Kanook --- yes , you are right that there were some Thompsons chambered in 10mm , I do not remember if it was for the FBI or not.
One time while I was working at a rifle range , two guys showed up about 15 min. before closeing --- I explained the fee was $25 for the day but that I had planned to do some IPSC practice after we closed to the public and they could stay and shoot by me where I could "watch" them. They were happy to agree and didn't start hauling out their gear till after I had closed the range and everyone else left ----- they then flashed LE badges and asked if they could run a few mags in full auto. I didn't have a problem with it and agreed.
One guy had a folding stock AK and the other guy had a H&K MP5 looking gun --- as I walked by , I noticed the mags were bigger/wider that any I had seen for a 9mm --- I asked " hey , is that a 10mm" ?? Guy said it sure was and I could shoot it if I wanted !!!! Well , it had a 3 rd. burst on it {didn't notice if it was full auto also } but I was able to put a 3 rd. burst with each trigger pull in the chest { A zone } on some IPSC cardboard targets at 50 yards EVERYTIME !!! He let me shoot about 200 rds. thru it as the way he put it , " I didn't pay for the ammo" ----- I want to say they were DEA but not sure.
That H&K was a HELLOF a gun !!! Zip for recoil , very accurate , and 100% reliable.
Moral of the story = it pays to be nice to people at the range !!!!!!