How Effective is an SBR?

Cover image snagged from CountryBoyPrepper on YouTube, here.

The trend in ARs has always been towards shorter and shorter barrels. Ever since the original 20″ version was created by Eugene Stoner, people have been dreaming about chopping that sucker down to a more manageable length. What constitutes ‘manageable’ seems to get shorter and shorter each year, although it seems the community has (temporarily) settled in the 10.5″-13″ range as being ‘best of all worlds’. Some would call it ideal, but I disagree. Ideal would be all the good of all types, but that’s just not possible.

Background

Why Muzzle Velocity Matters

The main concept is this: the .223/5.56 draws its effectiveness from speed.

For the back of the class, kinetic energy = ‘knockdown power’. That’s not entirely right, but it’s close enough for the discussion at hand. Here’s an article about knockdown power I’ve previously written. If you bring up momentum, I will ask you to leave. That’s for another day. Here’s what you need to take away from the equation:

• Double the mass of the bullet, you double the kinetic energy.
• Double the velocity of the bullet, you quadruple the kinetic energy.

Anyone who’s looked at the grain weights of more than one bullet can tell you that 5.56 has very light bullets. Even the heavy for caliber rounds are very light compared to anything else in the shooting world. Since the bullets coming out of an AR don’t have much weight to them, they are very velocity dependent. Now, what aspect of a gun controls bullet velocity? Barrel length. And what is the hallmark feature of a SBR and large frame pistol? That’s right, a short barrel.

Now, if you’re shooting some sort of chungus round, like a .450 Bushmaster or .35 Remington, you’ve basically gained all the potential velocity from the first 8″-12″ of barrel. Everything beyond ~12″ gains you diminishing returns in terms of speed. The .300 Blackout was designed from the ground up as being ideal out of 8″-12″ barrels (depending on the source). But the .223/5.56 is neither of those rounds. The .223/5.56 was created of off the preexisting .222 Remington, a born-and-raised varmint cartridge. The .222’s bullet weights are primarily in the 35 to 50 grain range, which is much lighter than anything available in the .223/5.56.

The bullet weight of the projectile also factors into muzzle velocity (MV) , and therefore has a double-effect on kinetic energy (KE). As the weight increases, KE increases, but the extra weight reduces MV, reducing KE. Which aspect exerts more influence over the KE of the projectile once it’s fired is determined by barrel length. Longer barrels let heavier bullets come up to speed, shorter barrels exasperate the problems of heavier bullets.

Weird how this whole ‘barrel length’ thing keeps coming up. I wonder if it’s important…

How does Barrel Length Affect SBR Effectiveness

A bullet’s affect on a target, known as ‘Terminal Ballistics‘, is a science unto itself, but the simple version is this: Bullets damage flesh by coming into contact with tissues and transferring more kinetic energy into those tissues than they can withstand structurally. More contact = more energy that can be transferred, and more energy transfer = more damage. So KE and surface area are important. We’ve already discussed how to increase KE, but what about surface area (SA)? The obvious answer is to shoot a bigger diameter bullet, but bigger bullets have their own issues that have to be dealt with.

If you aren’t willing to increase bullet diameter, then you have to get the bullet to get bigger after firing. Sounds dumb, but that’s exactly what the current defensive ammo industry does. The two ways to do that are to make ammo that deforms and ammo that fragments. For more info on this, see this article about bullet construction and why it matters. For the purposes of this discussion, just know that AR ammo primarily uses the deformation mechanism. Which finally leads us to the topic of today’s post:

Is A SBR/Large-Frame Pistol Effective for Defensive Use?

Before we get into this, shot placement is the most important single determining factor. A .22LR to the brain stem will stop a threat faster than a .50 BMG to the foot. Everything beyond this point will be under the assumption that the shot placement is not effected by the barrel length or bullet weight. How barrel length affects a person’s ability to get on target quickly is beyond the scope of this work.

Here are the things we need to keep in mind:

1. 5.56/.223 bullets damage through fragmentation inside tissues
2. Fragmentation is velocity dependent
3. Heavier bullet weights reduce muzzle velocity
4. Short barrels reduce muzzle velocity

How Velocity Determines Fragmentation

From some criminally obscure work by AR15.com members Tatjana Von E and Derek W. F. (source 1), we can say that fragmentation for M193 occurs reliably only above 2600 FPS, with the rates of fragmentation dropping off substantially as velocity decreases. We know conceptually that speed = good, but now we have real world numbers we can look at. And that number is 2,600 FPS. For 55 gr bullets, anyway. Unfortunately, the testing did not include heavier bullets. It would be great to know if heavier bullets need more/less velocity to fragment.

How Barrel Length Determines Projectile Velocity

Once more, I’m standing on the shoulders of giants. This time my source is from Rifleshooter.com (source 2). These absolute madlads have taken full length rifle barrels and test the muzzle velocity, cut an inch off, test again, repeat, all the way down to way too short lengths. The dudes deserve a lot of credit. Not only have they done it with .223 from 26″ to 6″, but also with a .308 barrel, a 7mm magnum barrel, and a .300 Win Mag barrel. Oh, and they also tested different bullet weights each time, only further increasing the usefulness of the results. In the .223 Rem they tested M193 NATO (55gr), UMC .223 spec (55gr), M855 NATO “green tip” (62gr), and Black Hills .223 68gr Heavy Match.

