Explosive nature of pinfire cartridges


Can you guys (& girls) help me with another pinfire related question?

I would like to know if pinfire cartridges posed any greater danger to the person firing their weapon (let’s say a revolver for the purpose of this post) than central fire or other cartridges if there was a blockage or flaw in the barrel of the revolver? I don’t know much about these things but hopefully I am right in saying that the unique way a pinfire cartridge worked was that the force of the hammer onto the pin caused a small explosion which propelled the cartridge down the barrel and out of the gun. If the barrel was cracked or blocked or whatever, what would then happen? I have in my mind a kind of cartoon image of the person firing the weapon having a blackened face with the end of the revolver peeled outwards like a banana. Would that be kind of right? What I am getting at is: would there be any harm to the person firing the revolver and would there be any debris littered around the scene? Or would the firer be okay and no shrapnel etc. be found? Hope that makes sense. If it makes a difference, I’m interested in 7mm pinfire cartridges specifically. Also, if anyone can point me to any good sources of information about pinfire cartridges generally please let me know.


I don’t get your point or question. In all conventional firearms (weapons that use a cartridge with a primer, powder, case and bullet) the “hammer” (firing mechanism) strikes a primer which makes a “small explosion” which ignites the powder, causing it to become a gas, the pressure of which “pushes” the bullet down the barrel and on its way to the target. The only way a pinfire cartridge differs is that the firing pin is built into each cartridge and is not part of the firing mechanism of the weapon itself. The overall function of the pinfire cartridge is identical to a rimfire, centerfire and I suppose other “patent ignition” systems.

I don’t see why an obstruction in the bore, which in a revolver, by the way, seldom causes damage to the shooter, but rather only to the barrel, and only then if a round is fired with the obstruction in the bore, would be any more damaging or dangerous with a pinfire system than with any other conventional firearms.

The first reason for an obstruction in a bore, a cleaning patch stuck there, a wad stuck there (in a shotgun), a bullet sxtuck there, generally does no damage at all. It is the retardation of the gas behind the next bullet fired that causes bores to bulge occasionally splitting a barrel.
The damage occurs behind the second bullet, not where the second bullet strikes the firest obsturction from the rear.

The only part that the “force of the hammer” plays is that it must hit the firing pin hard enough to ignite the internal primer, the same as with any other firing pin system. The force of the hammer itself plays no part at all in the expulsion of the bullet from the firearm, other than what was said - simply igniting the primer. It is the primer that ignites the powder charge in a pinfire, and the powder charge that propels the projectile, as with any other system involving a more or less convention cartridge.

I have probably explained that just about as clear as mud. There can be other factors involved, such as very, very bad condition of the firearm, etc., but the above is basic.


Thank you John, that is very helpful indeed. Please forgive my total ignorance of all matters relating to ammunition - it is all new to me. To sum up where I think we are:

  1. There is no difference between pinfire and central/rim fire cartridges in respect of the “explosiveness” (that’s fine, I just wanted to check).

  2. There is no danger to the person firing the revolver in the event of an obstruction (that’s also good).

But what would actually happen if there is an obstruction or a flaw in the barrel of a revolver which prevents firing? Let me put forward a scenario: I fire an old revolver with a slightly cracked or flawed barrel. My first shot successfully hits its target but firing the weapon damages the barrel beyond repair (although I don’t realise and pull the trigger again). What happens with my second shot? Does the revolver explode in my hand? Does it shatter into pieces so that there is debris scattered all around the immediate area? Or does it remain in tact? Will there be gunpowder everywhere? Will there be a noise? Or does nothing at all happen? That’s really what I’m trying to establish. (My scenario just happens to involve a pinfire revolver but if that makes no difference I apologise for introducing a red herring ).

Much thanks in advance for any more help you can provide!


Firstly, lets cancel out that statement “that there is no danger to the person firing the reovlver in the event of an obstruction.” If there is already an obstruction in the barrel and the gun is fired, GENERALLY, only a bulged barrel results. Bad Scenario, the barrel cracks. Worst scenario, bits of metal are flying about. When anything challenges the structural integrity of a firearm during firing, you cannot make the flat statement that there is no danger to the person firing it. Stuff happens! Generally, because of the gap between the cylinder and the barrel, there is created a “safety valve” if you wish to call it that, that allows the excess gas to escape and diminishes the damage effect on the gun (and shooter). But, you cannot rule out 100% that the shooter won’t be injured.

