Breech plug thread size for .50 cal 15/16" barrel

[tecum-tha]

I have a .50 cal 15/16" across the flats CVA barrel which was never used.
Got rid of the percussion drum part by cutting it off and after swamping it needs to get a flint breech plug.
I have a 9/16-18 plug tap, but had the wrong breech plug for a 7/8" across the flats barrel.
Is the 9/16-18 a good thread for a .50 cal or should I go up to 5/8-18 thread size.
Tap drill is 33/64" dia. for 9/16-18 and 37/64" dia. for 5/8-18.
Recommendations from experienced gunbuilders appreciated.
No CVA barrel safety bashing please etc.

[Daryl]

TC used 5/8x18, wasn't it? I used that as well on a Bauska barrel - seemed to work OK.

[tecum-tha]

Does T/C use 15/16 barrels? I don't know, as I don't shoot or own T/C guns.
9/16-18 would leave more barrel wall standing. That's why I am asking.
Oh well, I will order the 9/16-18 and see how it works. If it doesn't I just pick up another 5/8-18 breech plug at Friendship.

[bgf]

5/8-18 seems to be more standard for 15/16" commercial barrels. Might make it easier to find plugs in right size.

It works fine in my green mountain 15/16 .50...

[David Rase]

The minor thread pitch diameter of a 9/16 x 18 thread (depending upon the thread classification) is .494" which is less than the .50 caliber bore diameter.  Add .011" or .012" for rifling and you have a bore diameter of .522" to .524".  No way Jose. 

The minor thread pitch diameter of a 5/8 x 18 thread (depending upon the thread classification) is .556" which will seal a .50 caliber bore even with .011" or .012" deep rifling.

With that said, you will not get a seal at the face of the breech plug using a 9/16" x 18 breech plug.
David 

[bob in the woods]

What Dave Rase said.  You didn't say that you had a plug yet, and I don't believe that you'll find one too easily unless you go with the 5/8ths x 18  which are readily available.

[Dphariss]

5/8".

[Hungry Horse]

 This seems like it would be pretty obvious, that if you can't buy a 9/16th plug for a fifty caliber barrel, it probably isn't a good idea.

  Hungry Horse

[Daryl]

Does T/C use 15/16 barrels? I don't know, as I don't shoot or own T/C guns.
9/16-18 would leave more barrel wall standing. That's why I am asking.
Oh well, I will order the 9/16-18 and see how it works. If it doesn't I just pick up another 5/8-18 breech plug at Friendship.

In the 70's, the .45 and .50 Calibre "Hawkins" had 15/16" bls. while the .54 cal. renegades in the later 1970's had short 1".  They all had .0015"(1 1/2 thousandths) to .004" (4 thousandths) deep button rifling - by measure, not guess work.

Thus, 5/8 x 18 was standard for the .45 and .50's. I expect 3/4 x 16 was the thread and pitch for the 1" bl. on the .54's.

[tecum-tha]

5/8-18 it will be. No problem. Already ordered the drill bit/carbon steel tap and the plug will be ordered from ToW today.
I was also thinking that 9/16-18 would be problematic, just wasn't sure.
ToW sells their plugs for barrel sizes and doesn't specify caliber.
The 9/16-18 will work fine for a different .45 barrel I have which I may build sometimes later for another kid.
Thanks for the help.