How are airguns designed?
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
• How the Texan was designed
• Establish the power baseline
• The point
• Last point
I’m excited about today’s topic because it gives me a chance to do many things I’ve wanted to do. I linked this blog to the AirForce Texan because today’s topic arose from a comment posted to that report. Blog reader Kevin Wilmeth said the following.
“I would presume from that [my comment that the 34-inch barrel was selected by AirForce for several good reasons], that AirForce had their performance target identified beforehand, and built the barrel to fit it.
And I’m absolutely with you on kinetic energy having some significant limitations as a measure of hunting performance on medium game. I look forward to what’s coming!”
Kevin, you’ve been inside my head! You’ve made 2 really good comments here — each of which deserves an entire blog report to answer. I will address the comment about kinetic energy in a future report. Today, I want to talk about how airguns are designed.
How the Texan was designed
I was not a part of the Texan design team, but I did participate in the early testing of the rifle and did discuss the gun’s design with AirForce engineers at some length. Let’s start with the power.
Establish the power baseline
Dennis Quackenbush has pretty well established where the big bore muzzle energy bar needs to be set with his Outlaw Long Action rifles. To date he’s built and delivered over 1,600 of them. Compare that to the small handfuls of guns that other boutique big bore airgun makers have produced, and it’s easy to see that Quackenbush is the long pole in the tent. His .458 rifles produce around 500 foot-pounds. Yes, there are other big bores that produce even more energy than that, but as I’ve said, they aren’t being made in the numbers that Dennis’ rifles are. So, 500 foot-pounds is the established benchmark.
What about the Korean big bore guns, you ask? Haven’t they sold in even greater numbers than Quackenbush’s rifles? I really don’t know the answer, but I suspect they have. But the Korean guns have several drawbacks. First, they max out around the 200 foot-pound mark, give or take. And second, they are made with 0.451- to .452-inch bores that can only use pistol, bullets (.45 ACP bullets). Those bullets are ideal for 200 foot-pounds, but they’re not heavy enough to achieve the 500 foot-pound level we’re talking about.
Yes, those rifles will kill deer — that isn’t the question. The question is: What power level do American airgunners want from a big bore air rifle? The answer is 500 foot-pounds. If you don’t understand that, then the rest of today’s report will not make any sense.
I guess I’m saying that the energy level of a .458 big bore airgun has been established by convention — in the same way that the .223 Remington/5.56mm cartridge is so widely accepted by American hunters — despite its weak ballistics. Ten million ARs can’t be wrong!
Okay, so with the energy established, the next thing AirForce was after was accuracy. What do you expect from a .458 big bore air rifle? The couch commandos want half-inch groups at 100 yards, while the hunters are willing to settle for 3 inches. But if you’re about to launch a brand new big bore that you’re betting the farm on, you want to attract as many buyers as possible. So, how much accuracy is enough?
This is where the 34-inch barrel comes into play. AirForce tested other barrel lengths, and at close range all of them were acceptably accurate. But at long range, which I’ll now define as 100 yards and beyond, the 34-inch barrel had an edge. They figured that 100 yards would become the new standard, if it isn’t already.
Now, guess what length is the the maximum length that a barrel can be rifled with technology that’s affordable? Time’s up! If you guessed 34 inches, you would be wrong. As it turns out, 34 inches is beyond the capability of most rifling machines. I’m not saying it can’t be done. I’m saying that those companies that make rifle barrels and have hundreds of thousands of dollars invested in their barrel-making machinery can’t do it. Or at least they aren’t doing it.
Figure 80 percent of the barrel makers in the world can’t or won’t produce a 34-inch barrel. I’m not talking about old Zeke Graybeard up in Buzzard Hollow, West Virginia, who makes flintlock barrels that are 48 inches long. Zeke can make 34-inch barrels in his sleep. Takes him about a full day to make one barrel, and he charges $300-500 to do it. When AirForce comes along and wants to buy 500 barrels, old Zeke looks at the calendar and says he probably won’t live long enough to make them all. And AirForce needs those 500 barrels delivered over the next 6 months! So the Zekes of the world are out as far as suppliers of barrels for AirForce are concerned.
Now, Remington can turn out a barrel every 3 minutes on each of their 41 hammer-forging machines. And those machines can accept a mandrel (a hardened steel rod with a negative impression of the inside of the barrel) of up to 26.25 inches in length. You want to buy 500 34-inch .458 barrels from Remington? Fine, just give them $250,000 to install a specially built hammer-forging machine that can accept the longer mandrel, and, oh yes, you’ll have to pay to have that mandrel made, as well. Figure $400,000 to set them up to make your barrels.
Are you seeing where this is going? There just aren’t that many places that can supply 34-inch barrels. You can argue this point all you want and send me links to companies that supply barrel liners — I’m telling you what AirForce just went through to get 34-inch barrels for the Texan.
Okay, let’s say you locate a source of 34-inch barrels. What’s the ideal rifling twist rate for your new rifle? Well, the “books” say a lot of things, but the only way to know for sure is to test them all. So, that’s what AirForce did. I thought a 1:18″ twist would be the best, and they thought it would be either 1:20″ or 1:22″. In the end, the 1:20″ twist seemed to work the best. But it was a compromise of many things.