And again, if a barrel is obstructed, but no shot is fired before the initial obstruction is removed, then no damage generally occurs to the firearm.

Now, about firing a revolver with the barrel already cracked, no sane person would do that if he were aware of the crack. Secondly, all bets are off. The barrel would certainly be ruined - it is ruined before the shot is fired if it is cracked. You cannot predit with 100% certainly what willhappen after that.

Again, though, it matters not at all whether you are talking about a pinfire, centerfire or rimfire revolver. And, remember, we are talking only about revolvers, not solid -breech single-shot handguns and not automatic pistols.


I think to sum this up, if you do not know how safe a fire arm is, no matter what type it is, take it to a competent gunsmith and have him look at it to determine if it is safe or not. Trial and error may get some one hurt!


I think the basis of the question can be answered like this. The pinfire case was inherenlty weak and could be prone to gas leak even in normal use. However to offsets that the powder charges were feeble.

Actually black powder revolvers don’t usually blow up even with a blocked barrel because the gasses find other ways out but if you had a blocked barrel in a pinfire the case would almost certainly rupture even if the gun remained intact.


I cannot see why a pinfire case would “almost certainly rupture” from the effects of a blocked barrel. Much of the trapped gas in the bore escapes from the gap between the face of the cylinder and the forcing cone of the barrel. What does return into the cylinder would not match the initial pressure at firing, in my opinion. Since the cases seldom rupture upon firing, why would the reduced pressure of gas re-entry into the cylinder oor residual gas pressure in the cylinder, cause them to rupture? Further, the case would be expanded a bit from firing and would still be totally supported by the walls of the cylinder.

I have seen dozens and dozens of revolver barrels that were ringed from being fired with an obstruction in the barrel, including one where several shots were fired with the end result of the nose of a bullet visible slightly protruding from a huge crack in the barrel. I don’t think I have ever seen one, that was not bulged at the very rear and therefore bulged the frame slightly too, that could not be simply rebarreled into a completely serviceable revolver. We have all seen ringed barrels even in rifles, where no other damage was suffered.

It is still a crap shoot, of course. No one can predict with total certainty what will happen. Too many factors involved.

Vic had the best answer to all of this.


I’m afraid I have caused some confusion here due to poor drafting on my part for which I apologise. I want to clarify that I have no intention of firing any revolvers nor do I encourage anyone else to do so! Let me re-phrase this question:

What I want to get at is, this: Firstly, if someone fires a revolver with a cracked barrel, so that the cartridge is not expelled, is is possible that there will be some kind of explosion (however minor) or anything that makes a loud sound? Secondly, is it plausible that someone could fire such a revolver so that (a) there would be no injury to the person firing it and (b) the revolver remains in tact leaving no visible debris of any kind at the scene?

To explain further, I’m dealing with an actual historical scenario from about 100 years ago. Someone heard two shots but only one bullet was found at the scene of firing. My theory is that the revolver (or cartridge) exploded on the second firing but I want to be sure that no-one can come back to me and say: “hey, your theory is nonsense, if the revolver exploded then the person’s hand would have been blown off or burnt or otherwise damanged and/or there would have been pieces of the revolver scattered everywhere”.

Hope that clarifies it. I’m very grateful to you, John, Vic and Vince, for your assistance so far.


Could the second shot just have missed and the bullet went off somewhere where it was never found?


Thats a different question. With a black powder revolver espescially a pinfire, it is quite possible to fire the gun and the bullet get stuck in the barrel without any other adverse effects.
The reason I say espescially with a pinfire is because they only carried a pipsqueak powder charge and there were plenty of alternative ways for the pressure to exit. Black powder is much more forgiving than a modern powder would be. The revolvers also tended to be rather loosely made which meant they would leak a lot of pressure in the normal course of events anyway.

