I was thinking I saw somewhere you could buy an AR-like upper and use your .45 as the lower, but I don't remember where I saw them. Age...
I have a Mech Tech carbine on one of my Glock 21's and it has been perfect. If you are a Glock hater they make them for 1911s too and I think for the Springfield XD line now or very soon. I really like this (Glock+Mech Tech) combination.
MechTech - 1911
Images https://www.google.com/search?q=mec...kOpPhgwSL84GQDw&ved=0CDAQsAQ&biw=1920&bih=993
The 1911 CCU’s are available in the following calibers:
.45ACP, 10mm, 9mm, 40 Smith & Wesson & 460 Roland
Frame Size Compatibility: The only concern regarding what will fit and what will not fit is the length of the frame's dust cover. There are a few frames which have
dust covers longer than a standard Government model and these may not fit the CCU. All compact frames like the Officers Model will function correctly.
NOTE: Factory Loaded Ammunition Only. MechTech recommends only using good quality factory loaded ammunition in the CCU. The CCU is a blowback operated action and is capable of safely firing ammunition up to +P. DO NOT USE +P+ or 'hotter' loads.
When i was looking at a semi auto Uzi that had a long and a short barrel I was told I could buy it with either, but, to buy both I needed a ClassIII tax stamp. I was told that it was illegal to convert a pistol to a rifle or vice versa with out one.