A faster twist means more rotational friction on the bullet and slower velocities. A slower twist gains velocity but may not stabilize longer, heavier bullets out at longer ranges. What do you do? What if there’s a super-accurate bullet that weighs 300 grains and produces 415 foot-pounds in a 1:22″ twist barrel, but the 1:20″ twist barrel will also handle a 405-grain bullet and get you 500 foot-pounds, while it’s only a little less accurate with the 300-grainer? These are the kinds of decisions AirForce had to make with each design change they made — and there were hundreds of them!
And, really, when you get down to it, tradeoffs are a major part of design work. The first thing is to figure out how to make something work, then you have to figure out the best way to make it work and finally you have to figure out how to produce whatever you came up with.
So, the design is underway, and you have a prototype built. Now, it’s time to test it. Here’s a good question: How do you know during testing whether the person doing the testing is any good, or if he’s more of a hindrance than a help? How do you know the test is being done right? Well, the fact is — you don’t. Your head engineer may also be a lousy shot! You hope not, but that does happen. What you need are many tests by several people. To get that, you need to build several rifles — not just a single prototype. And it goes on and on ad nauseum.
I am going to stop here and make a huge point. Some readers may have thought that all this development work was done on CAD computers by a phalanx of scientific types in white lab coats. In fact — that’s an inside joke among airgun manufacturers! There is some of that, but what you’ve read to this point is by far the more accurate description of how this process unfolds. There ain’t no board of governors that writes up all the performance parameters so you know exactly what has to be built. It’s people like you and me making educated guesses. But the difference is that when these folks guess wrong, whole companies go out of business. So, people who have a record of guessing right are listened to, and the blue-sky dreamers are tuned out.
The other day someone asked me on this blog how far a bullet from a Texan would travel, and I guessed a 405-grain slug might travel between 1,500 and 2,000 yards. Right away, I was criticized by another reader who informed me there was no way a subsonic bullet could possible travel that far.
When I made that “guess,” I had in mind the 1992 Army experiment done at the Yuma Proving Grounds that used millimeter wave radar to track the flight of blackpowder bullets fired from buffalo rifles of the 1870s. A forensic scientist at Yuma had written a paper that claimed it would have been impossible for buffalo hunter Billy Dixon to shoot an Indian off his horse at 1,538 yards (at the second battle of Adobe Walls, June1874), because the 50-90 Sharps rifle he used could not shoot that far.
Well, the Army scientists discovered in this test that subsonic bullets will not only shoot that far, but many times farther! And my prediction of the range the Texan will shoot a 405-grain bullet turned out to be conservative. According to blog reader JimQwerty123, Chairgun software says it will shoot to beyond 2,300 yards.
The point is that I make guesses, but usually they’re based on my experience. Yesterday, I admitted to blog reader Claude that I was wrong about the penetration potential of a BB fired from the Colt Single Action Army revolver, so I’m not always right. But when I’m advising a manufacturer like AirForce on a project, I know how high the stakes are and try very hard to minimize any mistakes. If it’s a question of personal taste, like what I think of black rifles, I tell them my prejudices up front. But when it comes to what will work, and more importantly, what will sell in today’s airgun market, I’m very careful about what I say.
In 2006 I took an idea for a precharged airgun to Crosman. They were not in the precharged airgun business at that time, and in fact they had made some marketing blunders by rebadging certain European precharged guns in their recent past. But I knew they wanted to get into the precharged arena and thought I had the ideal vehicle to do that.
So, I made a PowerPoint presentation to them at the SHOT Show that outlined a precharged rifle I thought they should build. I thought it should fill to a maximum pressure below 2,000 psi to make it easier to fill with a hand pump. And there were a host of other performance parameters I thought it should have.
But I went farther than that. I felt Crosman should use this simple PCP to build their internal manufacturing capability to make PCP airguns in general. Stop buying from other companies and make them right there in-house.
I was met by a lot of skepticism. A 2,000 psi fill — was I out of my mind? Didn’t I realize that a .177 air rifle HAS to shoot 1,000 f.p.s. to be successful? I said that I did realize that, and that my rifle could do that on 2,000 psi air. I knew it could because I owned a USFT rifle that shot heavy .177 Kodiak pellets at more than 900 f.p.s. and got 55 shots on one 1,650 psi charge of air. But they weren’t sure.
A month later they flew me in to New Bloomfield, NY, to discuss the project further. I carried a device in my suitcase that I was going to use to demonstrate the possibility of my idea. It was a hose with an inline step-down regulator that attached to a scuba tank on one end and a Benjamin AS 392T CO2 rifle on the other. I would demonstrate how an air rifle could do all I’d promised right there in their war room (that’s what they called their conference room).
But I didn’t have to. Crosman has an engineer named Ed Schultz, who’s forgotten more about airguns than most engineers will ever know. In the month between our conversations, he’d prototyped two Crosman 2260 rifles — one in .177 and the other in .22 — and had them shooting exactly as I’d promised. Not only was the .177 getting 1,000 f.p.s. — it was also getting more than 20 good shots per fill!
Folks, on that day I was received like Moses pointing the way to the promised land! Every executive at Crosman was now convinced that this was the way to go and the rest of the story — well, that rifle became the Benjamin Discovery.
So — Kevin Wilmeth — do you see what you did? You scratched my itchy spot, and I had to respond. Airguns are designed in a number of different ways. Some are as simple as sitting down with a Chinese airgun factory and selecting models from their catalog, then adding the features you desire. The biggest decisions to be made are the graphics that will go on the outside of the box.
What I just described for you is the real way airguns get designed. Someone has an idea, and a lot of work is done to turn it into something nice. Just be sure you know what “nice” is before you spend the time and money to get there!