Milspec Challenge

Here’s one that came from Fishbreath. I’m a big AR-15 guy, and he suggested I spec one out as if I was going to issue it. So here goes. An issue AR-151, Parvusimperator-style.

We’ll start with the receivers. Both upper and lower receivers should be forged from 7075-T6 aluminum. Forged to keep it simple. The forging process is cheaper than the billet manufacturing process, and marginally stronger. Win-win. We’ll focus on the lower receiver and its components first.

The trigger group is a safe/semiautomatic/fully-automatic one. The AR-15 trigger group allows for a three-position selector without some serious modifications. The reader might wonder, ‘why not three round burst?’ My response is ‘why bother?’ The three round burst is the latest in a long line of devices designed at the urging of girly-man bean counters to force soldiers to not “waste ammo”. In the 1870s, the US Army didn’t issue the Winchester repeating rifles to the cavalry because the soldiers might waste ammo. Bolt action rifles around the turn of the 20th century had a magazine cut off, because soldiers might waste magazine after magazine of bullets. Both decisions were proven wrong. Discipline must be taught; it cannot be enforced by weapons. You might not always need ammunition, or fully automatic fire, but when you do, you really need it. So safe/semi/auto is the trigger group we’ll use. We’ll also specify an ambidextrous selector switch.

The receiver extension should be 1.14 inches in diameter, and should have six position detents on the underside. This part may also be called the “buffer tube” but it’s better to think of it as an extension to the receiver that gives more room for the bolt carrier to decelerate. It should be held in place by properly-staked castle nut. This will ensure that the castle nut will not back out on it’s own, but can be removed by an armorer if necessary. In terms of buffer internals, we’ll use chrome silicon wire springs (something like the Springco Blue model extra power spring) and an H32 weight buffer.

As for the stock, we’re going for the Crane3 SOCOM-pattern stock. This is heavier and sturdier than the standard collapsible stock, and comes with two waterproof storage compartments. These are perfect for storing batteries for the various issue electrics that will get attached to the carbine, as well as small bottles of lubricant to keep the gun running. It’s a convenient place to store some lubricant to make sure it’s available when needed.

There are a few other gubbins we need to specify. We’ll specify an ambi bolt catch and an ambi mag catch. Installing these in a convenient manner will take some reworking of the lower, but it can be done. Knight’s Armament and several of the fancy billet receiver companies have already done this. I’m not overly particular about most other small parts. Appropriate aluminum pins, etc. are fine. The one other thing to call out is the pistol grip. I hate the A2-type grip, so that’s right out. It’s too small and it has a pointless nubbin in a stupid place. Fortunately, there are lots of alternatives out there, and almost all of them are better. I’m a fan of the TangoDown BG-16 grip, as it fits the hand better and doesn’t have a stupid nubbin on the front in the wrong place, so let’s go with that.

That takes care of the lower. Onto the upper. We’re going to call for a 14.5″ barrel. Why 14.5″? Because it’s a military standard with lots of available data, and it’s shorter and handier than a 20″. This is a general issue carbine, so barrel length is going to be a compromise. Plus, that hot deathray M855A1 ammunition is designed around a 14.5″ barrel. It will have a 1:7 twist rate4 and a lightweight profile. Why a lightweight profile? Because I don’t see a point to anything heavier. A lightweight barrel was perfect for full-auto ambush drills in Vietnam. A lightweight barrel will take something like 900 rounds before it fails, and that’s if you reload as fast as you can and never stop firing. That’s about three times a basic ammo load for the average soldier. This is more than adequate for a select fire carbine. It’s not a support weapon built for sustained automatic fire. The barrel should be made of 4150 chrome molybdenum steel. And yes, Virginia, it will have a chrome lining.

The gas system should be a carbine-length one, or about seven inches from the receiver to the gas block. Again, this is the standard on a 14.5″ barreled gun. The gas block should be a low-profile unit held onto the barrel with a pair of taper pins. This is the most secure method of securing a gas block to a barrel. It should also have a bayonet lug at the front, since the length is right for a bayonet. And bayonet fighting teaches a lot of good things to troops, plus it’s occasionally very useful on the battlefield. Just ask the British. For they bayonet lug to work, the gas block has to be as long as a front sight block, but this isn’t hard to do.

For handguard, we actually want a 9″ long unit, even though our specified gas system is only 7″ long. This will cover most of our low profile gas block, giving us more room for a grip and accessories, but still let us access the bayonet lug. We want a quadrail handguard, because picatinny rails are the standard accessory mount. There are some alternatives out there, namely keymod and mlok, but they don’t totally replace the picatinny rail. We see no reason to adopt two mounting systems, so we’ll stick with the picatinny. I went back and forth quite a bit on which handguard design to call out here. I’m rather fond of the notion of a monolithic design, where the rail and upper are one continuous piece. However, this is generally heavy, and doesn’t let us change the handguard out at all. One of the joys of the AR design is just how modular it is. We’re using a very different handguard design than my father would have found on an AR-15, and we’d like our sons to be able to apply their own wisdom. So let’s skip the monolith, cool though it might be. We want a durable design though, and since we’re covering our gas block, we’ll need something tube-style, i.e. free float. No mall ninjas, we’re not getting a free floated barrel for accuracy benefits. It’s a service carbine. Don’t kid yourself–no one cares about tiny group improvements. We just need it to work with our gas block system. We’ll take the Daniel Defense DDM4 rail, in the 9″ length. This is an off the shelf product, just like everything else on our list. It’s lightweight, sturdy, and easy to install. It will still let us access the bayonet lug, but give us plenty of room for lights and lasers as needed. It also has quick detach sling swivel sockets built in, which is convenient. Saves us a part.

