September 23, 2010
By Patrick Sweeney
Stag's new Model 8 piston carbine might be the only AR you need.
By Patrick Sweeney
A lot of you know Stag as "the new guy" when it comes to making ARs, but that's not correct. When it comes to making ARs, Stag is actually among the oldest. In fact, it makes a lot of the rifles you see with other names on them. It is an old, experienced and capable manufacturer, and when the demand for a piston became great enough, the folks at Stag decided to investigate.
Now, I have not been at the forefront of the "the AR lacks/needs/requires a piston" crowd. As a professional curmudgeon, I've spent the last couple of years being dragged, kicking and screaming, toward that point of view. As a manufacturer, Stag obviously doesn't have the luxury of being a curmudgeon, but neither was it just going to jump into the pool because everyone else was.
Stag took a close look at the designs out there, and it rejected the designs that were complex, busy and required too much modification of the basic rifle. After all, if you're going to make things better, you usually don't accomplish your goal by shoe-horning a basketful of extra parts into a proven design.
I had an opportunity to see for myself when I visited Stag, which is located in a quiet business district of the general Hartford, Connecticut, metropolis. If you didn't know the address, you could drive right by and not have a clue ARs were being built there. And at the time of my visit, I had no clue Stag was beginning to produce a piston gun.
Inside, I was greeted by a sight that fascinates the mechanical engineer and gunsmith in me: big machines cutting metal. In several areas, the bins were filled with barrels and carriers that differed from the norm.
The piston tube is welded to the gas block, on top of which sits a folding Midwest Industries sight. At the base is a nut that permits gas regulation.
Stag does all the external work on barrel blanks, and it machines the front sight housing castings on standard AR barrels. Stag has its own CNC machines to carve carriers, so it was a simple enough change to make piston-specific carriers instead of the USGI carriers that it usually produces by the truckload. Having spent more than my fair share of time working on ARs, I immediately spotted the fact that I was looking at different parts and started quizzing my hosts.
They told me they'd received so many requests for a piston gun, they finally made room in the production schedule for a run, and since they had already been testing designs it was a simple matter to modify a few production machines for the new model. I was sworn to secrecy since Stag is not a company that tests the waters by announcing a prospective model and then gauging production by the orders that come in.
They fully planned to have as many piston ARs as they needed when the announcement was made. We'll see. Some say the bloom is off the AR rose, but I suspect the depth of interest and market demand is a lot deeper than many have predicted. Even planning ahead, Stag may be caught short with the thousands on hand they planned to have on announcement day.
The Stag piston system is a short-stroke piston, with a spring-returned actuating rod held in place over the barrel. The new front sight block/gas housing is held onto the barrel by means of both tapered pins and set screws.
A vented tube cycles the piston and bleeds off excess gas. The piston has its own return spring so the carrier does not have to shove it forward.
The pins hold the block on; the set screws are for securing accessories such as sling swivels. On that block, Stag has welded a tube for the gas system. The block is bored through, and the front gas system nut is both the gas flow adjustment and the system disassembly point.
The gas tube is cyclically vented. That is, when the piston is forward it blocks the vents and presents a closed system. As the gas pressurizes the system and begins moving the piston, the piston head uncovers the vents, and gas is blown out of the tube. This acts to regulate gas flow. Excess gas will begin to move the piston faster, but as soon as the piston head passes the vents, the gas is blown out, limiting any overworking of the system.
The piston body is smaller in diameter than the front, and the step-down is what holds the piston return spring in place. The rear of the piston is the same diameter as the gas tube on your standard AR, so the upper doesn't require any modifications.
By having a return spring on the piston, the Stag does not require the carrier to both feed and chamber a round, and push the piston back to the forward position. All the carrier energy goes to chambering and bolt lockup.
The carrier has been modified to have a piston thrust-shoulder machined as an integral part and not a bolted-on part. Additionally, the rear of the carrier has been increased in diameter as to negate carrier tilt
One aspect of the AR that early piston designers had to deal with was carrier tilt. If, instead of pushing the carrier back along the centerline as the original direct-impingement system does, you push the carrier on the thrust shoulder, it wants to tilt. Tilting bangs up the inside bottom edge of the buffer tube and is a sign of things not being right.
Stag made the carrier behave. In early testing, Stag figured it would gild the lily and coat the carrier with nickel/Teflon for a maximum wear, easy-to-clean surface treatment. What their designers found in extensive testing, however, was that while the Ni-T plating was harder than sin, it was also brittle and would eventually begin to chip and flake. In the end, Stag settled on good old Parkerizing for the carrier finish.
