Archive for the 'AR-15s' Category

Thoughts On Barrel Twist, Bullet Weight And Precision

BY Herschel Smith
1 week, 2 days ago

Shooting Illustrated had an assessment of the Barnes Precision Carbine.  It seems a nice enough gun, although for the price you’re getting into BCM and Rock River Arms prices.  I do like the looks of the hand guard, though.  But what caught my eye was this chart.

I find this fascinating.  I’m a wonk, I admit.  But even if you don’t find it as interesting as I do, I’d request that you [a] see this first and foremost as a fishing expedition, not a tutorial (because that’s not my job), and [b] keep track of similar data and send to me as you run across it.

So here’s my specific interest.  Technically, MOA is a measure of precision, not accuracy.  Accuracy can be modified based on sight (or optic) adjustments.  If you don’t understand the difference between accuracy and precision, without going through the mathematics of the Central Limit Theorem, you can see this article.  So now you’ll understand why I am using the term precision for this information.

The precision is lower for the 62 grain bullets than for 55 grain or 73 grain.  It would have been nice to see additional testing with Sierra MatchKing 77 grain OTM.  The difference above is nontrivial.

We saw in a previous post (not because I knew this information, but because I know how to find this information) that when the Army tested the M855A1 round, they were using accurized 1:8 twist barrels, not 1:7 twist as per MilSpec.  They got worse precision with the new ammunition with 1:7 twist barrels.  The M856 tracer round is 63.7 grains, and the Army had to show that the barrel could stabilize the round in order to justify the new bullet.

A twist rate that is too high can over-stabilize bullets, leading to “keyholing.”  We know that, and so it’s important not to overdo barrel twist.  It is fairly standard knowledge that use of the 1:7 twist leads to slightly less precision for the 55 grain, and maybe for the 62 grain green tip.  But it manages to stabilize the heavier rounds, including the rounds that are apparently in current use within SpecOps.

This stabilization is necessary because of changes made to the service rifle.  The original M-16 had a much milder twist rate than does the shorter barrels in use today.  The shorter barrels are a direct result of trouble getting into and out of vehicles for dismounted operations, going through buildings and around walls, and the general requirements of MOUT.  In order to make the ammunition work for these shorter barrels, the engineers had to monkey around with twist.

Now I’m to my main point.  I take interest in the fact that the precision is lower for the medium weight bullets.  I’ll stipulate that the variables are many, including perhaps the most important one, barrel harmonics.  I’d love to talk to some of the original engineers and test shooters for the newest Army round, but it’s likely that I’d never get the truth.

But what I can do is compile data of my own.  I’m wondering if this behavior stands up with other twist rates?  I’m also interested in whether barrel length plays a role.  Why did the precision decrease with medium weight bullets, and recover at the lower and higher ends?

At any rate, if you run across any data for 1:9 twist, 1:8 twist, or any more data on 1:7 twist, using different bullet weights, I’d be very interested to learn the precision of the groups and plot for future reference.

Thanks in advance.

Rifle Drills

BY Herschel Smith
2 weeks, 3 days ago

Shooting Illustrated.

I guess we all learned to shoot watching old movies because most folks emulate the actors and bring the rifle down off the shoulder, especially to run a bolt or lever. This practice wastes time and encourages failing to follow through, that is, re-acquiring the sight or the reticle after the shot. We should run the gun from the shoulder and be ready for the next shot as needed, and this takes a bit of practice. You can drill this by doing dry practice then setting up a target at 25 yards and firing a string of 3 to 5 shots, standing, working the gun from the shoulder.

I’m not certain what he’s saying here.  If he’s saying that you must shoot a bolt action rifle differently than an AR, I agree.

If he’s saying that the “plate-forward aggressive” stance for an AR must be corrected, I disagree.  I think his explanation could have used some work.  And I didn’t learn to square up against a target with an AR from watching movies.

Jerry Miculek With The Echo Trigger

BY Herschel Smith
2 weeks, 5 days ago

Do any readers have experience using the “Echo Trigger?”

Select-Fire Visits Daniel Defense

BY Herschel Smith
3 weeks ago

The Use Of Adjustable Gas Blocks To Tame Hot AR-15 Gas Systems

BY Herschel Smith
3 weeks, 1 day ago

This guy doesn’t like the idea.

I do not at all think that an adjustable gas block is the best answer to tame an over-active AR-platform gas system—and that means one that operates too quickly, too forcefully. When that’s at issue, gun parts and cartridge cases get beaten unnecessarily, and, if it’s “bad enough,” function problems result—most commonly ejection and feeding failures. Various barrel length, gas system length and cartridge combinations conspire to create this condition. It’s most common with shorter systems, such as carbines, and also with longer than usual barrels using a rifle-length system.

An adjustable gas block houses valving apparatus that’s user-adjustable. Some vent away or bleed off unwanted gas, others restrict or limit the flow of the gas into and through the system. Those that vent tend to hold up better longer.

Clearly, the idea is to install the adjustable gas block and then turn it “down.” Great idea! It’s easy, effective and it doesn’t last forever.

The valving in the block succumbs to the same heat- and pressure-induced erosion (flame cutting) as does a standard system (and as does the chamber throat and gas port) and will change, and it will get dirty, maybe corroded, and might lose its capacity for adjustment. Gas-induced erosion, by the way, increases space volume and that will effectively reduce pressure, so that means a tuned gun might not work after time unless the valve can be readjusted.

Adjustable gas blocks are best in the hands of competitive shooters, and are most wisely used only in rifle-length gas systems. One can work well to tune out percussion-induced rifle movement and will improve spent case condition. The farther ahead the block is from the rifle chamber, the less intense is the pressure and flame.

A recent article I wrote here discussed the importance and effectiveness of making essential “architectural” changes in the gas system to get closer to the original Stoner rifle spec combination of port location and barrel length (which works just fine, and dandy). However, I will quickly concede that valving might be the only reasonable solution. If we have to work within already-set specs that aren’t nearly ideal, then we may also have to work around that, and there’s the value of the valve.

[ … ]

I turn one all the way down, all the way closed, and then open it up until the gun works, plus a little more to ensure full function. The “full-function” test, by the way, and this is important, is that the bolt carrier assembly will go to lock-back on the last round. It takes a little more carrier velocity to get it back far enough to trip and set the bolt stop than it does only to strip the next round from the magazine. Don’t cut it too close!

I have to say I’m a bit unmoved by his dislike of this particular use of adjustable gas blocks.  Look, the original barrel length in the Stoner rifle is still out there and can be purchased, but it’s much less frequent to see a full assembly for sale.  Mostly, if you want one, you’ll need to order the barrel and replace the one you have, including the gas system.

So we’re dealing with typically shorter barrels now, some as long as 18″, some much shorter (14.5″ with a welded and staked flash hider, or even shorter for AR pistols).  To me, if your gun begins to malfunction and it’s obviously not the barrel or chamber, the first place to look is the BCG, the second is the buffer spring, and the third is the gas system (including the gas block).

Replacement of all gas blocks will be necessary given the second law of thermodynamics.  If gunsmiths or gun mechanics disagree with my assessment, I’m not married to it.  Please disagree, and state the reasons for your disagreement.

Surefire Enhanced Bolt Carrier Group

BY Herschel Smith
3 weeks, 3 days ago

I’d like some gun mechanics and gunsmiths to weigh in, but I’ve never had any of the trouble he’s alluding to.  If something isn’t broken, I tend to favor not fixing it.

This seems unnecessary, but I’ll let the reader decide.

You’re Not Legally Allowed To Sell The Assembled Magazines, Which Is Stupid Because A Monkey Could Put It Back Together Again

BY Herschel Smith
4 weeks, 1 day ago

News from Colorado.

A state law banning the sale and transfer of large-capacity gun magazines has not stopped the sale and transfer of magazines that hold more than 15 rounds of ammunition.

An undercover investigation by 9Wants to Know found examples of gun stores in Colorado either ignoring the law altogether or finding a loophole to get around the law.

“It’s shocking to see that people are doing this,” said state Sen. Rhonda Fields, D-Aurora.