I supplemented their data with the ballistic calculator over at Gundata.org (super useful, check it out), and am able to create a table of terminally effective ranges based on barrel length when used with 55gr bullets. I’d love to be able to expand this table to include 62gr and 68gr projectiles, but we must do what we can with what we’re given. I purposefully set the muzzle velocity as the measured velocity for a 16.5″ barrel from Rifleshooter’s test.

First observation: The range at which the 16.5″ barrel goes below the recommended velocity threshold of 2600 FPS is just after 160 yards. I’m calling this distance “Terminally Effective Distance”, or TED. That’s a little further than I expected, honestly. So, combining the above two sets of data, I compiled the following table of data. The two columns I calculated are the “=16″ @” and the “est. 2600 FPS Distance”. The first column is the distance a bullet from a 16″ barrel has to travel to slow down to match the muzzle velocity of that row’s barrel length. Example: A 16.5″ ‘s bullet travels 40 yards before slowing down to what a 14″ barrel even starts at. The second column estimates at what distance a bullet from a given barrel length drops below that 2600 FPS mark. For instance, since the 14″ barrel’s muzzle velocity is equal to a 16.5″ at 40 yards, then the 14″ ‘s TED is about 120 yards.

Barrel length	Muzzle Velocity (FPS)	~16.5″ @ (yds)	est. T.E.D. (yds)
16.5″	3187	0	167
14.5″	3067 **	32	135
14″	3039	40	127
13″	2974	58	109
12″	2919	73	94
11″	2834	98	69
10″	2767	117	50
9″	2662	149	18
8″	2520	192	N/A
7″	2410	226*	N/A
6″	2202	291*	N/A

All interpolation calculations were made using the interpolation calculator here. I mention that because it’s what I used to estimate the muzzle velocity of a pinned & welded 14.5″ barrel (marked with a **), which I’m not a big fan of. You, dear Reader, can also use that calculator to estimate these values for any arbitrary barrel length you can think of, like a 13.5″, 10.3″, 12.7″, whatever you can find. The last two values are marked with an * because they may not be very accurate. They extend beyond the data set I got from Gundata, and the interpolation got a little squirrelly.

What To Do With This Info

Well the main thing I see from the table above is just how worthless super short barrels are. The velocity and KE (and therefore, TED) drop off at an ever-increasing rate as you chop more and more barrel off. There is no uniform FPS lost per inch of barrel lost, every inch lost loses more than the inch before it. So, every inch you can keep on a barrel will make a significant improvement.

After seeing this, recommending anything under 10″ will be hard. I would also not worry about practicing much beyond the TED calculated for the barrel length you shoot. Granted, most self defense engagements are within 10 or 12 yards, so anything above an 8″ barrel should be fine. And that’s only if you plan on using 55gr as your defensive load (which I’ve always liked doing).

Expanding on that idea, I would recommend people get rifles with the longest barrel length they can justify. For most guns, that will be a 16″ barrel, which having a reliable fragmentation distance to almost 170 yards, is looking very capable. If you are very concerned about the gun’s length, take a good look at bullpups. They preserve that oh-so-valuable barrel length while still reducing the weapon’s overall length. In today’s market, there are several good options, even for left-handed shooters. If you absolutely have to have an SBR/pistol AR, be sure the shorter barrel is worth it.

What Not To Do With This Info

As noted in a previous section, what we can actually draw conclusions from is pretty limited. Performance from anything that isn’t a 55 gr FMJ cannot be accurately predicted with the data I’ve presented. A 62gr green tip with steel penetrator may act completely different, and I don’t know if it would be better or worse (my money’s on worse). A 77gr open tip match bullet will definitely act different. An expanding hunting round or barrier defeating LE round will both act differently than anything previously mentioned.

Hopefully the team here at Pro-Gun Millennial will be able to fill in some of those gaps in the future.

There are people out there who shoot SBRs at long distance to prove a point about the capabilities of the rifle. Nothing I put forth here has to do with accuracy, so don’t think that they or I are at odds. My whole goal is to examine what kind of terminal effect those bullets have once they reach their terminus

And, slow or not, don’t think that a non-fragmenting round won’t hurt you. It is still completely lethal. A slowed down .223/5.56 may not expand or fragment, but it’ll still put a hole completely through a person. Chuck Pressburg once said that muzzle velocity doesn’t matter when you put 15 rounds into a dude’s upper thoracic (paraphrasing). The main difference is the margin of error and how much more important shot placement becomes.

A well performing round is not necessary to survive a lethal force encounter, but it does stack the odds in your favor.

Final Thoughts

The AR-15 is a annoying gun to try and shrink down. The barrel needs to be long so it can accelerate it’s projectiles to effective speeds. The stock has to be long and cannot fold due to the buffer system contained within (excluding the LAW folder and others like it). The beefed up varmint cartridge it shoots and the in-line recoil system, which are hallmarks of the gun at this point, make it a real pain to shorten. Those design features also make the gun great to shoot.

Stay Dangerous, and I’ll see you next Friday. -S_S

Sources are linked below.

1. https://www.ar15.com/ammo/project/Ballistic_Gel_Experiments/BTAmmoLabsTest1/Test1.html
2. https://rifleshooter.com/2015/12/223-remington-5-56mm-nato-barrel-length-and-velocity-26-inches-to-6-inches/