Large or small size primers in pistol cases?


I’ve come across some fired brass headstamped CBC .45 AUTO but one has a small primer and one a large primer, both boxer. The small primer seems to be crimped too. Can anyone tell me why the different primers?


Finding small and large diameter primers in the same caliber is not unusual. I have seen that in .45 ACP, .38 ACP, .38 Special, 7.62X39mm, and .400 Cor-Bon, and there may well be others. In fact, I reloaded some .45 ACP cases with small primer pockets just last night. I don’t know for sure, but have suspected that it has to do with higher chamber pressure loadings, as small diameter primers would be better if chamber pressures are high, as they would not have as much tendency to back out under pressure. Maybe someone else has a better explanation.


Because of my doubts on some things said here, I contacted a dear friend of mine who is an engineer retired from a major American ammunition company. So there is no mistake that this information is from him, and not from the very limited resource of my own brain, I am going to quote him in my answer over the “why” of small primers in pistol cases that have been traditionally primed with large pistol primers. The thrust of my own study of ammunition has always been more historical than technical, and while I had some idea of the relationship of small primer use to modern “Green” ammunition, my knowledge was extremely limited.

Firstly, it has nothing to do with chamber pressures and primer back-out. To begin the quote:

"The reason you mention that appeared in your source is incorrect. The responsible companies all load their ammunition with SAAMI recommendations. Primer back out and chamber pressure is not involved in the consideration of which size primer to use.

"The trend that you noted began prior to WWII when most of the .38 Special and .357 Magnum shellcases were loaded with ‘large’ primers. The reason that they morphed to ‘small’ prmers is unknown to me, but see the next paragraph.

"For the record, SAAMI does not recommend primer sizes per cartridge per se, although the primed shellcase sensitivity is specified per cartridge. Small primers generally have better sensitivity than Large Primers because typically the primer cups are thiner. Most manufacturers use what is essentially a Small Rifle primer in 357 Magnum cartridges to better resist primer perforations with the relatively high pressure loads.

"Another benefit of Small Primers in pistol cartridges relates to shellcase head strength. The 40 S&W shellcase was designed with a Small Primer for that reason. If you have ever cross sectioned a 10mm Auto shellcase you will note a distressing lack of metal in the head. Same with a 25 Auto but in that situation the pressure is so low that the shellcase can contain the pressure.

"The 45 GAP was also designed with a Small Primer to provide more metal in the shellcase head. Most folks have never noticed that the extractor relief angle on the 45 GAP is much steeper than the 45 Auto. Again, to provide more metal over the feed ramps. ‘Gun Writers’ don’t usually notice such esoteric subtleties and many have written “The 45 GAP is just a shortened 45 Auto.” No, it is not!

"The last and mabe most important consideration is a condition known to insiders as “Breech Peening.” That is physical damage to some pistol breech faces caused by lead-free primers using DDNP as the initiator.

"The pressure generated inside a DDNP primer is momentarily much higher than the ultimate chamber pressure. The breech peeing can move enough metal on the breech face to interfere with firing pin movement, or worse yet, to lock the firing pin in the forward position.

"The better semi-auto pistols have heat-treated and properly hardened slides/breech faces and have never experienced the breech peening problem. Some of the lower grade pistols have ‘soft’ breech faces. They complain “There is nothing wrong with our pistols - the ammo has changed.” Maybe, but those manufacturers that don’t change with the times will fall by the wayside.

"The quantity of priming mix in a Small Primer is considerably less than that in a Lare Primer, thereby producing less damage on relatively soft breech faces. The factories can not just simply reduce the quantity of priming mix in the large primers because of potential misfires.

"Some Shooters (but probably no gunwriters) have noticed that the flash holes on shellcases destined to be loaded with DDNP primers are much larger than ‘standard’ flash holes. That is to relieve the ‘primer pressure’ quicker to minimize breech peening.

