I seem to remember someone makes these and they use grease gun mags... Anybody else?

Olympic Arms

I know the Oly haters will jump in immediately and bash them to no end, but they do make a variety of pistol cal uppers.  They make some new rifles that take glock mags too, the catalog is available on their site.

Okay here it is.   Oly is the ONLY .45acp upper available on a production basis.   If you use an aluminum lower you're stuck with using very expensive Uzi mags.   The .45/.41AE mags hold a whopping 16 rounds and sell for about $75 each.

The other option is buying a Cav arms Mk2 lower with hahn block and heavy buffer.   This will allow you to use unmodified (cheap) M3 greasegun mags.   A bit more initial investment since the lower is $165ish, buffer $80ish and blocks $90ish.  The payoff is the cheap mags.   I've got a bunch of m3 mags just for this.  

My avatar pic is my early configuration M3 mag setup before the buffer issue.   You must use the heavy buffer or the lower will be destroyed.

Wow,.... sounds like I'd have about as much in a .45 AR15 as I would in a UMP'd H&K USC.45.

Dave

Yes, but with much cheaper magazines.

I meant with the grease gun mod...