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Review: LMT LM8CSW300 Compressor

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This article originally appeared in RECOIL ISSUE 23
Photos by Straight 8

Shooters Can Shave Ounces and Inches With LMT’s Innovative New Stock System

Once you get talking with the gun geeks at LMT, you start to understand the meaning of the acronym. We toss around the term OCD as shorthand for someone who has a bit more interest in arcane details than we do, but in this case, it’s probably not far from its clinical definition. If the first step is acknowledging you have a problem, then we’re OK with saying that, yes, we can get excited about the 1907 Roth-Steyr’s firing mechanism bearing an uncanny resemblance to that found in a Glock. But these guys take it to a whole ’nother level.

Approached by a NATO member country that wanted a compact carbine for undercover security forces who worked in confined spaces, they examined the problem, then presented a solution that used several of their existing products, as well as coming up with a brand-new one. We got an exclusive opportunity to go hands-on with one of the first pre-production units, which coincidentally is one of the guns appearing on this issue’s cover.

In developing the Compressor, LMT took their proven L7SA1 upper receiver and stuffed a 10.5-inch, .300 BLK barrel into it. The receiver itself is a monolithic design, with no joint between the upper and handguard, making for a very strong unit with almost no chance of force being transmitted to the barrel nut from the rail system. It’s what the AR would have been if Eugene Stoner had access to five-axis CNC machines.


The .300 Blackout was chosen due to its versatility and ability to be effectively suppressed, an essential requirement of the end user. Due to their environment, if they’d lit off, say, an unsuppressed short barreled 5.56, communication between team members would have been compromised. Most of them would have been deaf after the first few rounds.

In addition to several advantages, the .300 BLK brings with it a few problems. We’ve messed around with enough examples over the past half-decade to be able to look past the hype and sycophantic fanboy-ism and realize that while it’s a decent compromise that enables the AR-15 to launch heavy, subsonic bullets, it’s not a magic death ray. When setting up an upper, it’s usually necessary to decide whether its primary ammo is going to be super or subsonic, as either longevity or reliability is going to be compromised if the wrong choice is made. Subsonic ammo is usually more reliable when teamed with a pistol length gas system, but firing supersonic ammo in the same setup can result in an upper which is over gassed — problems such as the bolt outrunning the magazine springs can rear their head.


Conversely, an upper using a carbine length gas system (which depending on barrel length is optimum for supersonic ammo) can be very picky about supersonic fodder and may produce stovepipes when fed ammo it doesn’t like. Add to this mixture variations in back pressure when a suppressor is mounted or removed, and you have an engineering nightmare.

The Blackout is typically fueled by a slow pistol powder, which by rifle standards burns much faster than usual. Because pressure spikes closer to the breech than with the original 5.56 round, by the time a bullet passes the usual gas port location, it’s running out of steam. The acid test for any .300 BLK gun is its ability to work with subsonic loads when unsuppressed, and supersonic loads when fitted with a can. These combinations produce the widest range of gas pressures and volumes, presenting a tough hurdle for any manufacturer to overcome.

The Internet would have you believe that a 9-inch barrel is optimum for this caliber, but in our experience, this is not the case. You can gain a bit more performance with a longer tube without producing an ungainly weapon. LMT seems to have reached the same conclusion, as the Compressor uses a 10.5-inch barrel, allowing the gas system to remain pressurized for a longer period of time than the industry-standard option. Offsetting the slightly longer barrel is LMT’s new stock and buffer system, which chops 3 inches off the overall length of the gun.


Despite its flaws, the AR-15 endures as the preeminent American rifle of the 20th and, so far, 21st centuries. RECOIL readers are no doubt familiar with the limitations of the multi-lug bolt, the craps-where-it-eats gas system, goofy charging handle, and tits-on-a-fish forward assist. Perhaps not as well known, but more important from a functionality standpoint, is the action’s short dwell time.

