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Barrel Length vs. Velocity

by G&A Staff   |  October 26th, 2017 0



“How much velocity do I lose if I shorten the barrel on my AR?”

The answer ranges from the precise, if wrong, “You lose 25/50/75 feet per second per inch” to the realistic “It depends on the caliber” to the resigned “I don’t have a clue,” which is at least an honest answer. To actually find out usually requires taking barrels of different lengths and testing them side-by-side. The problem is this: They are different barrels.

I built two identical.38 Super competition 1911s with barrels from the same production lot, both fitted by me. Everything was identical, not just similar. Yet one produced 100 fps more speed, with the same ammo than the other. Rifles are no different.

This test is different, a test of a barrel not just comparable, not just from the same company, but the same barrel. I’m destroying a perfectly good barrel in the name of science. And not just a perfectly good barrel — an expensive one.

The Barrel

The barrel is made by White Oak Armament (WOA), a well-respected barrel maker based in Carlock, Illinois. This 26-inch stainless match tube has a 1:8-inch twist and is notched on the lathe at 1-inch intervals to make it easier to cut off. Let me point out before we proceed any further that this is not something to be done lightly. It is a lot of work. It can be hazardous. And the knowledge gained ends up being pretty darned expensive to come by.

The Rifle In the beginning, the idea was simple: Grab a bolt, check headspace, plug the barrel into a spare upper. Take it to the range with ammo, chrono, Sawzall, power drill and Manson Tooling crowning tool. Do a basic sight-in, and then get to work. Chrono all the ammo, chop the barrel, recrown, and repeat.

I planned to keep going until I was left with a barrel that set fire to the shooting bench or had a bullet sticking out of the barrel stub when chambered. Some of you can see the problem with that. When cut below 16 inches, I’d be making a short-barreled rifle (SBR), usually a legal no-no. I have registered SBR, however, so no problems there. The barrel was installed in a spare Aero Precision upper and placed on my Primary Weapons Systems SBR lower. I didn’t even bother installing a dustcover or forward assist.

A second problem: I could not make it a self-loader, so this test mule AR would be a straight-pull bolt-
action rifle. I used a Beaten Zone charging handle, its Gen 3, with a side handle the size of a coat hook. The side handle is spring-loaded to close, and after a few runs I used duct tape to keep it open. I simply used the charging handle to treat the rifle as a straight-pull bolt action.

The Beaten Zone charging handle, with its spring-loaded extra lever.

The Beaten Zone charging handle, with its spring-loaded extra lever.

The Loads

I stacked an immense pile of ammo for the test, everything from lightweight varmint poppers to heavyweights, 35 to 77 grains. I limited myself to factory ammo, so no 80- or 90-grain bullets that High Power shooters fool around with. I limited myself to what loads I had on hand in enough volume to do all the testing of each in a single production lot. That still meant I would be testing 19 different loads in .223 Rem. and 5.56 NATO.

The Electronics Some may know that I am a serial chrono assassin. Name a brand, and I’ve shot it. During this test I did wing one of them. Yep, I used both my PACT and a new one that arrived just in time for this project, Labradar. It uses radar, not sunlight, and I could have chronoed by moonlight if I had so wished. I used them simultaneously, so if one failed to “see” a bullet, I had a back-up reading. The best part is the Labradar stands beside the muzzle, not out in front of it. You can’t shoot your Labradar.

The Experiment One of the first things I discovered is that a 26-inch barrel is too long for the .223/5.56. Shortening the barrel increased velocity in some instances. This should not come as a surprise. As the bullet moves down the bore, the pressure decreases. However, friction remains a constant. When the accelerating force of the pressure has reduced enough through expansion that it can no longer overcome friction, the bullet will slow down. It is probably even possible to estimate how long of a barrel it would take to stick a bullet in the bore.

The second thing I discovered was that barrels get hot. Snicker if you will, but this hadn’t occurred to me starting out. I mean, I’m shooting what amounts to a bolt action, doing chrono work, not practicing for the rapid-fire stage at Camp Perry. Well, it wasn’t long into the first test that I found the barrel a bit warmish, so I switched to a front-of-the-lower hold for firing. Once I’d finished the ammo for that barrel length, I picked it up to chop off a segment. Yow, that thing’s hot! (Enter blister number one.)

The next thing I discovered is that barrels are made of tough steel. My Ryobi reciprocating saw lasted for three cuts with the cheap hardware store blades. I asked White Oak about this, and it reported back that the barrel was made of 416R Crucible, a primo barrel steel. Being the techno-geek that I am, I had to go look it up. Wow. It’s a hardenable stainless steel with a good dollop of chromium in it, with good machinabilty and the ability to take a smooth finish. I didn’t tell that to the club members who were practically crying when they watched me chop off yet another inch of barrel.


Cutting Concerns

Don’t do this when tired. The downward pressure to cut the barrel translates into a swing toward your leg once the barrel is cut through. I didn’t cut myself, but I can see where someone might, and it wouldn’t be pretty. Another detail about cutting barrels: use a broom. I noticed I had built up quite the pile of shavings and thought, That would make for a killer photo op. So I went to sweep them up with my bare hands. “Ow! Ow! Ow!” Metal shavings as slivers hurt. They are hard to dig out, and they continue to be annoying while you test-fire the rifle.

