Breeching decissions

[Scota4570]

I am making an under hammer pistol.  The dimensions have stacked up to a non optimal situation.  Formerly I have screwed the nipple in the side of the barrel or made a patent breech.  My barrel is a 13/16" 45 cal.  IF I screw a nipple in the side of it ahead of the plug then the nipple projects into the bore, not good.  IF I trim the nipple, not good.  I can make a patent breech, not good for reliability. 

I could put the nipple into the plug like some do vent liners. 

See attached drawing, what do you think?

[Bob Roller]

Either will work with the light powder charges used in a pistol.The UNchambered breech design would be easier clean.
Bob Roller

[JPK]

Or braze a piece of metal to the outside of the barrel to act as a spacer so the nipple isn’t in the bore. It could even be made to look good.

[Gaeckle]

Why is trimming the nipple not a good idea?

[bama]

If you have 3 complete threads after trimming the nipple you should be fine. When I worked in the Nuclear Power Field, 3 threads was considered full engagement. I have seem several original breech plugs with nowhere near the thread engagement that we use today but they held up fine. If you don't feel safe with 3 threads that's understandable but as was mentioned, with the lighter pistols loads it should not be a problem.

[David Rase]

When I built my underhammer pistol I drilled the nipple into the breech plug like your top photo.  Never had an issue.
David

[JPK]

Why is trimming the nipple not a good idea?

The hole at the tip of the nipple is small but the rest of the hole from the cap to near the tip is larger. If you trim the tip you may get into that larger opening and get excess pressure pushing the hammer.

[Leatherbark]

When drilling the nipple hole use an end mill and stop before drilling into the barrel to create a flat bottom hole.   Your nipple will stop against this.  Drill a flash hole in the center of this flat-bottomed hole slightly larger than the flash hole in the nipple. This way there will be no force from the explosion against the nipple with its minimal thread engagement. All explosion forces will be against the small flash hole.

Bob

[smylee grouch]

Just curious but why would a patent breech be unreliable?  I have these breaches on several rifles and have never had a reliability problem with them.

[Bsharp]

I have made many with the patent breech style, threads all the way thru. Lets the powder reach the nipple.

[kutter]

Place a simple flat washer of sufficient thickness over the nipple thread so that when the nipple is threaded into
place, it's end does not enter the bore.

Same idea as JPK but just a small dia washer hardly larger in dia than the seat dia of the nipple itself.

If the bbl wall thickness / thread engagement bothers you,,then go ahead and thread the nipple into a patent breech.

As far as cleaning, either will clean up easily as any other M/Ldr with some warm water and a dot of soap in it.

[Dphariss]

Top one, use a ball end mill for the cavity. I would seat the nipple so it bottoms on a shoulder as shown. But then I generally do the same thing with vent liners.
I use a piloted counter bore of ther proper size to make the hole to tap so the bottom is flat. This is for a vent liner 12-32.
Need to grind a tap to get threads to the bottom.