Many years ago I went on a search for broad flexibility in ammo selection for a single auto pistol. Flexibility can be critical in times of shortage or because you've traveled into an area in which your preferred caliber isn't available. Some rounds, like 9mm, are broadly available now, but it wasn't always so. It took some impetus to move me into this search. I started this because a mentor told me I should – so I did. The world has since passed me by, but here's the tale on my efforts and a recent shooting excursion update. There really is little cartridge interchangeability in the world of the semi-auto pistol. The reason that some combinations can work is that certain critical dimensions are the same or are closely similar. Jim Cirillo, NYPD veteran and firearms expert recommended the Glock 20 as a foundation for a "convertible auto."

One gun with extra barrels and four different loads -- three different calibers: that's interchangeability.

It's a 10mm pistol – not the most common auto pistol cartridge in the world then or now. The case head of the 10mm is dimensionally close to the .40 Auto and .357 Auto. The cartridge overall length for each round is different, but allegedly not enough to cause problems in the Glock design. The .40 is broadly available because, for many years, it was nearly the most common US police service cartridge around. The .357 was easily had in some locales in 2004 – when I started down this trail. It was cheaply and commonly available in Texas at the time and wasn't exactly rare in Virginia – those states and a few federal agencies being hotbeds of .357 SIG activity in that era. Even now, you stumble across stores that "close out" .357 SIG – if you have a gun in which to shoot it – and manufacturers still make guns chambered for it. The .40 S&W is still broadly available. The reason a .40 S&W pistol can be easily converted to .357 Auto is that the cartridge case dimensions, particularly the case head, are the same. The case head is the part of the cartridge that comes into working contact with the breech face and extractor. Fundamentally, the .357Auto is a .40 necked-down to .357, producing a bottleneck case. In the Glock design, the .357 and .40 magazines are dimensionally the same and only a barrel change is required to change calibers. The 10mm is longer; the gun has a bigger cargo hold and the round has more power. The question was, could the 10mm magazine reliably feed the shorter cartridges and would the 10mm recoil spring system function with the shorter, non-standard cartridges?

Shooting .357 SIG out of a properly barreled Glock 20 large frame pistol is like shooting 115 grain 9mm ball out of a Glock 17 -- very light recoil.

According to Jim Cirillo's experiment (and Bill Jarvis' experience), the answer was yes. Bill Jarvis, of Jarvis Custom, provided barrels to Jim Cirillo and his experience with them was positive. I likewise contact Mr. Jarvis and made arrangements to try a couple of barrels for a police-trade Glock 20 that came through the shop some years ago. The gun was taken from a Florida police department that issued the big 10mm for a few years. The gun selected from the pile was a Generation 3 (there were some Gen. 2s in the group) and still had the original factory plastic sights. The guns all had the NY-1 trigger spring with the standard connector. This makes for a stout ca. 8 pound trigger pull. I didn't bother to change it out. The slide was sent to Jarvis Custom April 16, 2004 – according to my records. The barrels that were provided are shown. They're constructed of 4150 steel hardened to 40-44 RC. They feature conventional rifling and the cartridges are fully supported in the chambers of each barrel.

One gun, three calibers: a 10mm Glock 20 with spare barrels chambered for .40 S&W and .357 SIG.

Now for the update. The gun still has "ball in the bucket" sights. I had a selection of ammo for .40 and .357 but my 10mm supply was the new Federal 180 gr. Trophy Bonded JSP. This robust load is said to yield 1,275 fps – and it was certainly a jolt. I was unable to take the chronograph to test the load, but I'd be surprised if it was significantly less than factory specs. I shot a pair of loads through the Jarvis barrels – the .40 S&W Federal 180 JHP Personal Defense load and the .357 SIG Cor-Bon Performance Match 147 gr FMJ. I fired groups from 25 yards using a pistol rest. The target was the Birchwood Casey 12" x 18" "Shadow" target. It looked like the whole sheet would fit inside the 7-ring of the full-size, 50 yard NRA B-27 target – making it more like the B-29, a reduced size silhouette (nominally 11.5" x 22") used for shooting 50 and 25 yard PPC stages at fifty feet. I fired 5-shot groups. With the 10mm, the "group" was 4 ½" above point of aim and two-inches to the right. The group was 4 ½" without a flyer. Using the .40 S&W/Jarvis – The group was three-inches high and 3" to the right of point of aim. The group was 1 7/8" without the first-shot flier.

The .357 and .40 loads feed and chamber from Glock 10mm magazines.

The .357 SIG/Jarvis put the group closer to point of aim – only an inch over and 1 ¾" to the right of the sights. The group was vertical and nearly four inches in length without the called flyer. Without the two low flyers, the group was 1 ½". I shot on the fifty yard steel targets with 10mm, .40 and .357. I used some old Cor-Bon 140 grain DPX and had no trouble staying on the 11" disk at fifty yards. The Federal Trophy Bonded 10mm hit like a tank and it's not a round you want to shoot groups off a rest with. I believe it'd be a good hunting load. I set up another reduced size B-27 style target and shot the .357 barrel exclusively. The big gun with the plastic front sight filling the rear sight and the NY-1 trigger spring giving about 8 pounds of trigger press did well. The blast was pronounced but the recoil was like a G17 with 115 grain range FMJ ammo. I was still tending right on target, likely a function of grip/trigger reach for my hand. If not for the three rounds barely into the right-side 7-ring, they'd have all been inside the 8-ring. A full-size B-27 at over fifty yards would likely have shown the same results – the barrel is a product improvement. For information on barrels, see Jarvis Custom. -- Rich Grassi Grassi is editor of our companion service, The Tactical Wire (www.thetacticalwire.com)