Go ahead and fully retract the BCG on your AR. You’ll no doubt notice that compared to say an AK, there isn’t that much distance between the back of the magazine and front of the bolt. When it comes to reciprocating components, distance equals time, and that ¼ inch or so just doesn’t give the magazine much of it — in order to push another cartridge into position before the bolt runs forward after reaching the end of its stroke. We can slightly change how quickly the BCG returns by adding or subtracting mass.

One of the ways manufacturers have sought to reduce the incidence of bolt over base stoppages has been through increasing the number of sliding weights in the buffer, thereby reducing its tendency to bounce off of the end of the receiver extension. Any change to the AR’s design that fails to account for the action’s short dwell time will negatively impact reliability.

Which brings us to the LMT Compressor’s buffer system. Their engineers managed to shorten up the buffer tube without shortening the BCG’s stroke, by means of chopping down the buffer’s length — and they did this without losing any reciprocating mass or the deadblow effect of the original’s sliding weights. Which we think is pretty trick.

When you break the system down, you quickly reach a forehead-slapping moment and question why you didn’t think of it first. Immediately apparent is the abbreviated SOPMOD buttstock, which slides onto a shrunken, six-position buffer tube. Despite losing inches, there’s no loss of functionality. What’s hidden from the outside is the patented bit of the system, consisting of a tiny aluminum buffer, a flat-wound recoil spring, and a bolt carrier insert. It’s this insert that slides into the open rear end of a regular bolt carrier and houses a sliding tungsten weight. The beauty of the system is that it requires no permanent changes whatsoever to a stock AR-15 upper, and can be retrofitted to any existing carbine. Like we said, forehead-slapping.

Rounds Downrange

Lewis Machine and Tool will be introducing their own suppressor design in 2016, but it wasn’t ready for our range session. So we enlisted the help of SureFire, who sent along one of their SOCOM300 SPS cans. After slipping it over the matching muzzle brake, we slapped in a magazine of 220-grain subs and started hammering steel. So far, so good. Removing the suppressor and remembering to insert earplugs, we then shot it unsuppressed and noticed that brass was ejecting into the same pile that the first mag produced, indicative of both a low back pressure suppressor and a well-tuned gas system.

LMT compressor

The third magazine was loaded with 120-grain supersonic rounds, and we got through about half of it before the RECOIL curse manifested itself. We’re well known for breaking shit (or maybe it’s just that when stuff breaks, we actually tell you about it), and true to form, the prototype carrier insert decided to spontaneously disassemble itself, locking up the bolt. LMT’s representative was quick to jump on a call with their engineering department, and we discovered that several changes had been made to the production version.

In ours, the tungsten weight was retained by a simple threaded cap that had backed out, whereas the cap on newer ones are also secured by high-temperature thread locker and a couple of spot welds. Despite lacking the new improvements, our proto had still managed to last through several range sessions prior to its arrival in Arizona before letting go, leading us to believe the new version should hold up. But as always, we’d like to test a production unit before pronouncing it ready for service.

We still had ammo to burn, so rather than call it a day we installed the upper on a different lower and shot it with both supersonic and subsonic rounds. Because no one wants to leave a range with anything other than brass in their range bag. Apart from a slight problem caused by our reverse Midas touch, the session went as planned and no further problems were encountered.

We’re always conscious that being the first to try out new gear brings with a certain amount of risk, for both writer and manufacturer alike. We think the trade-off is worth it, as you get to see cool kit before anyone else and see potential pitfalls and what’s been done to correct them. In this case, LMT’s stock system is a novel and innovative approach to solve an end user’s problem. The Compressor stock will be available as an aftermarket package that can be fitted to any AR-15, and should bring down the collapsed length of a 14-inch, pinned and welded carbine to 26.5 inches — just above the legal minimum, and equal to many SBRs. What would you rather have: 3 inches of additional barrel or 3 inches of buttstock? Thought so.

LMT Compressor

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