The UD mags are identical in fit & function to the 45 mags, they hold 25 rds of 9mm. They are marked in same loc. as 45s, United Defense Supply Corp, New Haven Connecticut,SPC. They function great in a Thompson w/a 9mm barrel. A few yrs. ago I picked up a 9mm barrel, bolt & the UD mags from long time,been there, done that Frank Galati. Put it together & it ran circles around the 45. No muzzle climb to even notice. You could cut the head off a target without even trying. All good things can't last forever, After being completely wiped out by hurricane Katrina, the best stuff that i had saved on high ground were stolen from my temporary home by some dopeheads who broke in. I found out who it was & the local gestapo would not even bother. They had bigger fish to fry like bleeding FEMA for all they could get. If I come across as sounding a bit bitter about it it is because I AM. I will never forget how shi**y I was treated by LEO . Like OH is that your biggest problem, we don't have have time for things like that. Hide your goodies well, O'bummi's democraps will have the troops out looking B4 you know it.