I had this wild hair for awhile about using .45 GAP in a Sten mag and using a KKF Sten magwell block. I couldn't find anyone willing to make an AR15 barrel in .45 GAP. No problem, I got a Thompson submachine gun barrel and milled the end down so the chamber would fit a .45 GAP. I had a custom barrel extension made to fit the Thompson barrel to an Oly Arms .45 upper receiver. I spread the lips on a Sten mag to the same dimensions as a grease gun mag and DAMN if it didn't work. Feeds slicker than snot. I can get 17 rounds of .45 GAP into a 32 round Sten mag. Definitely needs a mag loader to get all 17 rounds in there.

I used 200gr JHP rounds. I have a sneaky suspicion that 185gr Truncated Cone .45 ACP rounds might fit as they are very close to the lentgh of the GAP round.

It still needs a free float HG and my Aimpoint will work until I figure something else out for sights. There is enough material on the barrel at the front of the HGs to mill it to fit a regular AR front sight base but that would add to the cost of my set up.

A regular non-milled Thompson barrel works with .45 ACP for anyone wanting to make a cheaper upper than Oly sells.

Cost:
Gun Parts Corp .45 Thompson 16 1/2" barrel - $98
Custom barrel extension - $90
Oly Arms .45 bolt assy - $132
Oly Arms pistol caliber ejector assy - $10
Sten mags - $5.00 each (no cost if you already have a Sten 9mm setup)
KKF Magwell Block - $65 (no cost if you already have a Sten 9mm setup)

Total cost for conversion parts - $400. Prices will vary depending on vendor but a hell of alot cheaper than $594 from Oly Arms.

Some of the pictures are out of focus cause my camera sucks but you can get the idea.















Edited to add cost.
Edited again cause I forgot about the cost of the mag and the magwell adapter.

That is very nifty..nicely done!!

That is really nifty.

What type of velocity are you getting out of these with the barrel?  How is the precision of that set up?

I don't have the ability at present to test velocity. Plus it's been around 3 degrees here every day. I still have to drill the barrel extension for the index pin but it is rock solid. I'll get out and test for accuracy when the weather is good and the range is clear.

Forgive me, but whats the advantage of having shorter cartridges? Do they feed better than a .45 acp upper? Is it a better magazine fit?

The advantage of a shorter cartridge is it can be fired out of a 9mm hand gun or in this case, use 9mm magazines that are abundant. The main problem with .45 ACP is the lack of a decent magazine that will fit a standard lower receiver. The .45 GAP, 200gr round is ballistically identical to a .45 ACP, 200gr round, so there is no loss in ballistics but has a larger permanent wound cavity than a 9mm.

If you want to shoot .45 in your AR15 but can't find decent mags, change your barrel to .45 GAP and use 9mm mags and have at it.

And no one else has done it.

How many .45 mags could you have purchased for the $400?

There are no good, high capacity, .45 mags that work in a regular AR magwell. The only mags currently available are the 10 round Oly mags. Why would I want to buy more expensive, crappy mags?

Oly wants $65 for a 10 round Uzi mag with a mag block welded on the back. Dalphon is out of business and they used highly modified 30 round grease gun mags with a mag well block on the back (can't use a mag loader).

The idea was for a 9mm AR owner to just change the bolt, the barrel, and open up the feed lips on a Sten mag and now he has a .45 GAP set up.

I had to spend $400 because I did not have a 9mm set up to start with.

When I first saw your topic I wanted to reply Why bother... too expensive for what you will get... then I see you already did it....  I would love to know how well those Sten mags work with your project and if this proves to be reliable...

This is for you ---->>>  

I cannibalized my Oly .45 upper to test fire it. It fed, fired, and ejected 10 rounds out of a Sten mag. Definitely needs a Sten mag loader, the spring is stout. I did see a lot of varience in the Sten mags. I bought 5 and none are the same dimensions. One is so narrow the entire mag passes through the magwell.

If you have an Oly/Sten set up, all that is needed is the barrel, a free float hand guard (the barrel can be machined for a AR front sight base at extra cost and use regular AR HGs), and to open the feed lips on the Sten mags.

Next phase -

pin barrel to barrel extension (wasn't pinned for test firing)
order a flat top upper
order oly ejector system
order a free float fore end
get front sight that installs on FF HG (or just use my Aimpoint)
get rear sight of some type

I'll update this as I get it set up as a stand alone upper receiver.

Great work.  It would be a great solution to use slightly modified Sten mags.

I did like others have and modified the Sten mags by taking the groove out of the back side.  It takes about an hour to do each one, including modifying the feed lips.  I load 16 to 18  rounds of 230 grain Winchester White box loads or reloads and they run like gang busters.  Modify a few $5 mags vs. the effort you went through..............


Again, great work, and a different approach.