The Sig Sauer P320 ASP (left) laid the foundation for future designs with its excellent polymer frame and basic slide design. The next level up is the M17 which adapts the frame, less the now unnecessary removable backstrap panel for inserting CO2. As this view shows, the M17 has significant improvements including a full slide and barrel lug interface for the blowback action slide, and a match to the centerfire model’s black finish removable rear sight optics cut that allows direct mounting of optics to the slide. The optics cut was part of the Army’s MHS requirements. Unfortunately, the CO2 model’s is not removable.
The role of the M17, as both a military pistol and as a blowback action CO2 pellet-firing air pistol, is very different than that of the Sig Sauer P320 models they are based upon. There are noteworthy differences in the centerfire and CO2 models, beginning with safeties. The P320 centerfire pistols do not have them.
The Sig Sauer P320 CO2 model comes in coyote tan similar to the M17’s finish. One of the most arguable features of this blowback action model was adding the mandatory safety to the left side of the frame. This alters the look of the pistol from the centerfire model. Also note the one-piece slide, ejection port and barrel lug, a single casting to reduce manufacturing costs but another gig for the gun’s authentic look.
All problems solved in the looks department with the slightly more expensive 2018 Sig Sauer P320/M17 ASP. The obvious differences include the manual safety matching the MHS requirements for the military version, and the correct profile white dot sights. The centerfire guns have Sig-Lite night sights.
The Sig Sauer P320 CO2 model, in complying with air pistol safety requirements, added a manual safety on the left side of the frame. Sig designers placed it where a manual safety would go, so technically, it is still a training aid, just not for a P320. Overall, placing it where a safety would normally be positioned on a semi-auto, rather than some obscure place on the right side of the frame as some air pistol manufacturers have done, was a better choice. The M17 does have a manual ambidextrous safety, so that issue is eliminated with this new model which is 100 percent accurate to its centerfire counterpart.
Comparing inert and functioning controls between the P320 ASP and M17 ASP (Advanced Sport Pellet) model are the aforementioned manual safety and more importantly the beautifully crafted takedown lever from the P320 now actually works on the M17, which can also be field stripped.
The slide release on the P320 was an accurate but inert component, since the slide did not lock back. This has remained the same on the M17 because even with its advanced CO2 pellet magazine design there is no follower, and the slide on the CO2 model cannot lock open, even manually. The disassembly lever, another well made but inert component of the P320 CO2 model, which doesn’t even a corresponding cutout in the slide for the lever to rotate under, is now a fully functioning take down lever on the M17, which can also be field stripped. And this is where a great deal of the CO2 pellet magazine and its interface with the firing system in the M17 CO2 model are revealed!
The slide on the blowback action M17 still can’t lock open after the last round is fired, it is held open by hand for this photo. Here you can see the opening between the back of the barrel housing, forcing cone, and the firing mechanism in the frame. This is where the pellet magazine fits when the gun is loaded.
When the CO2 pellet magazine is loaded, the top of the rotary magazine rests between the barrel housing’s forcing cone and the firing mechanism (CO2 valve). This is equivalent to the 9mm cartridges in the Sig Sauer center fire pistol’s magazine resting ahead of the striker-firing mechanism of a P320/M17. The air pistol’s firing mechanism rotates the pellet magazine’s belt each time the DAO trigger is pulled bringing a fresh pellet into battery.
Sig Sauer’s design for the M17 air pistol is almost as unique as its approach to the centerfire P320/M17 models. While they are a modular design that allows moving the fire control housing (firing mechanism and trigger which are the actual serial numbered parts of the gun) from one frame to another, the M17 CO2 model is a dedicated 4.5mm pellet firing duty-sized pistol equal in dimensions to the M17 military version with extended capacity magazine. The 9mm magazine holds 21 rounds, the standard capacity magazine 17 rounds. For the CO2 model, the extended capacity magazine holds 20 pellets.
To best understand the basis for the P320/M17 ASP you have to see the components that make up a centerfire P320/M17 model. For all intents and purposes the P320/M17 is the small component at the top housing the firing mechanism and trigger. From a purely legal standpoint, this is the gun; everything else you see is an interchangeable component; the very definition of a modular design. This allows a P320 or for the military an M17 to be reconfigured with different frames, slides and magazines from full size to compact and in three different hand sizes. The M17 ASP is based solely upon the full-size model as a single, complete gun with no interchangeable capability.
The internal design of the M17 is different from traditional blowback action CO2 models with its large integral fire control housing in the frame and a unique recoil spring guide rod design for the pistol’s blowback action and slide operation. You can also see that the M17 CO2 BB magazine is a two-piece design which we will explore in Part 3.
The internal design of the CO2 version is different from most pellet-firing CO2 pistols, and even the Sig Sauer P320 model. Unlike the majority of blowback action CO2 semi-autos the slide for the M17 houses nothing but a locking block and firing pin (or rather the CO2 version of one); the rest of the CO2 action is housed in the frame. And while the M17 CO2 pistol isn’t a modular design, the self-contained CO2 pellet magazine is!
The CO2 action uses an internal hammer (not striker fired like the centerfire pistols) activated by pulling the DAO trigger. When the slide is on the frame, the hammer, which is mounted to the frame’s firing mechanism, fits behind the firing pin. The U-channel in the slide goes over the firing mechanism much like putting the slide on a Walther PPK by slipping it over the barrel, pulling it back past the rear of the frame, lowering it, and sliding it on from the back.
In Part 3 we will look at this innovative magazine design that bridges the gap between separate CO2 and pellet magazines, load CO2, and run our initial velocity tests.