The P320 in military dress
By Dennis Adler
Blowback action air pistol designs and operation vary from as close to an actual blowback action centerfire pistol as possible, like the Umarex Walther PPK/S, to thoroughly accurate J.M. Browning short-recoil, locked-breech designs, like the Umarex HK USP, and internally contained, frame-mounted firing mechanisms (usually with stick magazines or reversible 8+8 pellet magazines) like the Sig Sauer Max Michel and Spartan 1911 models, or Umarex Beretta PX4 pellet pistol. The latter designs, if they have an open slide and barrel lug interface, only expose the top of the internal firing mechanism, rather than the top of the magazine when the slide goes back or is locked open. This configuration, used in the Sig Sauer Max Michel and Spartan 1911 models, formed the basis for the P320 M17 ASP’s internal design, but the definition of “basis” here is simply that, it was a starting point not the end result.
The M17’s construction allows a lot of looking into openings to see how things fit together and function in unison. You can’t look inside any of the aforementioned Sig CO2 models or even the P320, there’s just not that much to see, although the P320 is close (internally). The M17, however, is an open book; remove the magazine and look up inside the grip and you have a clear view of the internal operating system, the opening for the rotary pellet magazine to fit in between the CO2 valve and the forcing cone at the back of the barrel housing, and the connecting valve that fits into the magazine’s CO2 chamber. All very straightforward in application and totally dependent upon the modular CO2 pellet magazine to operate.
The M17 Magazine
This is the engineering marvel of the M17, a first-of-its-kind self-contained CO2 pellet magazine with a 20-round rotary fed “clip”. That’s the official term Sig Sauer has given to the modular pellet-loading device for the M17 magazine.
Loading the CO2 into the large main section uses a long lever that folds down away from the back of the magazine housing. This is similar in operation to the loading system Sig Sauer developed for the P226 ASP, which had a folding backstrap panel to accomplish the loading and seating of the CO2 cartridge in one motion. With the M17 magazine, once the lever is lowered (as shown) the CO2 is inserted. You then raise the lever and squeeze it closed against the back of the magazine. In one move the CO2 is locked, raised into place, pierced and sealed with the housing that fits into the pistol’s grip frame. This is part 1 of the 2-step loading process.
The pellets load into a compact version of the P320 rotary magazine; once loaded, the clip drops into a channel and locks into place. The CO2 and pellet clip are now one piece within the magazine and ready to load into the M17.
Sig Sauer will have spare magazines on the market before the end of the year and there will be two options, a complete magazine with 20-round pellet clip, or a set of 2 spare 20-round clips for quick reloading of M17 magazines. This is more efficient in terms of using up CO2 in one magazine, and a fine alternative to spending the money for a second complete magazine. For training, at least one spare magazine and clip would be recommended plus a set of extra clips for quick reloading of magazines during practice.
First shots downrange
To wrap up Part 3, I am going to do an initial velocity test with the freshly loaded M17 magazine and Sig Sauer’s own 4.5mm, 5.2 grain alloy wadcutters. A run of 20 rounds at 15 second intervals (to minimize CO2 cooling and a drop in velocity) returned an average velocity of 353 fps, a high of 387 fps, and a low of 344 fps. This is well below the high factory spec of 430 fps, but that figure has an * followed by “Results may vary based upon pellet weight, temperature, and altitude”. Even so, 350 plus feet per second isn’t bad for a blowback action pellet pistol. And it is hard to make comparisons because there are no other pellet pistols that share this design!
In Part 4, I will chronograph a variety of alloy and lead wadcutter pellets in the M17 and run the first 10 meter accuracy test.