trapdoor - 3 groove

[hiram]

I got a trapdoor from a friend and slugged the barrel. It was 3 groove. From the land across to the bore measured .455. The bullet I have to use is the Lee 405 FP HB. The bullet casts .459. I will use bp.

I think it should be fine.

Any opinions?

What would the barrel be if it were 4 groove?

[JeffinNZ]

Shoot the fattest cast bullet you can easily chamber. Your .459 is a good start.

[noylj]

Take the lead bullet, coat it black with a Sharpie, and use it to slug the bore. If you get rifling marks all around, it is large enough. There is also the tried-and-true method of shooting it and seeing how it performs. it is certainly not TOO large.

[Catshooter]

Trapdoors can go to .463, or so I've been told.

Wrap the slug with a .001 feeler gauge, measure, then subtract the .002 (feeler gauge). That'll give you the slug size.


Cat

[mpmarty]

If you could measure the height of the land you could add that to the measured diameter and ( I think ) get the bore diameter.

[Larry Gibson]

TDs can go considerably larger with 'good barrels" running .460 to .465. Suggest you take a strip of beer can 1/2" wide by 3" long. Mic the thickness of the beer can material. Slug the barrel with a very soft (Pure lead works the bes and lightly lube the bore first) bullet. If it is smaller than .466 the sqeeze it in a vise or set i base down and wack it a couple times with a hammer untill it swells to over .466. Now wrap the beer can material around slug from the barrel and pinch it together on one side tight enough that the material wraps suggly around the slug. While holding it tight mic the diameter over the beer can material. Subtract twice the thickness of the beer can material and that will give you what the groove diameter is.

The Lee 405HB is a replica of the M1873 arseneal 45-70 and 45-55 bullet. Cast soft of 1-20 tin - lead alloy it will obturate a little in the larger TD barrels if the full 70 gr BP load is used. The arsenal bullets were sized at .459 also. Cast softer of 1-30 or even 1- 40 and they do well in TDs. They will do better if they cast larger. I use a dipper and cast at 725 degrees with a 1-16 alloy and my bullets drop at .464 from my Lee 405HB mould. They shoot mmuch better as cast or sized to .4615 for use in my TD which has a .460 grove diameter.

Larry Gibson