Archive for the 'AR-15s' Category

MAC continues the BCM AR-15 testing.

I do find his videos very helpful and instructive.

We Are The Mighty.

Ultimately, the decision to shift from 7.62x51mm ammunition to 5.56x45mm came down to simple arithmetic. The smaller rounds weighed less, allowing troops to carry more ammunition into the fight. They also created less recoil, making it easier to level the weapon back onto the target between rounds and making automatic fire easier to manage. Tests showed that troops equipped with smaller 5.56mm rounds could engage targets more efficiently and effectively than those firing larger, heavier bullets.

As they say in Marine Corps rifle teams, the goal is to locate, close with, and destroy the enemy — and the 5.56mm NATO round made troops better at doing precisely that.

I include this not to start another caliber war, but to link up the PDF document, “Rifle Squad Armed With Armed With A Light Weight High Velocity Rifle.”

I had never seen that before and thought readers might find it interesting.

With Chris Costa. I would only find this position comfortable with knee pads.

I found this to be a useful video.  I enjoy and benefit from videos like this where an operation that could be complex is taken apart into its parts and explained thoroughly, with tricks of the trade explained.

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.

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.

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