Master Sergeant Bill Putnam, U.S. Army conducted a world wide search starting in 2002, to find a high capacity magazine that could be used for a .45 ACP conversion on an AR15 rifle that would also fit the narrow mag well on the AR15. Only three magazines were found that were .45 ACP and high capacity. The Thompson SMG, the Reising SMG, and the M3 grease Gun SMG. All from WWII. None of them would fit the AR15 mag well without major modifications to the magazines.

In 2004, while serving in Operation Iraqi Freedom, MSG Putnam had an epiphany. Why not make a new lower receiver that used one of the existing high capacity .45 mags. The first thought was to use the Thompson SMG mag since there were millions of them available and they were very inexpensive.  Problems arose quickly in the R&D stage. The upper receiver and bolt carrier would have to be modified to make it work. R&D continues today on the Thompson mag option though it will be more expensive to produce.

Next MSG Putnam tried the Reising mag. A successful prototype was made using a regular AR lower by squeezing the mag slightly. The main issue with Reising mags is the price. Original only held 20 rds and they were around $100 a piece. 30rd Reising mags were made by Ken Christie but these still ran $85.  Last was the M3 Grease Gun mag. The single feed mag worked best with an Olympic Arms .45 upper receiver. Olympic Arms is still the only factory making this caliber conversion.

MSG Putnam also conceived the removable carry handle with A1 sights vs. the standard A2 rear sight while stationed in Macedonia in 2002. He contacted Justin Halford of CNC Gunsmithing www.cncguns.com after seeing Justin's personal projects on AR15.com. After Justin made a short run of high quality A1 carry handles, MSG Putnam decided to contacted Justin again to see if it would be possible to make a dedicated .45 ACP lower receiver designed to mate with the Olympic Arms .45 upper, while using grease gun mags.

Problems were encountered with how to make a magazine catch that worked.  Justin deserves the credit for the successful design and creation of an unique magazine release that allowed the use of unmodified grease gun magazines.  The new magazine release also preserves the original AR15 manual of arms, by having the magazine release button in the same location as the standard AR15, unlike other conversions on the market where they use an awkward lever on the bottom of the receiver.  By preserving the manual of arms, there's nothing new to learn for the shooter.  These features made the conversion even better as no additional modifications needed to be made to the magazines.  Justin made a few personal conceptual prototypes to test the new magazine release.  After successful testing, the AR45 was born.