by B.B. Pelletier

I returned from the hunting trip yesterday afternoon. I looked at all the back questions and then answered just a few. The rest I didn’t answer, so ask them again if you really want an answer.
The hunt was very successful. I will tell you all about it next week, but tomorrow’s post will be that new item I promised. Now on to today’s report.
How pressure relates to velocity
The bottom line of this research is to provide some insight into how powerful the antique big bore airguns are by knowing what kind of air pressure they work with. There’s no formula to calculate such a relationship, and it may be such a complex relationship that there never can be; there’s a fair amount of information gathered from observation. For starters, let’s look at Quackenbush’s Brigand. On CO2, we get velocities around 575 f.p.s. with a .375 caliber lead ball weighing 83 grains. CO2 is nominally 900 psi. With the same rifle running on air at 1,200 psi, the velocity increases to somewhere between 730 and 775 f.p.s. Pressurize the gun to 1,500 psi, and the velocity drops to around 600 f.p.s., until the high pressure is lowered to the optimum range. Then it picks back up.
What we can say about the CO2 Brigand is that it has a valve that functions best with air pressured to between 1,000 psi and 1,200 psi–and it will function with limited results between 600 psi and 1,700 psi. That’s a broad range of pressure but a much narrower band of optimum performance.

Looking at the outside lock rifle built by Gary Barnes, a rifle I haven’t yet reported on, but will in the near future, we see an optimum range of performance from about 500 psi to 650 psi, with a working range of 300 to 800 psi. Oddly, although the outside lock functions at about half the pressure of the Brigand, it gets a few more shots on each charge of air. What’s involved is a combination of caliber, barrel length and weight of the projectile.

To achieve high efficiency with low air pressure, the valve needs to remain open longer to allow air to continue to push the projectile until it’s free of the muzzle. A large caliber provides more volume to lower the air pressure after it leaves the reservoir. The farther down the barrel the projectile gets, the more volume there is behind it, and big bores increase in volume faster than small bores.

To push a heavy projectile fast, you have to maintain the force pushing on it for as long as possible. That means a longer bore. A longer bore will diminish the air pressure behind the projectile very rapidly, unless the force is applied continuously.

What this means is that big bore airguns must leave their valves open much longer than small bore guns; to do that, they have to run on lower pressure. Note that when Quackenbush went from 1,200 psi to 3,000 psi, his velocity increased by only 118 f.p.s. (775 vs. 893). To get even that increase, he had to redesign the valve, because the standard valve wouldn’t have functioned at the higher pressure.

What can we learn from this? Well, when we see a vintage .36 caliber air rifle shooting a round ball at 675 f.p.s., we now know it’s about where it should be. Perhaps it might get up to 750 f.p.s. But a claim of 1,000 f.p.s. for a .65 caliber rifle with a 48″ barrel should be met with some skepticism because of what it would take to actually achieve such performance.

I hope you were not waiting for some magical air pressure/energy calculator. I don’t have one and I would doubt seriously anyone who said they did. A single gun and valve can be modeled fairly close, but once the design starts changing, all bets are off.
This process could be modeled, no doubt, but it would take more work than most people might imagine. There are variables that don’t even become apparent until you start trying to estimate performance of a real design. So be wary of the person who tells you this is a simple linear relationship, because it is anything but.

Summary
The way antique big bore airguns are designed, there’s no reason to over-pressurize them. They work well only within the narrow band of pressure for which they were designed and (sometimes) tuned. So, by knowing the specifications of the pump used to charge them, we can know what their operating range is, and that, in turn, reveals their performance in a general way.

My thanks to Dennis Quackenbush for providing the test data and the test pumps used in this article.