"It is not practical for an ammunition company to produce and inventory the same shellcase with different size primer pockets and flash holes so they standardize on the one that works with both lead-free primers and conventional primers.

“The different size primer pockets raises havoc with reloaders, but that consideration has never rated high on the ammunition manufacturer’s list of priorities.”

Well, there is an explanation from a professional “ammunition engineer,” and in the case of the gentleman who gave it to me so quickly on my request, a highly proficient one. I will admit right now to not knowning what “DDNP” stands for, so don’t ask me now. If someone knows, feel free to chime in. I will find out and if not already posted, will post it on this thread.

Hope this is of some help and interest to the question at hand.


Thanks for the explanation. I was unaware of the pressure differences with the DDNP primers. but I am familiar with breech face peening, especially in autopistols of the “Saturday Night Special” variety. These often have slides made of Zinc alloy instead of steel, and peening around the firing pin hole occurs in short order - maybe with as few as 100 rounds. It can eventually lock the firing pin in place - and that’s with standard ammunition, not lead-free. I have never seen that happen to a pistol (or rifle) with a steel breech face, even when lead-free ammunition was used in them nearly exclusively.

The idea of a case head being stronger with a small primer pocket had occurred to me, but I did not mention it, as I had not read that previously and did not know for sure.

DDNP means Diazodinitrophenol, which is the impact-sensitive ingredient in the lead-free priming mixture. In lead-free ammunition, it replaces lead styphnate. It’s nothing new, as it was used extensively as a primary explosive ingredient during WWII. The main downside of DDNP is that it is not as sensitive as lead styphnate at very low (below -20 degrees) temperatures, therefore, ammunition used in combat should retain the conventional lead styphnate-containing primer for low-temperature reliability. That problem may have been overcome, but I don’t know the latest on DDNP, as my experience with it goes back a few years.

By the way, I reload 9mm, .38 Super, .38 Special, and .357 Magnum cartridges that use a small primer pocket with Small Rifle primers. I started doing that many years ago after my wife sustained an eye injury (non-permanent) due to a perforated primer while shooting a .38 Special revolver. It’s worked fine for me over a great many years, and it simplifies my reloading supply inventory.


A question from the dim and distant Past:

Does this modern "small primer/Big calibre/Nontox priming compound"
discussion have any relationship or just happenstance with the original
.45ACP Gov’t ammo (FA) with the unique .204 Boxer primer? (Rather than the current .210 LR). Was it something to do with early balloon head cases vs. solid head/ or something else?

(Only Italy had a .204 primer, and it was Berdan, and a rifle ( 6,5x52 It.) primer.).
Note that European Loaders (pre- and Post- WW II) used a .199 (5.00mm) Berdan primer in .45ACP (Norway, Yugoslavia) and also in some 7,62 Tokarev and 7,62 M95 Nagant Pistol).

Doc AV
AV Ballistics.


FA used the .204" primers in 45 ACP to prevent the use of LR .210 " primers which would have caused high pressures. See Hatcher.
I have never saw a 38 ACP or 38 Special with a large primer.



Here is an example of a large primer in a .38 Special as used on this early high velocity loading. I also have at least one other large primer .38 Special (Peters maybe?) that is a standard velocity load from what I can figure.


Great information! Thank you for providing it.



During WWII, to make matters simpler due to war shortages I suppose, the .38 ACP was loaded with large primers by some companies. I have samples in my own collection. Some other calibers were as well, but I don’t have a list made up and no time this morning to make one (I could for auto pistol only, anyway) as I have to go away for a couple of days and am leaving soon.


Thanks to all of you for all the great information. I’ve copied all and will file.


The size of the primer in most pistol cases is not critical. We used to see a lot of .45acps with small primers. From one or other of the eastern European sources(?) It didn’t seem to affect combustion because the amount of powder in a .45acp is not more than in a .38 spec, give or take.

What it used to do was jam up our automated loading machines if we weren’t careful to weed them out.