Sig Sauer 556xi Adaptive Tactical Carbine
Tom McHale 01.04.16
“Be adaptive and be tactical” seems to be the number-one design goal of the Sig Sauer rifle family, and the now discontinued Sig Sauer 556xi in particular. If you really want to distill the raison d’etre down further, focus on the word adaptive. It’s kind of a shame that these rifles have gone the way of so many other cool firearms, but progress marches on.
Was the Sig Sauer 556xi Discontinued?
Replaced by the newer MCX line, the 556xi rifles were a combination of the most desirable features from the AR, AK, and Swiss 550 rifle designs. As such, they were piston-operated, so you get those pros and cons right out of the box. 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. See below for more on that. The new MCX rifles are much closer to a traditional AR platform, going along with the current market.
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.
Before I get into more wordy bits, here are the hard specs from Sig:
- Caliber: 5.56x45mm
- Action: Three-position adjustable, long-stroke gas piston with rotating bolt
- Overall length: 35.8 inches
- Barrel length: 16 inches
- Rate of twist: 1:7 inches
- Weight: 7.1 pounds unloaded
- MSRP: $1,466 to $1,599
Was the 556xi a Good Rifle?
To answer quickly, yes it definitely was a good rifle. After all, people are still looking for information on these rifles. Don’t believe me? You’re reading this, aren’t you?
Let’s do a deep dive into 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 lock and release lever. The charging handle can be swapped between the left- and right-hand side of the bolt carrier. Regardless of your setup, the gun always ejects out of the port on the right.
Like a standard AR-type rifle, the steel upper and aluminum lower receivers mate using two captive push pins. Pop the rear one off the receiver’s 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 buttstock 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.
Up front, the default model shown here comes with a slim polymer handguard. I love the minimal profile and especially the “memory bump” up front that keeps your hand from sliding forward onto a hot barrel. Clever idea there.
The 16-inch barrel has a 1:7-inch twist rate for the 5.56 model, while the 7.63x39mm 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 35.8 inches long and weighs 7.1 pounds unloaded.
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.
Is the Trigger Any Good on a 556xi?
The trigger is definitely a utilitarian “combat” type. At the range when I was shooting groups for accuracy, the pressure felt heavy and measured about 8.5 pounds according to my Timney Triggers Tension Scale (every gun owner needs one of these!). 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-nut 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 heart’s content. I really grew to like this feature the more I tinkered with it.
Does the 556xi come with 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 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.
Being an adaptive rifle, if you wanted to top it with an optic, one good bet is an EOTECH HHS II Holographic Hybrid Sight. This setup gives you the EXPS2-2 holographic site and the G33 Magnifier. Holographic sites are great. They’re better than a standard red dot, because the “dot” appears projected out in front of the rifle, therefore it doesn’t move with your angle like a red dot will. The flip-out G33 adds 3X magnification for longer distance shooting. This setup is not cheap, but you need good optics, and this system will allow you to adapt to the style of shooting you’re doing.
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.
For me, the most practical application for this might be portability. With the stock folded, the Sig 556xi fits perfectly into my Blackhawk! Discreet Carry Case, with a mag in place, and no one is the wiser.
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.
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 kit contains a modular suppressor setup that can be used on everything from a .22 LR pistol to a .300 Win Mag rifle and everything in-between, 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.
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. I’m itching to try one of the Sig Sauer SRD556 suppressors on this rifle if I can hold on to it long enough.
How Accurate is a 556xi?
I’ve generally found that direct impingement rifles are often just a hair more accurate than piston designs, but only by a hair if even that. 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 applications.
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 rest 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.
| Ammo | Group diameter in inches, five shots at 100 yards |
| American Eagle AR223 55-grain FMJ | 2.11 |
| Hornady Match 75-grain HPBT | 1.60 |
| Black Hills Barnes TSX 50-grain | 1.73 |
Can I still Buy a Sig 556xi?
There is a decent used market for them, and the prices are starting to reflect the market demand. After all, a Sig Sauer will always be a desired rifle. You’re not going to find these used for $150 any time soon, and even then it’ll probably be a fake.
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 encouraged. Sig used to have a website that showed all the things these rifles could do, but it, too, has gone away. Very few of the modular rifles have remained fully available, which is a shame, but understandable. In the rush to build a better mouse trap, the popular winner turned out to be the original in the AR-platform rifles. Who knows how that will go with the current political climate, which shifts more often than firearm tastes for sure.
Tom McHale is the author of the Insanely Practical Guides book series that guides new and experienced shooters alike in a fun, approachable, and practical way. His books are available in print and eBook format on Amazon.
