Just an FYI on the 15 rd MecGars vs the 17rd MecGars. The 15 round ones have a normal looking follower and spring and what seems like a perfect factory taper on the top where it transitions from the staggered stack to single round. The 17 round ones have a funky hollow almost like a cap follower and a collapsing into itself spring. This allows the column to be even shorter when collapsed, to squeeze in the 2 extra rounds. BUT the taper seems to have been tweaked a tiny bit, likely because of the funky follower. In addition, the floor plate on the 15 rounder is different than the 17 rounder. The lip for the base plate is normal on the 15 rounder (flanged in) and on the 17, flanged out. For those of you who know about the S&W "+2 adapter" this is why they can't be used with a 17 rd MecGar.
I discover the taper issue, as they both work fine in my S&W 5906 and 6906 pistols, but only the 15 round MecGar would fit in my KelTec P-11. I suspect the KelTec, designed while the 459 pistols were still being made, spec'd the interior grip dimensions to those measurements. Wild guess. I bet if I could talk directly to George Kelgren at KelTec, he would verify my guess. Oddly enough, the Beretta 92 full sized, flush fit 18 round magazine has the correct taper, and has the mag cut out on the other side, so I will be trying to cut new notches in 2 Beretta mags as an experiment. Why, you ask? Because the KelTec adapters for the longer S&W magazines are SLEEVES. They WILL work on an adapted Beretta body, and I can have a 3 fingered grip on a Beretta 92C 15 round magazine for a 15+1 pocket pistol when I use a 6906 to P-11 sleeve. OR, a 5906 sleeve for an 18+1 full sized grip on the 92 full sized flush 18 round magazine.
KelTec P-11s and the early, first generation Sub2000 carbines were made to use S&W double stack magazines. Back in the day, a P-11 was designed to be a back-up pistol for law enforcement carrying a 59xx or 69xx pistol, or earlier, a 5x9 or 669 duty weapon.
I purchased a P-11 because I already had a bunch of 5906 and 6906 mags. I brought up the Beretta mod because 69xx mags aren't available new, but there are at LEAST 2 options for substitutes. One is based on a MecGar KelTec P-11 12 rd magazine with a special inch long plastic spacer/base plate. MecGar is one of the OEM magazine makers for KelTec. This Beretta hackis just a 3rd option, and it will hold 3 extra rounds in the same length, and with the same pinky rest style floor plate.
The MecGar Beretta 92C mag you want is the 15 round, flush fit anti-friction coated MGPB92C15AFC
The MecGar Beretta 92FS 18 round flush fit anti friction coated MGPB9218AFC
Those are MecGar product numbers. You can get them from Greg CoteLLC. Great small business to do business with. The AFC coating was designed for over in the Sand Box in the military M9 pistols.
SO, if you want an original S&W magazine or two for your collection, great. For regular shooting, wear out a 15 round, or if the taper works in your specific pistol, the 17 round flush fit, or the modified 18rd Beretta magazine.
I hope this helps some of you. I know this is long, and sort of covers several sub subjects, but maybe the info will show up when searched multiple ways. I swear I cover the KelTec/S&W magazine info every year or two going back at least a decade. Now I add the Beretta info as an option.
This is all about saving the rare and/or expensive early mags, and wearing out current, common and inexpensive modern production ones.