Question regarding tool for breeching barrels.

[Rolf]

I'm getting ready to breech a 14"long, caliber 0.62 smoothbore pistolbarrel. I have access to a large good quality lath. I have bought a 43/64 tap drill, 3/4-16 plug tap and  matching die.

I plan to drill and tap the barrel in the lath. After looking at the tap drill and tap, two things occurred to me.

1. The tap drill will make a shoulder that's on angel in regards to the bore (not 90 degrees).

2.The plug tap won't thread all the way to the breech "shoulder".

How do you get the breech plug to fit snug against the breech shoulder??



Best regards
Rolf

[D. Taylor Sapergia]

I don't have any more equipment than you do Rolf, so what I'd do is go ahead and drill your breech, then grind the end of the drill flat, and finish off the seat in the barrel with that.  Tap the barrel with your plug tap, and when it has bottomed, grind the end of it off very slowly until you have treads like you want.  Then, again very carefully, run the tap in a little at a time to bottom those threads.  I have purchased extra taps so that I can do the work on other projects without regrinding all the time.
When I grind off the tap, I leave the slightest taper, so that the tap's thread is not so fragile, and dress the plug as it nears the bottom, to match the thread I've cut.  The plug must bottom against the inside shoulder, so there's nowhere for fouling to accumulate or for the patch to grab when you clean the thing.  And the threads must be as full of steel as possible to prevent gas cutting and barrel pressure in a thinned section.  There are others on this site that can give far better instruction that I - but that's the way I do it.

[snowdragon]

I've done it the same way as D. Taylor, and I have done it where I talked a machinist into cutting the recess (minor diameter) with a cnc mill.  I have two 3/4-16 taps, one of which I ground flat for a bottoming tap. I run the tapered tap in as far as I can, then use the homemade bottoming tap to thread to the shoulder.

And like previously mentioned, you need to be really careful with the ground off tap, those front teeth take a lot of stress. I did break off a tooth once, but just re-ground it flat again. Use a good cutting oil, and cut a little at a time.  I always chase factory breech threads with the bottoming tap, so it comes in handy even with a pre-threaded breech. Bill

[Dennis Glazener]

This from a non-machinist I did the same thing as Taylor but not wanting to grind forever to get the tap flat on the bottom, I put the tap in my lathe, strapped my Dremel tool on the tool post ala tool post grinder and ground the tap down to bore size. Ended up with a piloted bottoming tap using a couple of Dremel cutoff wheels (two works better than a single wheel).
Dennis

[ironwolf]

  Good idea Dennis.  To Taylors point of "Full" threads,  Most tap/drill charts are set up for a 75% thread engagement.  Consider this when choosing drill sizes.

  KW

[Rolf]

Runar sent a couple of pictures and a description on how he breeches his barrels.
He does not grind his drill flat, but keeps the drill angel on the "breech shoulder".

The pictures show how he grinds his taps. Taylor, is this what you mean by tapering?





He then files the breech plug so it matches the angel on the breech shoulder. Runar is interested in comments and thoughts on his way of breeching.

Best regards
ROlf

[westbj2]

Depending on the amount of material you are removing with the 43/64 drill, remember that the drill will result in a hole slightly larger than the nominal dimension of the drill body.  Likely something in the .003"-.005" range.

Because you have access to a lathe here is a  better option.  First, determine which class of thread your tap is designed to cut.  Second, check in a resource such as Machinery Handbook to determine recommended hole diameter for that thread size and classification.
Then put barrel in the lathe and indicate it to center.  Using an internal boring bar instead of the drill will result in an accurate hole diameter and at the same time a 90 degree bottom at the bore intersection.  Your tap will have a centering hole at the end...use a center pilot on the tail stock to align the tap and start threading with the lead on the tap and finish by grinding your tap as noted by others.   Hoping for the best with a drill and a tap is OK for a plumbing job but the sequence noted above will result in a superior fit of the plug/breech threads.
Jim Westberg