In 2013, a Democratically-controlled state legislature passed four comprehensive bills dealing with guns, including the bill sponsored by Fields banning magazines that hold more than 15 bullets.

The bill, signed into law by then-Gov. John Hickenlooper, banned the sale, transfer and possession of a large-capacity magazine as of July 1, 2013.

Yet, six years later, magazines that hold more than 15 rounds of ammunition are still being sold in Colorado. Legislators thought that they banned them, but gun store employees describe a loophole in the law.

“It’s just poor wording with the law, which benefits us. We can work around it,” said an employee at Centennial Gun Club at 11800 E. Peakview Ave. in Arapahoe County. “It’s a loophole in the wording that they gave that lets us keep selling.”

Our undercover investigation found gun stores selling these “parts kits” in Arapahoe, Douglas, El Paso and Larimer counties. The kits are large-capacity magazines sold in pieces, ready to be assembled after they are purchased.

“This is a 30-round mag; we have to sell it as parts,” said the employee at Iron Horse Armory. “That’s one of Colorado’s retarded laws.”

Brown, a Democrat elected as sheriff in Arapahoe County in 2018, was aware stores were selling parts kits before viewing the undercover video.

“I went in to purchase these in uniform and he took them apart right in front of me. And I said, ‘Well, you don’t have to do that, I’m a law enforcement officer.’ They say, ‘We do this with all of our magazine sales,'” said Brown.

Law enforcement officers are exempt under the 2013 law.

Of course “the only ones” are exempt.  Don’t you just love the shock expressed by the pols?  Shock!

I especially loved the part about taking it apart right in front of the LEO.

Small victories, you know.

And remember, there isn’t anything hard about making a box with a spring in it.

Brownell’s On Straight-Walled Deer Hunting Cartridges For Your AR-15

BY Herschel Smith
4 weeks, 1 day ago

AR-15 Armorer’s Tools

BY Herschel Smith
1 month ago

Gun News Daily has an interesting list of recommended AR-15 armorer’s tools, including Brownell’s armorer’s wrench, and others.

I’ve used a Magpul castle nut wrench before, and it’s very good.  The Starrett pin punch set looks interesting, and especially interesting is the Wheeler Engineering AR-15 armorer’s set.

Do any readers have good recommendations for others, or perhaps having worked with these, do you have preferred tools?  I know that Brownell’s has a full AR-15 armorer’s kit.

AR-15 Gas System Maintenance

BY Herschel Smith
1 month, 1 week ago

Shooting Illustrated.

Direct-impingement (DI) gas tubes—such as those used on most AR platforms—should be checked periodically to ensure the hardware that secures them is present and tightly fitted. If you can access that area, removing the roll pin (or screw) will allow you to inspect the forward end of the gas tube for any cracks where the pin passes through. This is not a frequent check, but it is worth doing anytime you remove a gas tube. Any cracking around the pin hole requires replacement of the tube. Remember to re-pin the gas tube after reinserting it into the block.

Gas tubes benefit from periodic internal cleaning, too. Short, large-diameter AK- and SKS-style gas tubes can be scrubbed out with an appropriately sized brush and solvent, then swabbed dry with a clean, lint-free rag. Ensure that the locking cam that holds your gas tube in place remains fully engaged when assembled and that the tube itself is not badly dented or misshapen.

AR-type tubes can be cleaned out with long, purpose-made pipe cleaners and some bore solvent or a .063- to .076-diameter spring wire. You can do this from inside the upper receiver, eliminating the excuse of not wanting to remove the gas tube. Judging by past comments I have received on this subject previously, the concept of cleaning gas tubes is taboo in some circles. As long as you do not stick something in there that will get stuck (like the end of a cotton swab) cleaning it out is both acceptable and recommended. Use a flashlight if the dark space scares you. Just clean it out and move along, little fella. Check gas-tube ends for damage from moving parts, such as bolt-carrier keys or locking cams. Replace the tube if it has been beaten up in this area.

I’ve never cleaned gas tubes, and I’d like some gunsmiths to weigh in with their experience with this.  Is this really necessary?  The gas velocity in the tube is extremely high.

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