How Fast Should I Go?
 

About two (could it be three?) years ago I bought a 13/16" .40 caliber Green Mountain barrel with the intention of putting together a simple iron mounted "Southern Style" long rifle for plinking and target shooting.

Well, that barrel is still sitting in the closet untouched. I never got up the enthusiasm for the build. I think it was the realization that a .40 caliber is not something I would use much when my .54's and .58's are so much fun to shoot.

But (big BUT) I do love those heavy lead .45 caliber conicals that Ed Mehlig produces. And I would be mighty pleased to have a trim little long rifle that shoots them well.

So it hit me this morning when I was once again moving that .40 caliber barrel out of the way to get to something else . . . "why not have it rebored for those big conicals?"

Those pure soft lead bullets from Ed are actually .452 according to my micrometer. So I want the bore diameter to be .452 for a slip fit.

But what about groove depth. I don't want deep grooves as would be appropriate for patched ball shooting - but how deep for a soft lead conical?

And rate of twist??? My 1:30" GM/LRH hunter barrel shoots Ed's bullets well. But what would be optimum? 1:28"? 1:26"? 1:24"? 1:20"?

What say you? All input is welcome.

I myself would go with a groove depth of around .006" and a 1:20 twist. If you planned on those 465gr cannon bullets I'd go with 1:18 though 1:20 would probably be enough. You don't want to go too deep with the groves because of gas leakage which would cause some serious leading problems. I would also go with 5 groove cut rifled.

13/16" would be considered too small at the breech end for heavy conicals based on Original Match rifles of 45 cal that were 1.2" at the breech. The majority of these match rifles had 1:20 - 1:18" twist for bullets in 500-550gr +/- weight.
Also, do you know what steel you're barrel is made of? Be careful with any conical or heavy loads with 12L14 steel.

Good point 52bore. The Green Mountain web site does not specify the steel used for these barrels. I just sent them an email asking for that info. They may well use 12L14 for their muzzle loader barrels because of it's machining characteristics. Though 12L14 is somewhat controversial for smokeless barrels, I don't think it's an issue with black powder and black powder pressures, even with conical bullets.

No!!!!!!

One of the best things I ever did was make my .45 flinter a .40 when I made my .54...Its a hoot to squirrel hunt with a .40 caliber...Drop down to about 25grs FFF, take head shots and have a ball!!! That nice crack a .40 makes with that small powder charge is about like a .22 mag...

:s2::s2::s2: Yeah, I know Hawkeye. My first muzzleloading rifle was a .40. I know what you mean about that .40 crack. And I must admit it's a fun little caliber. But these days I get more of a kick (no pun intended) shooting bigger balls.

I have a .32 for small critters and short range plinking. I'm not sure a .40 will get much use by me. But another .45 conical shooter sure would.

Here's the answer on the barrel steel.

Here's a comparison of 12L14 vs. 1137.

Good deal, now we know.
I know the metallurgist who was involved with the Douglas RB lawsuit and problems with 12L14 - he's always told me to stay, especially bullet guns.