General primer question


What causes primer to explode, a shock wave from the physical impact of the firing pin or the deformity of the surface/volume of priming compound? If it is the 1st (physical impact), would not firing pins benefit from being larger (wider) to deliver greater shock and brake less often?


The priming compound is impact detonated (you can crush a primer in a vise and it won’t go off, but smack that same crushed primer with a hammer and it will detonate)
It takes a given impact force to detonate, increasing the size of the pin would reduce the force (PSI) for a given spring.
The pictured example simple looks like the pin didn’t line up with the anvil (if that case even has a anvil (might be missing)


That rifle is in very poor condition looking at the eccentricity of the striker. It is nowhere near the central Berdan anvil. It could be general wear in the boltway or possible the wrong bolt for the rifle (the serial number of bolt, barrel and receiver should all match). Such a badly fitting bolt may be only supported by a single locking lug, a dangerous situation if it did happen to fire a cartridge.



It was very common to make the striker punch excentric. You will see that at german military rifles. The Berdan anvil shouldnt be hit directly and deformed because of reloading.
In the land of Boxer priming it might look strange :-)


That cartridge was probably chambered in a .303" Lee-Enfield. The intention with this rifle was always to have the strike central on the primer. There was an allowable tolerance on eccentricity before the rifle needed to be condemned until the defect could be rectified. I don’t remember how much the strike was allowed to be off-centre but it was nowhere near as much as shown on that cartridge. As a rough guide we used to say that the striker impression must touch the centre of the primer, this would allow half of the striker diameter maximum eccentricity, no more.

UK fired cases of that era were not reloaded so there was no need to “avoid hitting the anvil”. If the cartridge headspace and striker protrusion are within specification then the anvil should not be damaged anyway.



Reloading good quality brass employing the Berdan primer will not damage the anvil in any reasonable number of reloadings, and this despite the fact in most arms the firing pin blow is generally well centered on the primer. The anvil is more likely to be damaged by improper use of an awl-type decapper. Jack


Coming back to the OP, its internal friction within the priming mixture that does the job, coupled with an extremely volatile substance or compound. The microscopic crystals in the primer are ground together by the blow, squeezed between the identation and the anvil. In early primers it was Mercury Fulminate but modern primers are much more complicated.

Ground glass was often added to the mixture, so was brick dust, to increase the effectiveness. These materials stayed in the bore acting as an abrasive when susequent shots were fired. That explains, at least in part, why a lot of old guns have so much wear on the barrels when viewed today at gun shows. Rounded off rifling and the like. In those days you really could shoot a barrel out even though they were only using lead bullets at comparitively low velocities and pressures.

They still use brick dust in a lot of muzzle loader caps. You can see it round the nipple after a number of shots have been fired. A sort of brownish residue. After about 1000 shots the bore of a muzzle loader is mirror smooth for that very reason.People drop a bore light down the barrel, see how shiny it is and say “the barrel is good” but more knowledgeable people will realise its probably worn.


I think it is beneficial to read the book “the chemistry of gunpowder and explosives”

(I own the book , but you can also download it from the above link , look at the end for the primers)


This thread reminded me, I acquired a fired .303 case the other day that must have been fired in a really loose old rifle. The primer has a single strike in the same place as Vlad’s round, and it is backed out of the pocket by .022" (0.52mm). I wouldn’t like to be standing near anyone shooting that particular rifle. I suppose this was either caused a a really worn bolt and action or the wrong bolt? The round is an Indian made .303 headstamped “OK 83 7”.


That book is Tenny Davis’ “Chemistry of Powder and Explosives.” It was published back in the 1930’s and covers a wide range of information, not only about propellants and explosives, but also fireworks of all kinds. It includes many “recipes” for making explosives in the laboratory. I don’t think there is any other book quite like it, even though it is now somewhat dated. I worked in an explosives QC lab for awhile, and it was one of the more-used reference books in the lab library.

I had mentioned this in another post, but one of the most authoritative and exhaustive treatments concerning primers is to be found in George Frost’s “Making Ammunition” published by the NRA. I do not know if NRA still sells it, but it should be a “Must Have” for anyone interested in ammunition manufacturing technology.


Falcon, The bolt on a Lee Enfield is rear locking not front locking as on most rifles and indeed all modern rifles. To me that is a design weakness. Wear, although it would have to be excessive, or the wrong bolt and you have a problem because the business end of the bolt can float around. Its all a matter of the extent.

Most Lee Enfields have off centre strikes, mine has, next time I see you I will bring along a fired case from my rifle to show you.

There is a lot about primers and how they are made in the Narramore book I gave you.


I have seen alot of .303 cases with slightly off-centre strikes. In fact, very few rifles seem to strike dead centre from the fired cases I have seen.

The case I have is far more off centre than most, and with how far back out of the pocket the primer is it makes me think that the bolt was excessively loose.