Aluminum Breech Fix?

[TTD]

Early last year (before I purchased my own small lathe) I had a fellow make me a repeating breech for an airgun project I was working on. Overall, he did a pretty nice job except for one detail - instead of drilling the 1/2" hole the entire length of breech in one shot, he drilled in from both ends (said he didn't have a long enough bit) and as you probably already guessed, the two holes don't line up perfectly. Barrel section is dead-on straight with dovetail rail so that's good, but bolt section is off by about .015" - .018". I attached a photo for your viewing pleasure showing the offset between bolt/barrel....



Between being super busy at work and having numerous other projects on the go, I tossed the breech on a shelf and pretty much forgot about it until now.

Breech is basically useless in its current form, but rather than scrap it and start over, I'm going to try and fix it if possible. I have a 10" long 1/2" bit here now so I was thinking(?) of boring/drilling the rear section of breech out to 5/8" or so, plugging it with a length of aluminum round bar held in place with high strength retaining compound, then re-drill it with my long bit coming in from muzzle end to keep everything in line. Also going to sleeve the bolt section of breech with some thin-walled brass tubing for a smoother action while cycling the bolt. Then just re-mill the excess aluminum/brass out of the bolt handle and hammer pin slots.

So my question is - is this the right way of fixing the issue or is there a better way? I was just going to go ahead and do it, but thought I better ask someone first who would know better just in case I'm missing something.

Thanks for any advice/tips you can offer!

 

[rgray]

Plugging and redrilling it probably a working solution. The long bit you have will be little help though. Get a gun drill and drill it that way. That or fashion a D bit. There has been much discussion of using D bits on this forum. Try a search to find some threads on them. I have never made or used one. But I have a few gun drills and they work well.

If you want to plug and redrill you will need a straight bore in order to plug it. a boring bar would probably be the way to enlarge/straighten the hole so it would accept a plug or tube.

 

[TTD]

Thanks, Russ! The "problem hole" is actually bored straight, just on a slight angle so I wouldn't think there would be a problem fitting a plug if I decide to go this route?

Although, sitting here staring at the breech I just had another "possible" fix pop into my grey matter which would eliminate the need for a plug and a bunch of extra machining - do you think I could get away with mounting the barrel in breech (barrel is a very snug fit and secured by three 8-32 set screws), insert the other end of barrel in my 3-jaw (breech would sit tight to jaws of chuck) and, providing I take light cuts, just use a boring bar to enlarge/align the rear hole? Then I would just make a new bolt to the correct diameter (which I was going to have to do anyways).


Even though it seems to be quite rigid when I did the mock-up photo above, that's a LOT of material hanging off the end of the chuck though...I'm a bit worried about it flexing the instant that the boring bar made contact.

Any thoughts?

 

[AJB]

Todd,

I am on the low end of experience compared to most of the people who post, so take my suggestion as only a thought and not a recomendation.

If I was going to try to bore the action as your photographs suggest, I would insert a bar in the chuck and turn it to the dimensions of the barrel, then slide the receiver onto this duplicate piece. At least you would know that the receiver is aligned in the lathe properly before you begin to bore.

That is a lot hanging out so I think flex could kill you considering it would be interrupted cuts and an unbalanced piece.

Good luck!

Tony

 

[rgray]

4 jaw chuck would be handy. With out one Tony's point is a good one. Definitely some indicator work well need to be done. The barrel bore should be concentric with the outside diameter of the barrel but that should be checked. Shims can be used under one jaw of the 3 jaw to get it running true. Might be to much flex in the assembly to get a smooth cut with a boring bar but definitely worth a try. You'll want a good sharp tool for sure and light cuts.