The utility of handguns is obvious. Compact, durable, reasonably powerful and holding a useful amount of ammunition, handguns have been with us for as long as gunsmiths could craft them. However, there has also always been a desire to make them more useful while not losing the good parts. I don’t believe there is a single military organization through the centuries that hasn’t experimented with handguns-as-compact-carbines. Most failed, some miserably, some with a vestige of dignity.
Famous examples such as the Luger carbines and Artillery Lugers, C96 Mausers and their buttstock-holsters are truly lust-worthy—and ferociously expensive. Even Samuel Colt took to fabricating shoulder stocks for his revolvers. Alas, such approaches fall short for many of us on two details: Adding a stock to a handgun greatly increases its size, and doing so requires paperwork and a federal tax of $200. (More on that in a bit.)
But there is a way to make a handgun a more stable aiming platform and not incur the transfer tax needed to make your handgun into an SBR (Short-Barreled Rifle): the SIG Sauer Adaptive Carbine Platform.
The ACP is a housing into which you install your handgun. Obviously, it accepts SIG handguns, and the Mk25 SIG, in 9mm, was the test gun I used to try out the ACP. However, the SIG engineers were far more clever than that, as making such a housing such that it only accepted SIG handguns would have been short-sighted. No, they made it adaptable, and as a result it will accept any of a long list of handguns.
There is a catch, however, and that is that the handgun has to have an integral accessory rail on it. No rail? No go. OK, let’s take a quick look and get a sense of how this works. The ACP is an aluminum shell, with accessory rails of its own, and if you don’t demonstrate a bit of restraint, you can bolt entirely too much extra gear onto it.
The ACP kit includes a large-ish polymer block, the rail adapter. Take the rail adapter and slide it onto the accessory rail of your handgun (unloaded, of course). Once the rail adapter is on the frame, and you have the rail’s locking slot lined up with a clearance slot on your handgun’s rail, press the locking bar through. That bar locks the rail adapter to your frame. Next, slide the rail adapter (with handgun attached) into the front of the ACP. The shell is slotted and shaped so the rail adapter only fits one way. It doesn’t need force, but it will be a bit snug. Once it’s in, press the bottom locking pin up and you’ve got the handgun held securely in the ACP at the front.
Now take the rear cap and one of the other polymer adapters in the kit. You have two rear adapters, the round one and the flat one. Which do you use? Simple: Is the rear of your handgun—the tang—round or flat? For the SIG Mk 25, with its rounded tang, use the round adapter. If you are locking a flat-tanged Glock (for example) into the ACP, you’d use the flat adapter.
Slide the adapter onto the post of the rear cap, and press the cap onto the ACP until the spring-loaded plunger locks into the top rail.
At the rear of the cap you’ll see an Allen-head screw. That screw adjusts the bite of the rear adapter, the pressure it exerts on the tang. You want to screw it in enough to make the fit snug, but not so much it binds the frame or presses the frame hard enough to alter the point of impact.
On this part you’re going to have to set-and-try and perhaps adjust as you go.
It is a bit fussy, but that is part of the price you have to pay that makes the ACP adaptable. SIG could have made it easier, and a more elegant design, but they could do so only by locking each ACP model to a single handgun. This way you can adapt your ACP to any of a slew of handguns in your safe (or potentially in your safe) instead of just one.
Once it’s installed, you work the controls much the same. The magazine catch, slide stop, safety, etc. all work as they do otherwise. Working the slide, however, is different. Since the ACP has your handgun enclosed in the shell, you can’t reach the slide. No problem. On the side there is an operating handle, familiar to users of the FAL and other rifles. If you need to work the slide, grab the operating handle and yank it back.
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At first, you’ll find the task a bit easier if you also push forward with your shooting hand, but after a bit of practice you will quickly get the hang of getting the job done with just your left. Southpaws, the process will be a bit different. For you, rotate the ACP clockwise, reach over with your right hand and work the operating handle while the magazine is horizontal to the ground. Unless, of course, you take the easy route and reassemble the ACP so that the operating handle is on the right side, the correct side for left-handed shooters. Yes, it is that adaptable.
The op handle bears on the front of the slide but does not reciprocate. So when you fire, the slide cycles inside the ACP but does not cause the operating handle to move, too.
I can see that some of you are still a bit confused at this point. “I’ve enclosed my SIG inside of this aluminum housing, made it heavier and bigger. How does this help me aim?” To start, you put a red dot sight, such as the ultra-compact SIG STS-081, on the top rail. Then you dig through the ACP box and pull out the sling that was in there. (A slight digression: SIG offers the Standard and the Enhanced ACP. The Enhanced includes a red dot sight and sling. With the Standard you’ll have to source your own sling.) Take the QD socket, and plug it into the rear cap of the ACP. Adjust the sling, and put one arm and your head through it so that as you extend your arms out to aim, the sling is a bit short and you tension the bungee as you extend.
That’s how you enhance aiming.
As near as I can tell, the exact invention of using a sling this way is lost to history, but it became well known when it was used by the SAS in the 1970s and 1980s. They used it with the HK MP5K, the stockless version of that ubiquitous SMG, and mostly because they were interested in aiming and firing while wearing gas masks. What makes it useful to us is that while increasing stability and making aiming easier, it avoids the transfer tax and hassle of getting a handgun “papered” as an Short Barreled Rifle.
The sling length that works for you may not work for someone else, and you want to have it adjusted so the last six inches of arm extension are working against the bungee. Too short and you are working hard against the bungee and don’t have attention or strength to work on aiming. Too long and the bungee won’t tension and thus won’t aid your aiming.
I’ll admit I went a bit overboard with the ACP, but in my defense SIG sent all the goodies. Along with the ACP and an Mk25 to go in it, they shipped one of their compact red dot sights, an STL-900L Tactical light and laser, and a CPL-RM Classic pistol laser. The CPL-RM in particular is so compact that I could mount it forward of the red dot sight and not block the field of view. Oh, it showed a bit in the bottom of the tube, but not so much to be a problem. The ACP accepted all those with rail estate left over.
The ACP has a hand stop on the bottom rail to keep your hand from getting too far forward. My mitts are big enough that the hand stop was in the way, and I choked up on the ACP to get a good hold. Be aware that the muzzle of the handgun is deep inside the ACP, and if you get your hand out front, it won’t know to not fire.
Were I going to equip a handgun (SBR or not) with an ACP, I think I’d double down. Using the ACP to turn a handgun into a PDW is cool, but I’d also install a longer barrel, if at all possible. Something long enough to protrude from the ACP just a bit would add velocity and give me a tactile reminder that there’s a barrel up there.