Sig Sauer 556xi Adaptive Tactical Carbine

This article originally appeared at OutdoorHub.

One of the features I really liked about the Sig Sauer 556xi was the front hand guard.

The key words here are “adaptive tactical” as that seems to be the number one design goal of the Sig Sauer xi family. If you really want to distill the raison d’etre down further, focus on the word adaptive.

These rifles are a combination of the most desirable features from the AR, AK and Swiss 550 rifle designs. As such, they’re piston operated, so you get those pros and cons right out of the box. Well, the only potential cons I can conjure are perhaps a little extra weight, a couple more parts, and maybe, just maybe an impact on accuracy. While we’re on that topic, yes, I generally find that direct impingement rifles are often just a hair more accurate than piston designs, but only by a hair if even that. And from a practical perspective, the difference is irrelevant. Whether a gun shoots 1.2 or 1.4-inch groups with standard ball ammo at 100 yards doesn’t matter a hill of beans when you start thinking about real and practical application.

Anyway, the idea behind the xi family is to build modular systems around a serialized upper receiver. That’s right, the lower is just a part that the ATF doesn’t particularly care about. Switch out anything else you want including barrel, bolt, lower, stock, forend, and whatever else you can think of to your heart’s content. For example, you can switch between AR and AK style lower receivers on the same serialized upper receiver. Cool.

A quick tour

The rifle discussed here is the 5.56 model with the appropriate lower. If you’re familiar with earlier Sig 556 rifles, you’ll notice a new, more rounded pistol grip, but you can use the old style too. Controls are fully ambidextrous except the bolt charging handle and lock / release lever. The charging handle and lever are both on the left side while as the ejection port is on the right.

With the exception of the bolt charging handle and release,controls are ambidextrous.

Like a standard AR-type rifle, the steel upper and aluminum lower receivers mate using two captive push pins. Pop the rear one of the receivers hinge for cleaning and basic maintenance. Pop them both and the two parts separate. While we’re talking about this area, you’ll see a reversible and rotating single point swivel attachment between the butt stock and upper rear of the receiver.

The butt stock on the standard model is fixed length and foldable. It’s got a snap in cheek riser to provide improved cheek positioning if you’re using taller optics.

I really like the included flip up sights – they’re plenty durable for permanent use.

Up front, the default model shown here comes with a slim polymer hand guard. I love the slim profile and especially the “memory bump” up front that keeps your hand from sliding forward onto a hot barrel. Clever idea there.

The front sight post adjusts for elevation and is well protected against dings.

The 16” barrel has a 1:7-inch twist rate for the 5.56 model, while the 7.63x39mm Russian variant has a slower 1:9 twist. This 5.56 model will handle big 77-grain bullets without trouble, and I’ve not yet run into any issues related to overstabilization of lighter weight bullets in the 1:7 barrels.

In the “standard” configuration, this rifle measures 38.5 inches long and weighs 7.1 pounds unloaded.

The cheek riser panel is a snap on or off component.

Got all that? Now forget it. Remember the adaptive part and take a look at all the things you can do to the upper receiver components.

Trigger

The trigger is definitely a utilitarian combat type trigger. At the range when I was shooting groups for accuracy, the pressure felt heavy and measured about 8 ½ pounds according to my Timney Triggers gauge. The pull was smooth, but took some pressure, and I had to focus to get good groups. This is just fine, however, and actually desirable. Here’s why. On these rifles, Sig Sauer uses a plunger to control the second stage pull weight. It’s that little post and but you’ll see extending from the pistol grip right into the rear of the trigger face. You’ll notice when you press the trigger all the way, it activates the plunger, pushing it inside of the pistol grip body. This plunger does nothing to break the sear – that’s all handled by the trigger itself. To make a long story short, you can adjust this weight by adjusting the piston nuts. Want a heavy trigger press like the default for combat use? No problem. Want to lighten it up for competition or general range fun? No problem there either. Tweak it to your hearts content. I really grew to like this feature the more I tinkered with it.

Note the piston behind the trigger. That regulates second stage pressure.

Sights

The iron sights included on the standard model are plenty stout for permanent use. Yeah, we get spoiled by all the fancy holographics and scopes, but in reasonably lit conditions, iron sights are still pretty darn effective. These are folding and appear to be made of steel, at least according to the magnetized bottle opener stuck to the side of my fridge. Pop pop them up with a finger, and they lock into place. Large unlock buttons on each allow them to be placed back down in the storage position. The operate positively and are easy to work with gloves or even mittens on your hands. Elevation adjustments are done with the front post while windage adjustments only are handled by the rear sight.

Shooting the Sig Sauer 556xi

Since I don’t kick down doors for a living, I don’t have a “serious” application for shooting this rifle with the stock in the folded position, but it’s interesting that you can. Since it’s a piston-operated design, there’s no technical need for that solid buffer tube behind the receiver, so Sig includes a fixed-length folding stock. A large hinge button on the left joint breaks the lock free, so the stock rotates to the right side and friction locks into place along the side of the receiver. The folded stock covers the right side magazine release button and safety lever, but the ejection port and trigger are still perfectly accessible as the rifle is designed to shoot in this configuration. Yeah, it was fun to do.

It’ll shoot with the stock folded and locked into place.

For me, the most practical application for this might be portability. With the stock folded, the Sig 556xi fits perfectly into my Blackhawk! Diversion Racquet Bag, magazine loaded and all. You can carry it back and forth to your home, car or range with no turned heads.

In my few weeks with this rifle, I found reliability to be a non-issue, which was expected from a gun like this. I ran about a dozen varieties of .223 Remington and 5.56 through it with no hiccups. I never did get it filthy enough to require use of the “adverse conditions” gas setting – everything ran on the standard lower gas default. Just know, that if things get foul, you can rotate the gas port 45 degrees and basically double the gas going through the system to make this rifle run regardless of gunk level.

The standard three-prong flash hider.

I did notice the ejection pattern was interesting. Spent cases were lobbed fairly high in the air in a two o’clock direction.

Just for fun, I tried it out with a Griffin Armament Optimus suppressor I had in at the time thanks to my friends at Silencer Shop. This Lego block kit contains a modular suppressor setup that can be used on everything from a .22LR pistol to a .300 Win Mag rifle, and all in between, for the most part, provided the projectile is less than .355 inches in diameter. I set it up with the Griffin three-lug muzzle device on the Sig, so I could pop the Optimus on and off at will without tools. Fun stuff. With the piston operation, functionality was perfect, gas in the face was minimized, and most of the blast of the loud .223 Remington was mitigated. I’d keep a suppressor on the Sig 556xi all the time. Because why not? I’m itching to try one of the Sig Sauer SRD556 suppressors on this rifle if I can hold on to it long enough.

Using different three-lug muzzle devices, I used a Griffin Armament Optimus suppressor on these two Sig rifles – a 556R (bottom) and 556xi (top).

Accuracy

Because inquiring minds want to know, I shot a pile of five-shot groups from 100 yards to see how this rifle would shoot. Each number shown below is an average of multiple groups. I used my very stable Blackhawk! Sportster Titan III rifle rests from a permanent bench. For this, to eliminate any optical sighting error, I used a Hawke Optics Sidewinder Tactical IR 10x fixed mil-dot scope. Here’s what I found.

Closing arguments

This rifle is going to get me in trouble. I constantly find myself tinkering with reconfiguration of guns that aren’t even designed to be modular. In this case, it’s not only designed, but encourage. Take a look at the SigEvolution website to get a full appreciation of what you can do to these.