By John Kleespies
I’m currently in the process of building a new .308win rifle and went to Lapua.com for some new brass. There, I discovered that they have two offerings for my rifle: “.308win” and “.308win Palma.”
Curious, I reached out to Lapua to learn the difference between the two types of brass, and I was informed that while standard .308win uses a large rifle primer, the Palma version uses a small rifle primer. Other than that, they are identical.
The reason for this second offering, it turns out, is that Palma precision rifle shooters were requesting small rifle primers in .308win due to their belief that there was less variance in ignition power with small rifle primers versus large rifle primers.
Having never heard of this “primer eccentricity” before, I did a quick Internet search and found this statement repeated here and there.
The thing is, none of this “confirmation” was backed up with, “based on my testing,” or “as I have personally witnessed” or the like, so I couldn’t tell if anybody was actually experiencing this phenomenon, or if everyone was just repeating “what everybody knows” because that’s the rumor around the shooting benches and the chat forums.
So… asked an extremely well-respected primer manufacturer for an answer.
I can now report that I have an “official source who wishes to remain anonymous”—woohoo! – that confirmed with actual engineers that this observation about primer consistency is correct.
Before I get too deeply into the explanation, a very important fact needs to be pointed out: both large and small primers use the same chemical compound, just in different amounts.
This being the case, my anonymous source’s anonymous workplace’s R&D department took large primers and tested them in a series of five decreasing charge weights, from a standard (large) charge amount to less than what would be found in a small rifle primer.
Each decrease in charge weight resulted in a decrease in power variation.
This scientific testing shows that small primers do, indeed, exhibit less variation in power output than large primers!
Kudos to the Palma shooters who requested this, because when it is taken into account that the primer ignition ispart of the overall charge of the cartridge (ie, primer + powder) then going with the small primers can result in tighter groups.
Now, if I may theorize my own explanation of this phenomenon… I think this is, at least in part, simply an outcome based in arithmetic. Basically, if a single substance is used, and it has a +/- 10% variation, and it is put to use in two amounts—say, 10 units and 20 units—then the 10 unit will variate by a factor of +/- 1, while the 20 unit will variate by a factor of +/- 2. When this power factor variance is added to (or subtracted from) the power of a constant powder charge, then the smaller unit will exhibit a more consistent total in the overall.
Hence, small primers have less power output variation than large primers simply because they are smaller.
It’s just math.
I also wrote a separate article on small versus large primer reliability in cold weather, using the same anonymous source for data, and in both of these articles I feel that I should include the disclaimer that this information is being derived from only a single manufacturer and probably should not be construed to represent the entire industry.
That said, I highly doubt that you’ll find any primer manufacturers who use a different chemical compound between their different sizes of primers. As long as the chemicals are consistent within a manufacturer, then it stands to reason that the reactions amongst the sizes of their primers should exhibit consistent properties, as well.
So, if you’re looking for the most consistency in your loads – and there’s an option between large and small primers – then go with the small.