So far, so good. There are a few more notes that we’ll make for the upper. The muzzle device should be a three-pronged-style flash hider, since these seem to work the best. It should also be a suppressor adapter. Since we haven’t specified a suppressor, we won’t specify a muzzle device. But a good example would be the Surefire SF3P, which would work with Surefire’s excellent line of suppressors.

You may have guessed we’ll be using a “flat-top” or “A3-style” upper receiver, and you’d be right. That’s prime optic mounting real estate. And we’ll have the standard folding dust cover on there too. Yawn. Tell me something interesting, you say? Fine. We’re specifying our uppers to not have a forward assist. Why? Because it’s a stupid, useless, protruding, weight-adding piece of junk. Get rid of it. It makes the design simpler. It makes the design lighter. It’s how Stoner originally intended it to be. And stupid Army was wrong to insist on such a device. If you’re in a situation where you need to force the bolt closed, then your gun is FUBAR, and unsafe to use. Any case so screwy that you have to force it into the chamber because the buffer spring won’t do the job has no business in your gun. Yes, you can force a fucked-up, bulged case into an M1903, M1 Garand, or M14, but that doesn’t make it a good idea. Or even a military requirement. The M1918 BAR has no such capability, but troops loved the BAR all the same. Look, when both John Moses Browning and Gene M. Stoner don’t think it’s needed, it’s not needed.

Didn’t see that coming, did you?

Now for the guts of the rifle. But first, the charging handle. Surprise, the stock one is fine. It doesn’t need ambi anything. It’s already ambi. It’s on top of the freaking gun, for crying out loud. Fine as is.

Anyway, the guts. Or, more specifically, the bolt carrier group. We’re going to call out somewhat nonstandard parts here. No, we’re not going to retrofit a piston. There’s already one there. It’s inside the carrier, you dumb idiot. We’re going to use a specialty bolt carrier and bolt, specifically the Lewis Machine and Tool Enhanced Bolt Carrier and Enhanced Bolt. Why? Well, let’s take a look at our system, here. We have a shorter gas system length than Stoner originally specified, so we’re getting more gas pressure than Stoner originally designed around. The M193 ammunition that was originally used had an average chamber pressure of 52,000 psi. But we’re calling for something like M855A1, which takes advantage of more consistent modern powders to increase the average chamber pressure to 63,000 psi, which of course means more gas pressure for the system. Plus, we like suppressors, which means yet more gas pressure. What does all this gas pressure mean? Well, it means we’ve got more velocity on the bolt, which increases the likelihood that the bolt will try to unlock to early, which puts a shear and bending load on the bolt lugs. Surprise, this is bad for bolt life. The Enhanced Bolt Carrier has more vents and a revised cam path to reduce pressure and slow unlocking of the bolt. The revised bolt has a different extractor, using two springs instead of one, which is designed to give longer spring life. The lugs are redesigned for added strength as well. The end result is a longer-lasting system. Plus it’s a slightly not-stock part, so I’m not just picking a rifle off the rack here and calling it good.

So how is the rifle issued? Well, it comes with an issued optic, which we haven’t chosen yet. But there will be an optic. There will be a two-point, quick-adjust sling. More convenient than a standard carry strap type sling that your grandfather had on his M1 Garand. There are three rail panels issued. I prefer the Tango Down units, but the exact model isn’t very important. Just something so that the picatinny rail doesn’t cut your hands up after lots of campaigning. The rifle is also issued with a set of folding back up iron sights. Specifically the Troy industries M4-style front and their standard (non-dioptic) rear sight. The rear sight is “A1 style”, i.e. it’s has two apertures, but is not adjustable for elevation. Nor should it be. It should be durable and simple. It should be zeroed and kept safely in reserve. It should not be complicated. It is a backup sight on a carbine, not iron sights on a National Match rifle. Keep it simple.

The carbine is issued with iron sights and optic fitted and properly zeroed. Since we’re not dumb, our weapons racks are designed to hold the carbines with optics mounted, so soldiers can trust that the carbine they draw from storage has a good zero.

Some of you may be wondering what I would do if you made me get an AR-15 that I couldn’t piece together as above. What if you had to buy something off a shelf, all-up, Parvusimperator? Simple. I’d call Knight’s Armament Corporation, and ask if they give a bulk discount on large carbine orders. So there. Smartass.

1.) I’ll be using AR-15 as the generic term here, so as not to sound like I’m specifying brand or model details. Also, the AR-15 designation came first.
2.) 5.6 oz weight.
3.) NSWC Crane designed, produced by LMT and B5 Systems.
4.) I.e. one complete rotation in seven inches of travel

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