The Mech-Tech conversion upper for a Glock 20 can be had in .45 for the Glock 21 in that caliber, and I am told that the .45 MT conversion on a Glock 20 frame will transform it into a .45 carbine, a good deal easier to find ammo for, and in which a Glock 21 25 or 27-round magazine will work.
A .40 S&W conversion barrel for the G20 is available from Lone Wolf, and uses the 10mm Glock parent frame's factory magazines. That makes the Glock 20 both very powerful for a semiauto, gives it the capability of enjoying the extended sight radius and 25+ round capacity of a light pistol-caliber carbine, and would allow it to utilize those 50 billion or so rounds of citizen-killer bullets that Homeland Security has amassed.
I have two very knowledgable pals, one a retired game warden and the other a former Army NCO with previous duties guarding and transporting nuclear warheads and related material; they both picked the G20 though I have not, but I do not think they chose poorly.
Can you say “truck gun” ?
Do you own a GLOCK? A 1911? Well if you do, MechTech Systems of Kalispell, Montana has a C.C.U. for you. That’s Carbine Conversion Unit and it does just that — converts your pistol into a carbine with an ATF-legal 16″ barrel, a stock, and rail space for optics and accessories. It isn’t a firearm, which means it can ship right to your door, all without changing the Federal classification of your handgun. Sounds great on paper, but does it actually work? . . .
http://www.thetruthaboutguns.com/2014/12/jeremy-s/mechtech-c-c-u/