Heat Mitigation

Next, if you are only casual about controlling temperature, things get crazy. I walked back down to the firing line, having chopped the barrel yet again, loaded up, and the chrono registered velocities some 200 fps faster than the previous set. How hot is that barrel? Yowch! (Enter blister number two.)

All the ammo went into the shade, and the rifle got a sopping-wet towel to rest on between loads. Then I poured water down the bore to cool it after each load. (Interesting data point: If you leave water in the bore, the first shot of the next load will be 150 fps slower than the average of the next four.) After a couple of dousings, swab the water out of the barrel. And be careful in handling the rifle while pouring water. (Enter blister number three.)

On the third day (yes, this took three full days of range work), I brought a bucket and water jugs with me, and simply left the bolt open and dumped the rifle, muzzle first, into the bucket. Then, chamber brush, swab the bore, and back to shooting. It took less than a minute in the bucket to bring the barrel temp down to normal.

The lessons just kept piling up. If you pour water down the bore, you wash lube off the bolt. Working the charging handle becomes a real chore, so you must lube the bolt, but not until after earning a blood blister on the left hand. And the middle finger of the right hand, taking the weight of the rifle on each bolt-pull, gets its own blister over time.

Flash Effect

Then there is the muzzle flash. Even before I got down to the legal minimum of 16 inches I was seeing flash. There’s a good reason for flash hiders, and those of us who were shooting ARs back during the wretched “assault weapons ban” remember all too well how flashy barrels were that lacked them.

When you get down to the really short lengths, the muzzle blast and flash, even on a sunny day, become impressive. They were so ferocious, in fact, that I think I worked myself into a flinch at the shortest lengths. I was basically setting off a flash-bang not much more than a foot in front of my face again and again. From other testing I’ve seen, the pressure in the bore on bullet uncorking with a 7-inch barrel can be as much as 17,000 psi. The muzzle flash is also warm at these short lengths. I could feel the flash as a warm puff of air on my face after each shot.

Since this is a one-time opportunity, I made sure to record all the details, including the lot number of each factory load. What I didn’t do is turn this into an endless ordeal. Ideally, I would want to chrono 10 rounds for the best average and standard deviation numbers. But that would have meant making an already big pile o’ ammo into a pharaonic pyramid of ammo. I also did not check accuracy. That would have made this a yearlong project, and I’ll leave that to someone else.

Knowledge Gained

This is the 32-inch-deep hole Sweeney’s chrono efforts dug into the backstop on the last day.

This is the 32-inch-deep hole Sweeney’s chrono efforts dug into the backstop on the last day.

The first thought many will have is to see what the trajectory changes are when you take a bullet usually launched at 3,100 feet-per-second and instead hurl it downrange at 2,100. How much drop do you get at 200 yards or more? But the real consideration here is the threshold velocity.

Every bullet has a minimum velocity at which it will expand. In the case of most bullets for the AR, the method of terminal effectiveness isn’t expansion but yaw and breakup. For instance, the typical 55 grain FMJ, used in bazillions of .223/5.56 rounds, needs to be going faster than 2,300 fps in order for it to yaw and break. Slower than that, and it is basically a high-speed knitting needle. If your barrel is so short that you can’t generate that kind of speed, even at the muzzle, you are not shooting a .223 anymore. You’re shooting a very loud .22 Hornet. What complicates this problem is that the bullet makers are not all that forthcoming about what the threshold velocity of their bullets is.

One company I got an answer from is Black Hills Ammunition. Jeff Hoffman has done extensive terminal testing in ballistic gelatin, and he reports that the 77-grain OTM with cannelure has a minimum velocity-fragmenting threshold of between 2,100 and 2,200 fps. The results it tell us that you have better have a barrel of at least 10 inches or else your distance-to-fragmentation range will be across a large room. Past that, no fragmentation.

One load that continues to impress is the Federal XM193. It is fashionable in some circles to disparage it, claiming it is not military surplus but military reject. My correspondence with Federal indicates that’s not the case, but if it is rejected ammo, reject it my way. Given a fragmentation threshold of 2,300 fps, the XM193 gives us that much speed out of a barrel longer than 7 inches.

Robert Browning once said, “A man’s reach should exceed his grasp, or what’s a heaven for?” We all like to think we’re good shots and can hit what we’re shooting at. But if the bullet isn’t going to perform“out there,” then we haven’t reached heaven. With the short barrels, the AR turns into a 100-yard rifle. Beyond that, it is only creating knitting-needle-like wounds, and you have to know that. Otherwise, you’ll be disappointed.

Length Matters

That’s one great big take-away from this research. As cool as the shortest barrels may be, they don’t perform much beyond CQB. The 7-inch-barreled ARs (and I have one) are not going to give you the terminal performance you hope for, but they all drill dirt. In the course of test-firing on the last day, I used the same spot as my aiming point and produced a hole in the hill. When I was done, I poked a stick in there out of curiosity. I didn’t feel bottom. I finally found a straight stick long enough to get to the bottom of things. Thirty-two inches is how deep a hole I drilled in one day of chrono work.
So, how many fps do you lose shortening a barrel? It depends on where you started. Going from 26 inches down to 20 inches actually picked up speed, not lost it. In the regular zone, between 20 and 16, the velocity loss could be anything from 12 to 50 fps. When we started down into the SBR zone, we sometimes saw velocity losses of more than 100 fps just from a 1-inch change in length. The short answer to the question of how much velocity do you lose per inch? It depends.

Click on thumbnail below for our velocity table.


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