Interest in .45 ACP conversions has grown by leaps and bounds in the last year due to a number of new options available to builders. Here is a base of information as I know it. Please feel free to correct any bad info and add any info not mentioned.
Olympic Arms is the only manufacturer of a factory built .45 ACP conversion. Their upper receiver is designed to use a standard AR15 lower receiver with modified Uzi .45 ACP magazines. Oly ran out of Uzi mags and there is no current .45 ACP mag that will work with a standard AR15 lower. Oly is planning on releasing a new .45 mag that does fit in a standard AR lower receiver in December 2007.
The Oly .45 upper receiver is a blowback design with the weight of the bolt carrier holding the round in battery when fired. The ejector system in a spring set up mounted in the upper receiver. Any AR15 upper receiver can be used but 3 small holes for the ejector need to be drilled and the ejector installed.
This is the Oly ejector set up installed in a regular AR15 upper receiver.
Here is the print for the holes for the Oly ejector installation. The .125 hole is drilled on the opposite side as well.
The AR45 lower receiver is a three way enterprise between Tango Chaser Tactical, CNCGuns, and Khalan Weaponry. It is a billet lower receiver designed to use unmodified grease gun magazines. The magazine release is in the standard AR15 location and uses standard AR15 lower parts except for the magazine catch assembly. Around 100 have been made and sold to date. Future plans call for a forged lower to reduce cost and a complete AR45 carbine will be available spring 2009.
Bazooka Bros .45 lower receiver is designed to use grease gun magazines and use an Oly .45 upper. The lower receiver is a two piece receiver mating a grease gun magwell to the back half of a standard AR15 lower receiver. Early versions required a new magazine catch slot to be cut into the magazines. The BAZ45 has been on the market for some time and is reported to be very reliable.
Cavalry Arms makes their MKII polymer lower receiver that is widened in the middle of the magwell to accommodate the use of a grease gun mag to work with the Olympic Arms upper. A Hahn magwell filler block is required to hold the grease gun mag in the magwell and uses a lever type mag release. The Cav Arms lower also requires a very heavy buffer like the Slash buffer to prevent damage to the lower receiver from firing the .45 ACP round. The primary drawback to the Cav Arms lower is it's one-piece design. Users can not change the buttstock or the pistol grip.
This shows the Hahn magwell block installed.
LaFrance made a AR15 type carbine that used Thompson SMG magazines. It has been reported that LaFrance sold only to law enforcement and military is no longer in business.
MGI is reported to have a removeable magwell that uses grease gun mags for their CQB weapon system but no release date has been given.
Grease gun mags vs Thompson mags. Grease gun mags are the easiest to use for .45 ACP conversions as no other mods are needed to the Oly upper receivers. Thompson mags require the clearance cuts to the bottom of the bolt carrier and inside the bottom of the upper receiver to clear the magazine. Mag catch slots also need to be cut into the magazines to work with any of the lower receiver systems on the market.
Some arfcomers have machined a standard AR45 lower to accept the Thompson mags with a magwell block but the additional modifications to the upper receiver were still needed.
Grease gun mags are too wide to fit into a standard AR15 lower without extensive modifications to the magazines. Dalphon used to sell such modified mags but is no longer in business.
I don't have a website for Bazooka Bros.
Moderators, please tack this thread as these questions get asked about 2-3 times a week. The .45 conversion will become much more popular as shooter attain knowlwdge as to how to build a .45 set up.