Keep The AR Cycling Happily
I know the way I write must sound like I think the AR-15 gas system is just corrupt engineering… I’ve spent quite a few words here and elsewhere going on about taming it down so the rifle behaves better.
Overview: An “impulse” gas system is regulated only by tube and hole dimensions. More or less, port pressure (never to be confused with chamber pressure) is the amount of gas pressure at the gas port during firing. Port pressure mostly determines the “timing” of system operation. If too much pressure gets in too soon, the system has no choice but to function. Port hole size and port location down the barrel has much to do with this timing.
Carbine-length systems, in particular, can produce problems. Essentially, if too much gets in too soon, the bolt will unlock too soon as the system begins moving the bolt carrier to the rear. Then, the cartridge case gets yanked while it’s still expanded inside the chamber. This creates the “extraction” problems common to carbines (16″ or shorter barrel). It’s not an extraction problem, really, but a timing issue.
Additionally, an overdose of gas creates overly high-bolt carrier velocity going back against the buffer. It can get so high, and again this is most symptomatic in carbines, that the carrier will “bounce” off its rearward stopping point and rebound overly quickly, going back ahead. Sometimes this appears like a “short stroke” or weak function but its cause is actually just the opposite. Overrides (failure to pick up a round from the magazine) and failures to lock back against the bolt catch or stop can result. The real issue is the carrier is outrunning the other part systems, the magazine specifically.
We are in milliseconds with respect to “fast” and “slow.” Virtually all the influential firearms functions, including breaking a shot, are measured in milliseconds.
So, there is help for all this; altered port locations and sizes (only done on custom re-barreling projects) or regulated gas blocks—or both—make big differences. So, too, do heavier-weight carriers. Those resist initial movement for a speck longer, giving internal pressures longer to subside. The easier means are related to the “back part” of the system, which, let’s say, is the buffering apparatus. It can be altered to influence bolt carrier movement, in both directions. The direction that matters most is going back after firing.
With any buffer change, make sure there is enough overrun (gap) between
bolt and bolt stop to ensure bolt stop function. A nickle’s width is the minimum.
By the way, a trick that can help bolt stop breakage problems in big-chassis
rifles is intentionally reducing this gap. Easiest done with the little stick-on
buffer pads from Brownells. Only for competition use though! Don’t cut it
closer than suggested, but stock guns usually have an excessive gap.
If you’re going to tune a spring by cutting coils, count them. Don’t
measure the spring. Rifle “standard” is usually 43, and it’s 37 for
carbines. So, if you want a carbine spring from a rifle spring, count
down six coils and zip it right there. I use a Dremel cutting wheel,
and safety glasses! The side of the wheel can smooth the ends afterward.
Never be afraid to cut coils to get function. Sometimes a lot of mods
to carrier function can require it, but that’s a good thing. Just make
sure the spring is the problem, meaning the cure.
Tools Of The Trade
The buffer itself has a few options on the market. One of the foibles of carbine architecture is the shorter buffer, that’s also a lighter buffer. Increasing buffer weight is effective. Doing this softens carrier movement rearward. The more weight, the harder to push.
Effectively increasing buffer weight, which is a pretty much accurate way to say it, can be done with a different buffer spring. I’ve decided for myself there’s really no such thing as a “standard” spring for rifles or carbines. There are differences in how different examples compress and rebound, and if you measure enough of them you’ll record different lengths. More checks will show, though, that most have the same number of coils. Any more I just trim rifle-length buffer springs to make them work.
David Tubb, umpteen-time NRA High Power Rifle and Long Range Rifle champion, markets a “flat-wire” buffer spring that works very well. It’s made from Chrome Silicon Alloy, a radically better material than music wire. Since the coils are flat, the compressed length or “solid height” of the spring is much shorter than a round-wire spring can attain. That means, if needed, an effectively stronger spring can fit into a carbine buffer tube without hitting solid height and impeding function.
The longer flat-wire spring adds a little extra pressure to aid in keeping the carrier still a tick longer, even though I don’t really see it as an “extra-power” spring. One will increase “in-battery load” about 25 percent. That’s significant.
Another very good product is from ITT (Enidine), The “AR-restor” incorporates a hydraulic component into its buffer. Works like a shock absorber. Works like a charm. Once again, it just slows things down. I’ve been running these in my competition guns for a few years now. It just replaces the standard carbine or rifle buffer (ITT also offers one for “big chassis” rifles like SR-25 or AR-10). Running a rifle with one of these installed is a noticeable difference in feel as well as function.
The only thing I’ve encountered (but only a couple of times) with replacement buffers is making sure there is enough “room” for the bolt stop to function when it all bottoms out. If not, the fix is removing a little material from the back end of the buffer (after determining that it’s not spring length creating the problem), which is easily done since it’s polyurethane.
Any and all of these changes have to be qualified as appropriate for use under one set of conditions. Not all perform the same on different rifles with different ammunition. Make double sure any change in ammunition, especially, is thoroughly tested for function with your setup. Very (seemingly) small changes in pressures, for instance, have big effects, or can.
By Glen Zediker
David Tubb’s CS Flatwire Buffer Spring. This spring will last the
life of your AR-15, and your next AR-15….
200 S. Front St.
Montezuma, IA 50171
(Virtually all parts mentioned are
available from this outlet.)
Superior Shooting Systems Inc.
800 N. 2nd St.
Canadian, TX 79014
ITT Enidine Inc.
7 Centre Dr.
SHAMELESS SELF PROMOTION
Information in this article was adapted from The Competitive AR15: the ultimate technical guide, published by Zediker Publishing. check www.zediker.com or call (662) 473-6107.