If you go to the NRA museum at Bisley they have there a Webley revolver with six bullets stuck in the barrel. The firer and the gun suffered no ill effect, in fact the firer was unaware anything was amis until the cylinder refused to turn.

The revolver is now sectioned and on display in the museum. Thats your perfect proof if you want it.


Falcon - the firing was in a small confined space, with police on the scene and, while I get the impression the police were pretty useless in those days, it would have been impossible to have missed a bullet hole.

Vince - that is really very helpful and interesting about the black powder. So perhaps I need to ask a different, more basic, question. In the case of a revolver, what is it that causes the sound of a gunshot? Is it the hammer hitting the cartridge and the cartridge exploding? Or what? (forgive my total ignorance). My point is that I thought the sound heard by the witness must have been the revolver exploding but, from what you say, am I now right right in thinking that it was, in fact, more likely to have been the normal sound of the revolver firing, just that the cartridge was not expelled?


With a modern rifle there are two sounds produced when a rifle is shot. One is the pressure wave of the expansion of the hot gases caused by the powder burn and the other is the CRACK of the bullet traveling faster than the speed of sound. With older pistols there is no sound barrier being broken so you will only hear the sound of the expanding gas. Actually there will be two sounds produced, almost at the same time, one from the cylinder’s escaping gas and the other from the barrel. You will hear this as one sound. Hope this helps


Excellent, thanks Vic, I think I’m finally understanding this! So, in the case of a pinfire revolver with no cartridge expelled due to flawed barrel, when the trigger is pulled there would still be the sound of the escaping gas and that would create a “bang” sound which would sound to an observer like gunfire?

You say that there would also be another sound from the barrel. Can you explain this further? I mean, would this sound still be emitted if the cartridge remains in the chamber? (i.e. so that there would be no difference at all in sound between the two shots).


No cartridge is expelled from any revolver at the time of firing. It is removed manually at a later time by the operator. Do you mean the bullet?
As for the noise, think of a champagne cork leaving a bottle and multiply it by ten ( or whatever). Or blow up a paper bag or a ballon and burst it.

The sound from the barrel is just made as the high pressure gases generated by the burning of the powder hits the still air at the end of the barrel. But with a revolver there is a gap between the cylinder and the barrel and some (actually quite a lot) of the propelling gases escapes through there, and reaches the air that way. With a stuck bullet in the barrel it would all go out that way but the bang would be the same. Maybe slightly different to a really trained ear but otherwise indistinguishable. Most pinfire revolvers would have been loose enough to allow that to happen with a light load of black powder without detriment to the gun or firer. The pressures were really low by modern standards.


Yes, indeed the bullet (to someone like myself who knows nothing about ammunition I’m afraid I tend to speak of the bullet and cartridge as the same thing but yes, of course, the cartridge remains in the chamber). So just to clarify, if the trigger is pulled, the noise is the same regardless of whether the bullet leaves the barrel. Have I got that right? If so, I think I have my answer.


Yes, I have edited my previous post to include that. I presume you are researching this for a book. Will we get to see it?


Perfect! I really appreciate that Vince.

And yes I am writing a book - it will be my first and probably self published - I will post back when it is available.


[quote=“DavidB”]Perfect! I really appreciate that Vince.

And yes I am writing a book - it will be my first and probably self published - I will post back when it is available.[/quote]
If you get a taste for it contact me at some time off forum and I have another old story you might be interested in getting your teeth into.



If I follow this thread right, you are exploring the scenario of an event where a pinfire revolver was fired twice, two reports were heard and only one projectile was accounted for. A theory that you are following is that the second shot resulted in a lodged projectile or another situation occurred where a projectile was not launched in a normal fashion but a report was still heard (gas venting from the cylinder gap or otherwise). I assume the revolver was not later available to verify such a malfunction.

May I suggest that perhaps the second cartridge fired was a blank with no projectile?



Hi DaveE, yes that’s exactly it and, if I might say so, rather better expressed than my own effort. Your assumption about the revolver is correct. On the facts of the case, there was definitely no blank cartridge. Thus, I am assuming that gas venting from the cylinder cap is the most likely explanation for the sound of the report.