Bore Pressure and Silencer Nerd Stuff

 I'm not an engineer or math dude. I just really like guns and silencers. Silencer's from manufactures are rated for different cartridges by name. This is usually a good practice to that the every Joe and Jane know that the silencer they bought will safely work on the rifles and pistols they have. I think it's vastly more complicated than just that.

I have a Dead Air Wolfman as my primary centerfire caliber silencer. Dead Air only rated that silencer up to 350 Legend. They didn't put in barrel length restrictions but did say it's not full auto rated for 5.56 or 300 Blackout. It is full auto rated for 9x19.

I want to use the can on a Marlin 336 in 30-30. While the manufacturer doesn't "rate" the silencer for the caliber, they do rate it for calibers with higher peak chamber pressures. 5.56 NATO is around 56,000 PSI and so is 350 Legend. 30-30 is around 42000 PSI peak chamber pressure. It should be safe right? Well it's more complicated than that. 

Gemtech did a study a while back that you can read here. That slide show/PDF shows that the pressure inside the barrel reduces as the bullet moves down the barrel. Sort of a duh statement but the values are interesting. The study took at 24 inch barrel and put sensor ports at 1 inch intervals. They would move the port to the last hole and fire 5 rounds. Then cut off a 1 inch section and move the sensor back 1 port. This way they could measure the pressure close to the muzzle of the barrel. The bore pressure, which is not peak chamber pressure, reduces the further away from the chamber the bullet gets. I think that makes perfect sense as if you make the volume of a given container larger, the pressure reduces for a given amount of air or gas. 

The numbers they got was that at 5 inches long, the muzzle has around 25,000 PSI but at 10 inches, the pressure is 12,140 PSI. That's less than half. The case capacity of 5.56 NATO is around 0.1128 cubic inches. From the throat to the 10 inch mark should have a volume of 1.576 cubic inches. That's a lot more than twice the chamber volume. That surprises me since the pressure is less than a quarter of peak chamber pressure. There doesn't appear to be a nice linear formula to figure all this out. I'm sure someone has a formula out there. 

That said, the Gemtech study does show some interesting data. What I would be interested in finding out is what the pressure rating is for the blast chamber of the silencer. The Wolfman does work on 10.5 inch barrel 5.56 guns. While I don't think the chamber is rated for something like 12,000 PSI, it might be. I say that because the pressure at the muzzle will reduce once it dumps into the blast chamber. What ever the pressure rating is for the silencer, I figure, if you know what the pressure is at the muzzle for a given barrel length and caliber, you should be able to user your silencer. 

The short and sweet? The TL:DR? I think I can use the Wolfman on a 20 inch barrel 30-30 and be just fine. I might even be able to use it on a 308 Winchester with a 24 inch barrel but I won't be doing that.