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Old 08-29-2013, 10:51 PM   #1 (permalink)
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Join Date: Dec 2008
Location: Houston, TX

Making a Hammer7 HH breech from an MR2 fin

Seemed like a good idea. The fin is a little worse for wear, but it is still smooth and straight on the inside. I have a hammerhead set and when I saw woouulf's MR2 backs, they looked long enough to make a breech out of. This lets me use FS rounds directly into the bore without taking a bore step down further in the barrel. Small paint will push out of the magazine (no detent) and double feed, but FS rounds work well. It wasn't easy for me, but maybe one of our machinists could do this with more reasonable effort.

The MR2 back has built in guides. The six grooves in the front are the perfect orientation. Before I received the back, I calculated that the D port was equivalent to a cut of 120 degrees (124.4 actually, but w-e). The grooves give you lines for each side of the D and the center.

I used a 5/8 bit to make the front of the D, but I placed it 0.375 from the front. The wobble of my drill press and the finish out ended up in the right spot. I drilled the bottom holes and used a thin dremel disc to make the D back.

I kept the o-ring for an automag-ish breech fit. The back end of the threads were parted off to make the depth match the Spyder breech (about 0.277" edge to edge). I made the collar to match the shoulder requirement of the original. I made it too tight to ano and had to re-trim it. That was a tough but ugly ano layer I had first. I dremeled out the rest with various bits.

Got it re-ano'ed and red loctited on.

The breech fits the body and magazine well.

The bolt fits well. I hope that helps the efficiency. I can see that play testing with FS rounds is not going to go as fast as regular paint.


Noticed that if I tighten the breech locator screw too much, it causes binding with the bolt.
With the vert loader, the tighter bore works just fine without detents.
Also, the velocity adjuster was designed to be turned in at least three turns so that it overlaps the power tube end. That means I can't use a longer spring without a longer adjuster. I went back to the stock spring alone, but the extra porting I put in the ASA and lighter valve spring seem to be keeping my regulated pressure down to about 500 psi. I must have been losing air at the power tube/adjuster fit.


I removed that tiny lip at the back of the D on the breech that is visible in the picture above. I had the magazine stick a couple of times feeding, and I hope that will fix it. With the paint (marbalizer) I had, I chrono'ed about 290. With the same settings, I chrono'ed FSRs at 320, so that was nice.
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some things are stickier than others

Last edited by Spider!; 10-15-2013 at 01:47 PM.
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Old 10-20-2013, 04:48 AM   #2 (permalink)
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Join Date: Dec 2009
Location: Toronto

this is very cool, great job
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