I have the Air Force conversion unit, since that is the one I used in the service, I wanted it.
It has worked flawlessly, several thousand rounds and full power ss109 in between. I have never had a jam, knock on wood.
Mine is the type with the white plastic mags that look like a 30 rounder, but hold 10 rounds of 22lr.
I have even taught my youngsters with this weapon and used Aguila 22 Colibri rounds, fed one at a time. It wont cycle the action with these, but makes almost no sound and shoots them great. I use Remington gold hps with this setup, and it eats them great. Nice groups at 25 yards.
My weapon is a preban Essential Arms shorty with collapsible stock, 11.5 barrel and long flash hider.
I have never used the M261, but around here, I have had no trouble finding mags for my Air Force unit. I bought 2 at a gun show for $25 and I still see the same guy with them about 3 x's a year at local shows, and he always has them.
Good Luck
P.S. I had one of the first Ceiner conversions for Glocks in my area, and I got lucky with it too, because I very rarely had a problem with it. Maybe 2 jams in 3,000 rounds. I attributed these to Aguila 22lr ammo that shot very dirty. I don't shoot that type anymore.
Remember, a weapon that jams is nothing but a very expensive club.