Installing a breechplug in an off-center barrel

[rickw]

I'm currently working on a .36 cal percussion underhammer pistol.
I got the barrel from Numrich. It's a .36 cal,15/16 across the flats, and the bore is off-center. That doesn't bother me, as I plan on putting the off-center part at the bottom of the finished barrel.What I was wondering if I would have trouble centering the breechplug on the outside of the barrel instead of the bore.The breechplug has 3/4-16 threads on it. I will be cutting the internal threads in the barrel on the lathe.
Should I try to drill the tap drill hole in steps, trying to get it to center, or could I just use the 11/16" tap drill bit?
I'm going to turn the barrel to round along most of the barrel, and I"ll have a octagon-to-round transition.I intend to turn it between centers for that operation, and hold it in the 4-jaw chuck to thread the breechplug.
Any advice anyone could give me would be appreciated.
Thanks,
Rick

[Acer Saccharum]

For a .36, you really don't need more that a 1/2-20 plug. (or even 7/16-20). You'd have to make a special plug with 12/-20 thread.

If you are dead set on such a large plug, then increase drill sizes gradually, by 1/16 increments, say. If you are trying to CENTER the plug on a barrel, and the hole is NOT, then you will have to BORE the hole until the hole enlarges enuff to be centered. THEN switch to drills. Why? Because a drill will follow a pre-existing hole.

[rickw]

Thanks for the quick reply.
The reason I chose such a large breechplug is that I had a 3/4-16 bottoming tap. I'm using a long-tang breechplug, and that size seemed the best fit between barrel size and the tap I had available. But maybe not.
I was afraid that the drill bit would follow the existing hole.Should I use a boring bar to center the hole, and then use the drill bit to drill it to proper depth?
Thanks,
Rick

[David Rase]

Rick,
I can't remember the last time I used a drill for boring the hole in a barrel for the female threads.  I have always dialed in the barrel on the flats in my four jaw chuck and then used a piece of hi-speed steel ground into the shape of a boring bar cutter to open up the hole and deepen it to the length of the breech plug threads.  Once I have the hole to the right diameter I tap the threads and then continue on timing the breech plug until I get it to bottom and shoulder out where I want it to.
Dave   

[Acer Saccharum]

To add to what Dave says, I work on the breechplug FIRST, trim the end of the plug to length I want, face it off nice and smooth, polish it. I also bevel the last few threads at the end to mimic the contour of my tap. If I leave the plug abrupt ended, but the tap has the last two threads beveled, the plug will gall threads before it bottoms out, and tear threads upon extraction.

Then when you're doing the barrel work, your plug is ready to be fit.

If I had two lathes, the sequencing of the work wouldn't matter. I hate like $#*! to have to take the barrel out of the lathe to machine the plug, then put the barrel back in.     Done that often enuff that I know better now.

[rickw]

Thanks for the tips.
I think I will try using a boring bar to open up the hole for the breechplug. I'll also use a smaller breechplug. I'll have to send my present one back, but I'm not in a hurry.
Thanks again!
Rick

[Pete G.]

The shipping to send your current plug back will be just about equal to the cost of a new plug. Keep your current plug for another project.
Stay with this stuff long enough and it seems that you will always end up with another barrel, a spare lock, etc. After a while you will be thinking up projects to use the spare parts. There is no such thing as building just one gun.

[Acer Saccharum]

There is no such thing as building just one gun.

No one told me this in the beginning....... and I refuse to count the years, and the pile of parts.