The question is around from time to time, why one shouldn’t use smokeless powder in black powder gun? There are some who even try it.
The majority understand that the old, antique guns do not withstand as much as the modern arms. But what is the case with the recently made muzzleloader replicas? Many believe that the notice „Black Powder Only” is just a bullshit. But it’s definitely not.
There are many reasons why one mustn’t use smokeless powder in guns made for black powder – including the modern, in-line black powder rifles as well. Out of these causes I detail some below, which justify the prohibition by themselves individually, and even more if they are combined with each other.
1. The strength of the raw materials used for highly stressed components
An engineering, manufacturing baseline is that we select raw material and production technology for the particular function and strain of the application. Black powder typically generates just the fraction of the pressure and mechanical stress, which forms in a smokeless powder gun having the same caliber and general characteristics. This also mean that provided the same wall thicknesses of barrels, cylinders, materials with less tensile strength can handle the strain caused by black powder, then needed for smokeless nitro powders.
If there is no necessity for the more expensive high alloy steels or for extra heat treatment, the product will be cheaper. If the raw material is softer, the machining is faster, the tooling last longer, the cost of production is less. The manufacturing technology is also tailored to the strength requirements, the stressed part which is made from forged raw material for smokeless powder guns, may easily be produced from castings while making a BP gun (even if some manufacturers do not make this compromise). And not only the manufacturer but the customer also benefits from this, the savings are shared between them.
Conclusion: The materials used for the vast majority of black powder guns have been selected for dedicated black powder use. There is no warranty that they will survive the first shot with smokeless powders or will withstand the use of the nitro propellants in a longer term.
2. The design of the guns
There are plenty of such black powder gun actions, which are not suited for the stress caused by smokeless powder, even if the best raw materials used. Just think of the open frame Colt muzzle loading revolvers, where the joint of the frame and the cylinder pin take all the forces which want to rip off the barrel from the frame upon firing. It is adequate for black powder, but more than hazardous with smokeless propellants. Or check carefully the thread of the nipple or bolster of many muzzle loading firearms. Will the .44 handle the fivefold strain 22000 psi instead of 4400 psi?
Conclusion: The design of several black powder firearms is not compatible with smokeless propellants.
3. The primer cup and the flash hole – a special case of the design-related reasons
At the central fire, metallic cartridge guns the blowback of the propellant gases is sealed by the cartridge case supported by the chamber and the breech face, while the gases which would come through the flash hole are blocked by the primer cup. The thickness and the construction of the primer cups ensures gas tight seal if it is supported by the breech. In metallic cartridge guns the firing pin seldom pierces the primer, but occasionally it happens. The gases breaking through the primer cup may cause significant damage to the gun. It is a common design feature of the bolt action guns to have gas escape holes through the bolt body and the receiver. A simple, lucky primer piercing only sets the hair of the shooter by the „wind”, but I also saw a tilting barrel double rifle, where the gases coming through the firing pin hole blew down the buttstock, and not in one piece but it was fragmented into several splinters. I could list several similar cases, but I think we can agree, that the gases blowing back to the action due to a pierced primer is not good for any smokeless powder gun.
Well, compared to these the traditional muzzle loading firearms (not talking about the modern 209 primer initiated in-line rifles) are more exposed to the gas blow-outs. The flintlock guns even while used with black powder split aside prolifically. If we load a flintlock gun with smokeless powder, we multiple the pressure that will burn out the flash hole very fast. Staying on the gun’s right side is surely not a fun. The blowing gases at these speed cut like a knife. But of course, it is just a bad joke that one would ignite the nitro powder main charge at a flintlock gun with a black powder primary charge…
With percussion guns the situation is much worse. The thin primer cap bursts into pieces even when used with BP. It was designed to act like this, as if it would be too strong, one could not fix it securely on the nipple, would fell down, and on the other hand it could hardly be scraped off the nipple after the shot, which is not the best in the middle of a battle scene. The hammer protects the shooter fairly well from the pieces of muzzle loading primers coming apart on the predicted way. But the system will not serve as well with the gases of smokeless propellants blowing out as the effect of way higher pressures. As I wrote above, at the modern cartridge arms, all the measures are taken to prevent gas blow outs, leaks. The breach supports and holds the primer in place, and it’s still an issue if the primer pierced. The majority of muzzle loading guns are not designed with the scope to hold the primer cup on place. In the case of their smokeless powder use, the primer cup pieces may burst out from below the hammer with dangerous velocity, and the expanding gases might kick the hammer back with such a force that it breaks. In case of percussion revolvers it can easily blow in the shooters face.
Conclusion: The ignition system of the muzzleloading blackpowder guns was not designed for the effects of the smokeless powder, and such use of them is dangerous.
4. Internal ballistic reasons
Who attended one of our reloading courses, or spent time with studying the properties of different propellants, learned that the burning speed of the black powder does not change significantly with the change of pressure generated in the gun – practically it is almost independent from the pressure. But the burning of the smokeless propellant speeds up with the growth of the pressure.
Secondly if we push down the bullet right on the top of the black powder, this is the safe and proper method. No air gap should remain between powder and ball. When the powder is ignited and the bullet starts to move, the burn rate of the propellant will not increase.
But in the case of smokeless propellant, the initial combustion volume (in cartridges the room available in the case below the bullet) is critical from the perspective of the generated pressure, and from the point of the intensity of the pressure rise. The smaller the initial combustion volume is (the higher the loading density), the faster is the burning and generating of gas of the smokeless propellant. The gas pressure will increase steeper in the 99% of the cases. In our video-series recorded for reloaders we compressed a 9 mm Luger cartridge 1 mm shorter than the factory original was, and we immediately measured 4350 psi gas pressure increase (this was true only for the particular ammo, with other propellants and calibers it may be more extreme!). And then we haven’t talked about the effects of extraction force and the bullet weight…
Conclusion: In muzzle loading guns the initial combustion volume can only be kept uniform if we push the wad or bullet right on the top of the powder. This works with black powder but with the use of the majority of smokeless powders it increases the pressure significantly, which is simply dangerous!
Summarizing the above I can’t find any reasonable argument for anyone to try smokeless propellant in a gun manufactured for black powder. The reason why we buy muzzleloaders is that they’re fun. We love their muzzle report, smoke and feeling. We can legally use them for sport or recreational shooting and for hunting as well, similar to the smokeless ammunition guns. What may motivate someone to give a try to smokeless in a BP only gun? It is not possible to neglect or override the knowledge and decisions of the engineers who design and put these guns into manufacture, nor the laws of nature – including the laws of internal ballistics – in a long term, scot-free. The „Black Powder Only” inscription on the gun really means ONLY!