After a recent podcast with John Paul of JP Rifles, I realized that the material was so popular that my readers might benefit from more of the same. This material was originally printed in the book (and training program) Your Defensive Rifle Training Program.
The short barreled rifle (SBR) has become steadily more prevalent and recently exploded in 
popularity due to the BATF ruling on the SIG arm brace allowing a designated “pistol” AR-type weapon that may be legally shot from the shoulder. I own several myself. They’re fun to shoot, more accurate than you’d expect and pretty handy for some applications. However, would I bet my life on an AR-type weapon that has a barrel much less than thirteen inches? Definitely not. Here’s the why of it and a bit of the applied physics to back it up.
First, remember that the AR-15/M-16 was originally designed as a 20” rifle with what we now call a “rifle” length gas system. This created an ideal port position resulting in optimal timing of the actuation of the operating system and also optimized dwell time of the gas impinging on the bolt/carrier system. The simple fact is that the further we diverge from this original format, the smaller what I like to call the “operational window” of the system becomes.
There is a reason the M-4 configuration has a 14.5” barrel. It is about as far as we can stretch the concept [short-barreled] and still have duty reliability. (Duty reliability: someone’s life depends on it going bang every time.) I know what you’re saying. “But I have a 7.5” shorty that works all the time.” Sure you do. As long as everything is perfect, as in you always use the same type of ammo and the stars are in alignment.
Here’s the real truth of it. The SBR by its very nature requires the gas port to be moved to some compromised position that still allows the rifle to cycle (most of the time). This is a lot more complicated than it sounds.
The closer the port is to the chamber, the sooner the operating system actuates, unlocking and attempting to extract the fired round from the chamber. The fired round needs time to depressurize and “release” the chamber. Remember that the purpose of the brass case is to seal the pressure in the chamber during the ignition cycle and it does that by forcing itself against the walls of the chamber with near perfection. Proper metallurgy in the case allows it to form itself to the chamber with virtually no gas leakage under the tremendous pressure of ignition. It also allows it to assume the shape of any minor irregularities in the chamber such as pits, dings, scratches, reamer marks, fouling and even microscopic anomalies in the chamber. Trust me; there is no “perfect” chamber.
The more imperfections that are present in the chamber, the more difficult it is for the case to release the chamber prior to extraction. The closer the port is to the chamber, the more perfect the chamber must be to allow the case to extract when you’re trying to pry it loose before it’s ready to come loose! That is the essence of the SBR, trying to extract a case that is not ready to be extracted. Even 16” rifles with carbine port positions are prone to extraction failures due to fouled or bad chambers.
Second, the less barrel you have AFTER the port, the less dwell time you have on the operating system. The operating system functions best if it is pressurized gradually, not with a sudden spike. Remember that the 20” barrel with its port position offers the ideal combination of timing and dwell time to reliably cycle the weapon under a wide variety of environmental conditions. It has plenty of barrel ahead of and behind the port.
The shorter the barrel becomes, the further back towards the chamber the port must be moved and the closer it gets to the muzzle. It is the worst of all worlds for a gas-operated system. Now in order to get enough gas into the bolt/carrier group, the port must be enlarged as there is no longer enough dwell time on the operating system due to the lack of barrel between the port and the muzzle. This has the effect of over pressurizing the Bolt Carrier Group (BCG) and is very inefficient at the transfer of kinetic energy to the BCG and may even blow a gas ring seal eventually. Not to mention that it is very abusive to the cam pin and cam pin hole in the bolt itself. This is a common failure mode on SBRs – bolt failure in the cam pin hole. It is common with these short-barreled configurations to see a stuck case with the rim ripped off by the extractor. All the above-mentioned conditions lead to this eventuality.
The SBR operating system actuates before the case has had time to relax and release the chamber. The case has formed itself to a less than perfect chamber. The BCG has started moving to the rear with such force due to the fact that it must be over-pressurized to work at all that it now just rips the rim off the case that has decided NOT to be extracted just yet.
Oh, did I mention about all the people that have injured or blown off fingers with shorty SBRs? This is another dirty little secret of the ultra-short SBR. If you can get a finger in front of a muzzle, sooner or later you will. I let one of my highly experienced team shooters use my 7.5” SBR one time and if he wasn’t wearing a glove, he would have taken most of his left index finger clean off as he had it up next to the muzzle device. His personal shooting style was to point his support hand index finger at the target while gripping the hand guard. I know from my law enforcement contacts that this is not an uncommon occurrence.
OK, now you’re saying that “But I’ve got a piston gun and it solves all of that stuff.” Not so fast, buddy. Piston systems have their own set of issues and I will not even go into that here, but suffice it to say that extreme SBRs based on the AR platform in 5.56 are nice toys and great novelty items, but I’m not betting my life on one if I have a choice.
Does this apply to 9mm or blow back pistol cartridge ARs? No, that is one of the exceptions. A blow back system is not subject to any of the aforementioned issues; you can have pistol cartridge chambered ARs with barrels as short as your pistol and they still work. The 300 BLK is also a departure as it operates at near pistol chamber pressures with a precipitous drop in the pressure curve as the bullets pass the port. This allows for configurations with pistol port positions that will function quite reliably with both super- and subsonic rounds, interchangeably.
The other consideration here is ammunition. Not all ammunition is created equal. and the ultra-short SBR by its very nature will be ammunition sensitive. By that I mean, if they work they will require a round that delivers a pressure curve that is compatible with the compromised port position. I’ve seen some that will literally only function with one particular round.
Over the past few years, we (JP Rifles Inc.) have walked away from sizable orders from law enforcement agencies that think they have to have a 10.5” (or shorter) SBR. This is probably a result of watching too many action shows with movie weapons. Unlike other companies that must take every order no matter if it makes sense or not, we are sufficiently financially secure that we don’t have to. That’s why our credibility is so high. I don’t want to hear that some officer had a rifle go down at a critical moment because it was a poor compromise in configuration.
What we do offer is a 13.5” SBR with our 12.5” modular hand guard system. For the departments that are a little more open-minded, once they test this configuration against the ultra-short SBRs, they see the light. It is still very easy to manage in and out of vehicles and tight confines, but has the full real estate on the hand guard tube for any realistic compliment of accessories. And you will not accidentally get a finger up by the muzzle. They are sub-minute accurate with high grade ammunition and deliver external ballistics very close to an 18” carbine, not to mention a recoil impulse (sight recovery) that is right in there with our race guns. In fact, Hornady 53 grain Vmax SuperPerformance ammunition clocks about 3000 feet per second maximum velocity out of this configuration. Now that’s rifle performance, and isn’t that the idea?
This year, just to prove a point, I shot one of our off-the-rack 13.5” SBR LE patrol rifles at the JP Rocky Mountain 3 Gun World Championship. I had no problem with engagement out to 600 yards and finished high super senior. Sorry for the sales pitch but that’s the price of this little dissertation.
John Paul,
JPE
Pingback: Weekend Knowledge Dump- July 24, 2015 | Active Response Training
Pingback: Anonymous