As for the rest of the rifle, it is your basic top-drawer AR carbine. It sports Midwest Industries folding front and rear sights, 16-inch chrome-lined 1:9 twist barrel, 5.56 chamber, mil-spec forged upper and lower and six-position M4 telescoping stock.
Internally, the lower is box-stock AR/mil-spec parts, and since Stag grinds the sear surfaces of both hammers and triggers itself, the trigger pull is clean and smooth while
being within mil-spec standards.
The upper has been so lightly modified by the addition of the piston system that it would fit and work on any lower, but for right now, Stag plans to offer complete rifles only.
As the Stag Model 8 is brand new, and I had to return the test rifle, I haven't had a chance to see what railed handguards the piston system would clear. However, since the piston design does not ride any higher over the barrel than the stock gas tube does, I imagine that this will clear many if not all of them.
The piston thrust shoulder is an integral part of the bolt carrier, and the rear of the carrier has anti-tilt pads milled into it.
For those of you already contemplating such a modification, I have to point out one detail you'll have to account for besides piston clearance: vent radius. The self-regulating design of the vented gas tube won't do you any good if you clamp it shut with a tight-fitting, railed handguard. So if you are going to install a railed handguard, look closely at the area of the vents. You may have to do a bit of extra surgery for vent clearance.
In shooting the Stag Model 8, I had the usual piston-system experience: A correctly gassed and sprung piston system is a bit softer in recoil than a direct-impingement system. That's not because piston systems are superior but because the AR, as a military design, is over-gassed. A bit more recoil and a slightly shorter service life is a small price to pay as far as the military is concerned--as long as the rifle works all the time, everywhere. I'm sure that if the military does ever adopt a piston system, it will be similarly over-gassed.
The Stag Model 8 is also smooth. In rapid-fire, the sights just hung on the target as I crisply slapped the trigger each time. Feed and function were flawless, regardless of the ammunition I used. For general test firing and drills, I just flipped up the Midwest Industry sights and had at it.
Once I'd determined that the Model 8 was reliable with USGI and Magpul mags (as if there was ever any doubt), I gave it a go with the new magazines from Tapco. They also proved reliable in function.
For accuracy testing, I clamped one of my trusty Leupold scopes in a LaRue mount on the top rail and settled in at the bench for some accuracy work. What I found was that the 1:9 twist of the Model 8 happily shot all weights--from standard 55-grain FMJs to heavyweight bullets--into small clusters. I've found some 1:9 barrels to be not so happy with 75- and 77-grain bullets, but the Stag shot them as well as everything else. All firearms show a preference for some ammo over others. For this Model 8, it was Hornady 60-grain TAP.
When it came time to disassemble the rifle, the true joy of the piston system came to the fore: The receiver, bolt and carrier were relatively clean. I say "relatively" because there is still some powder residue and carbon that gets back into the receiver.
Each time the empty is extracted, the chamber is opened and residual gases can leak into the receiver. Also, the case itself carries gunk back into the receiver, but nothing like that of the direct-impingement system.
The piston system makes cleaning the receivers and their parts a snap, and a little bit of lube gets it back into action. A quick chamber brushing (which is also a lot easier than on a direct-impingement carbine) and the receiver end of things is clean enough for more shooting.
Up front, you'll need a pair of cartridges or a cleaning rod and small punch. Depress the retaining plunger and unscrew the gas piston nut. The nut is drilled through so you can use a bullet or the end of a cleaning rod as a lever to get it started.
The Stag Model 8 comes standard with a collapsible six-position M4 stock.
It may take some work because despite the exhortations of the piston crowd, a piston system simply diverts gas; it does not make it go away. All that gas flows through the gas block, which will be hot, and messy. If you keep it lubed, it will be a smeary mess, and if you don't, it will be a baked-on mess. But a simple, cylindrical piston is a lot easier to clean than a gunked-up carrier, and the gas block itself is a snap to clean, especially if you use an aerosol cleaner.
The big question is this: Does the world need another piston-driven AR? If you had asked me that question not too long ago, my answer would likely have been "It doesn't need any." However, the Stag Model 8 is a clean, elegant design that requires no modification to the base rifle. It works as intended, reliably and without making demands on the shooter.
If, after shooting it enough to wear out the barrel, you decide you are happier with the old direct-impingement system, that won't be a problem. You simply remove the shot-out barrel, install a standard AR direct gas impingement barrel, a new carrier and you're good to go.
My bet is, that by the time you've worn out this barrel (no easy task, as it is chrome-lined) you're going to want another one just as good, and with the same piston system on it. There's only one way to find out: Get shooting.