Monday, March 9, 2009

Derrick's Crosman Adapter for AR Muzzlebrakes

So, I'd bought a couple AR-15 muzzle brakes with the idea of adapting them to fit various air rifles in the pile. AR muzzles are threaded 1/2"-28. I procured a thread cutting die and thought I was in the AR brake business, so to speak. My original intent was (and still is) to thread the airgun barrels and screw the brakes directly to the guns. Then, as always, I got sidetracked--this time by my ever evolving Crosman/CAR 2250 project. One slight hitch, Crosman 22XX barrels are only 7/16" diameter OD-- 1/16"smaller than the threads in the brake. A simple adapter was in order.

I found a perfect piece of aluminum rod in my metal supply box. It was 0.910" OD, already had a 1/4" hole drilled through and an external taper cut on one end.

I chucked it in the 3-jaw and used the tailstock to drill it to 7/16" to fit over the Crosman barrel. I didn't drill it completely though. Just a bit more than 1" deep. Drilling it completely through for the 7/16" barrel would have left less than 1/32" of metal to hold the brake threads. Too thin. I want the barrel to bottom in the adapter, then have about .375" of aluminum with just the .250" hole for the pellet to clear, THEN the threads for the brake. It's that critical .375" section that acts as the supporting structure for the brake.

I then faced the end and started turning down to match the 0.875" OD of the muzzle brake.

A quick test fit with a spare Crosman barrel looks like a go.

Got the OD to within a couple thousandths. No measurement taken. Used a comparison caliper on the muzzle brake and got it to match. Then, flipped the adapter around in the 3-jaw. This is the muzzle end that will get the 1/2" threads for the brake. It gets taken down to a couple thou under 0.500".

Shaving away. Going to cut the 1/2"-28 threads about 3/4" in length. When I was at size, I put a very slight taper on the end to act as a starting guide for the threading die.

One of the Taig accessories is a die holder that mounts to the tailstock. This lets you rather easily cut threads concentric to the centerline of the part in the jaws of the chuck.

Another view. Using a 1" diam. thread cutting die. Typically, round, adjustable dies are used to cut new threads while hex shaped dies are for chasing (cleaning, realigning) existing threads. The die holder is turned by hand with a short tommy bar in the side holes as the headstock is held fast by hand. Lathe power is not switched on.

I then used a small cutter to relieve the last 2 threads so the brake could seat flush. A parting tool is also good for this operation.

Flipped again. Gingerly held by the newly cut threads, I buffed the jaw marks off the adapter.

A quick trip to the drill press. Used the ruler trick to find the center of the curve and drilled though.

I'd better clean this up. Gotta love the way aluminum cuts on the lathe!

The final step. Tapping threads for the setscrew that will hold the adapter to the Crosman barrel. I used an M4 x 0.7mm tap and lots of Rapid Tap cutting oil.

Adapter goes here.

Last look. The muzzle slides inside to about the center of my thumbnail.

Slide on till the barrel bottoms. Snug down the M4 setscrew with a 2mm allen wrench.

Thread on the AR muzzle brake.

There's probably a few blogs in here...

Try as I might, I couldn't help but feel a bit like James Bond when I threaded the brake onto the gun.

Nick wondered how much clearance I had between the adapter and the sunshade. About that much, Nick. About that much. Total luck. Sharp eyed (and long-time) readers will perhaps notice the Carter barrel band.

Say hello to my little friend.


Anonymous said...

What's the ruler trick to find the center of the curve?

derrick38 said...

The ruler trick lets you easily center a drill on a curved surface, like a cylinder. Clamp the workpiece in a drill press vise, with the power off, bring the drill press chuck down to almost let the bit touch the workpiece. Place a thin strip of flat steel --like a machinist's ruler--between the bit and the workpiece. Adjust the workpiece until the ruler is level. When you're on center, the ruler will lay perfectly horizontal (level) when pinched between the tip of the drill bit and the top of the curved surface you want to drill. Remember to use a short, rigid, center drill to stat the hole. A typical drill bit will be too long and will flex off center and miss your target by a substantial amount.

Makun said...

That's kickass! Wish I had precision tools like that so I can complete my custom 2300kt (like the 2250 but better parts and metal breach) mods. Any change you'll be taking orders? If so, how much? Thanks!

derrick38 said...


Sorry, at this point, making stuff for others isn't in the cards for me. I've got a huge pile of projects already on my own list. Hopefully the blog will inspire you to create some cool stuff, too.

Maybe once I retire. Can you wait another 25 years?