Wednesday, June 26, 2013

M261 dedicated .22 LR upper build Prototype 1

The M261 .22 LR conversion is great for what it was designed for....general familiarization of the weapon platform with accuracy taking a backseat to trigger time and dynamics. With this in mind more and more "designated uppers" as they are called in .22 LR have been manufactured. Some of these are costly and take proprietary parts which may be hard to obtain from time to time. My idea here is to make my own dedicated upper in .22 LR using the M261conversion bolt as a host bolt and a "demilled" A1 upper receiver.  I acquired a pair of these A1 upper receivers cheap and needed to put them to use . I wont say my idea here is original, however I have not seen an example in person and I'm pretty sure this has been done using the M261 bolt before.  I'm starting with a Marlin 60 take off barrel I happen to have a few of in the old parts box. The idea is to use M16A1 upper parts to build a trainer rifle that is more accurate than just the barrel conversion used in the M261.

Since barrel length is not so much of an issue with .22 LR we will go with a 16.25" barrel. I've kind of always liked the looks of the dissipator AR-15 sight radius and shorter barrel length but have never felt the need to have one in .223/5.56 I thought the design could be applied to my project here.

Below is mostly pictures of my yet to be tested prototype. I need to spend another hour or two on it and it will be up and running. I put this all together in about a day...most of that time spent hunting for tools and putting the shop back together as we just had a new garage floor poured.

Shortened barrel with muzzle end turned to fit M16A1 triangle end cap and Front sight.
Bushing needs fitted, final length determined and threaded. 

 Above and below we see the start of what is to be the modifications needed to morph the old Model 60 barrel into what we need it to be for our project.

Barrel being turned to fit M16A1 front sight and end cap

 We started with 1 1/4" mild steel round stock for the barrel bushing (pictured below) Since the shank on the barrel was close to 5/8's I chose to drill the center of the bushing to 5/8's and then fit the dedicated barrel closely to that.

Roughed out barrel bushing ready for final fitting.

Final turning of the bushing collar.

Above we see the bushing being shortened up a little, Here is where my prototype took a wrong turn but more on that further down.

Test fit

Ready for a barrel

Barrel bushing fit is very important as it must be tight since this is where the accuracy comes from. Note on above picture the upper receiver is missing the area under the ejection port, this was bashed in to "demil" the receiver, why? I'm not sure. I have it on good authority you can not bend and weld 7075 alum so I removed this lower section, it should not greatly hinder anything with this particular build.

Barrel press fitted into the bushing, for future designs this should be pinned into place.

Note barrel bushing stick out into upper rec.

The place where this prototype took a wrong turn was in the distance the bushing goes into the upper receiver. I measured wrong somewhere and made the bushing nearly 1/4" too short, what happens is the bolt goes too far forward and binds on the magazines. This will be a easy fix 

Upper mocked up to take a look at the progress. The "Ohio" hand-guards were the only spare ones I had around, they were left over from team shoots.

Threaded for 1/2x28 

Now politicians can worry.......its the dreaded bird cage. 

The flash hider and threaded barrel are really not needed for this particular build but I chose to thread it and put one on as it makes the build look more complete and is a nice finishing touch. The fact that this is a trainer upper also pushed me to put one on as when I'm working with trainee's this will be closer to what they may find "off the shelf" also it protects the muzzle which tends to find its way into the floor from time to time if shooting prone.

Mocked up again with the flash hider installed. Rubber bands make a dandy quick delta ring.

So from here we will need to mill the barrel bushing to accept the M261 bolt, drill and pin the front sight base in. The fix for my mistake can be done one of two ways, either make a whole new bushing which wont be hard or further modify the barrel to extend a little farther into the receiver. either way I will have to pin the front sight base a little farther forward than initially intended as the distance between the delta ring and the end cap would be too short. I plan on making a few more of these uppers once I get all the kinks worked out. Recycling has never been so cool......


Anthony Teodosio said...

It's 70 series aluminum, not 60... You can't weld 70 series.

Wonderwolf said...

I stand corrected, it is in fact 7075 Aluminum. I've had 6061 on the brain so much recently for another possible project it slipped my mind it was 70 series.