve had great success with both primers.

The results of my first (new brass) and second (1XF) sessions at the range are shown below.

First the new brass:

Then the 1XF brass:

I apologize for the color of the targets, they look way better in real life, I had some light issues indoors. I developed this target with Rite in the Rain. It is on waterproof stock with true 1.047 MOA green, orange and yellow dots. The color provides excellent contrast and you can still see your impacts on the paper. Rite in the Rain sells these targets as well as paper that you can print your own targets on. To learn more about Rite in the Rain and their line of targets, click here.

Now that we have some data, what does it mean?

Good question. After sharing my data set with a couple of shooters we decided that it would probably be best to present the information in the form of bar graphs, so I generated the graphs below. In the graphs below, the large rifle primer loads are colored blue and red, while the small rifle primer loads are orange and green. To compare data gathered at the same session, compare blue to orange and/or red to green.

Primer size versus group size

This graph compares primer size to group size, smaller is better (unless you like shooting AKs). In most cases (not all), I think it would be safe to say the small rifle primer loads tended to produce small groups.

Primer size versus standard deviation

This graph compares primer size to standard deviation, smaller is better. In most cases (but not all) the large rifle primers produced lower standard deviations than the small rifle primers.

Primer size versus muzzle velocity

This graph compares primer size to muzzle velocity (feet/second), larger is better. This was actually pretty surprising since I hadn’t anticipated seeing much of a difference between the two primer types. Further, across the board you’ll note increased velocities in the 1XF brass over the new brass. In most cases (but not all) the small rifle primer brass had higher velocities than the large rifle primer brass. I speculate the increased velocities of the 1XF brass may be a function of neck tension, work hardening necks, or both.

Closing:

Did anything surprise you? Yes, I was a bit surprised by the tendency for the small rifle primer groups to have higher muzzle velocities than the large rifle primer groups.

What about sample size? You can always have more data, while comparing sample sizes of ten rounds is certainly better than comparing sample sizes of five, it is always possible to do more.

Why didn’t you shoot another primer? For instance why not a CCI large rifle v. CCI small rifle? I used the primers I normally go with. If I was going to buy 6.5 Creedmoor brass, those are the primers I would use. I suspect if I ran CCI standard primers the results would have been more disparate and favor the small primer more, however, I can’t be sure.

To learn more about Starline brass, click here.