I can tell you what happened to me a little while back I had listed a MP82 Holmes machine pistol on the Guns for sale, didnt sell it there but did sell for my price and I loaded some 9 by 19 shells with lead bullets to send the dealer a video of it in action, shot the video and emailed him the file and my Grandson came by and wanted to shoot it before it shipped out so I loaded the last 10 rounds I had in it and got ready to video him and the first shot did not fire, you get very goosey when that happens with a full auto so I cleared it and I had gotten a .380 casing in with the 9mm's and it would have been the same with your 9x19 in a 9x21 chamber the .380 was about .060 to short. This would be much better than the 9x21 in a 9x19 chamber. Jeff