Boring out a barrel

[roundball]

I have a question about barrel boring as it relates to an existing barrel's breech plug:

If a .58cal rifled barrel is bored out to a .620" smoothbore does the boring job have any affect on the breechplug and/or threads?

Or does the boring stop just short of the threads and the same breech plug continues to be used?

 

[paulvallandigham]

It depends on the breechplug used, RB. Most Breechplugs today for large bores come in a minimum thread size of 5/8" x 18 pitch. That 5/8", as you well know, happens to be .625", Which, conveniently is just about right for a .62 caliber smoothbore. I would think it would depend on the exact bore dimension for the new gun.

Reboring and tapping the barrel for a larger plug and thread size might not be possible if there is not enough barrel there.

 

[Bubba45]

If it was originally breeched with 3/4" plug it would be fine .
And it should be bored end for end but wouldnt effect threads.

 

[Zonie]

It depends on the breechplug used, RB. Most Breechplugs today for large bores come in a minimum thread size of 5/8" x 18 pitch. That 5/8", as you well know, happens to be .625", Which, conveniently is just about right for a .62 caliber smoothbore. I would think it would depend on the exact bore dimension for the new gun.

Reboring and tapping the barrel for a larger plug and thread size might not be possible if there is not enough barrel there.

Paul: You have the right decimal equivalent for a 5/8 thread but that is the major (or largest) diameter of the thread.

If the outside diameter of the internal thread in the barrel was .625 and someone bored a .620 hole thru it there would only be about .005 of the thread left.

Fortunately, if roundball had a .58 caliber gun, the breech plug threads would likely be a 3/4-16 UNC thread. At least that's the size Green Mountain uses and these things are pretty much standardized if the barrel is made in the U.S.A.

A internal 3/4-16 UNC thread has a minor diameter of .658 so any bore smaller than that could be used without cutting into the threads.

As for boring to a shoulder which stops before reaching the breech plug I doubt that anyone would want to machine such a barrel.
Even if they did the step would serve to collect crud and making the barrel more difficult to clean.

If the barrel was going to be rifled a undersized step at the bottom of the barrel would make cutting the grooves very difficult to do.
Most rifling cutting depends on the cutter being able to pass completely thru the bore.

 

[paulvallandigham]

I measured the breechplug on my 20 ga. fowler, and you are correct. It is a 3/4" diameter plug. The 5/8" is what I find on the two rifle barrels I have, but they are .36, and .50. I had a lot of difficulty fitting the plug to my .36 cal. barrel, and ended up measuring the plug and barrel diameter several times, and sent back two other breechplugs that were too tight to turn into the barrel. That is the source of my mistake.

Thanks for noting the difference and correcting me, Jim. :thumbsup: