The Lee cutter with replaceable case length gages in the center that give exact, repeatable results is pretty slick. If you've got a way to chuck it into a lathe or drill press you can zip through them relatively quickly. Sure don't know what it'd take to get 1.2", but there's probably an existing case length already - even if it's not exactly 45 caliber.

http://www.midwayusa.com/product/476992/lee-case-trimmer-cutter-and-lock-stud

And here's a universal chuck that works with it as well, might be handy in a lathe or drill press.

http://www.midwayusa.com/product/352653/lee-zip-trim-case-trimmer-universal-3-jaw-chuck...

Your post reminded me of my own 45 caliber shotshells. The M15 has a recommended length of 1.8 inches. To trim that many cases would be easiest if you got the RCBS die made for making these shotshells. Push the case into the die and use a hacksaw to cut off the case across the top of the die. The dies are hardened so the saw won't damage them. Finish with the suggested Lee trimmer or whatever you choose. Below is a link to a very interesting page of information on making 45ACP shotshells.

http://avr-developers.com/45shotshell/

A lot of guys cutting .223 brass down to .300 blackout length are rough cutting them with a cheap little Harbor Freight chop saw. Several guys are making aluminum jigs to hold them to the proper length. Final trimming is done after rough cutting. You could just clamp them in the vise or make a simple jig like this one

http://img829.imageshack.us/img829/3170/hpim1075u.jpg

lock the carriage and go.

Or for high volume, a Dillon 1050 with two modified Dillon power trimmers, each removing half. That's the ticket for 300BLK.