So I just slugged a 12.7 Swede and the bore measures .517 groove to groove and .476 land to land. I have several molds on hand for shooting in an 1858 Smith. They drop boolits at 515 to 520 and from 340gr to 380gr depending on design. I have one that drops at 517 and is a slightly modified 50/70 design. Is this difference between land and groove normal? Would it be ok to load the 517 modified boolit for the Swede?

Sorry but I'm a bit of a noob in BPCR reloading but I've done lots of conventional centerfire and muzzleloading.

This gun has has already been rechambered to 50/70. I could see the rifling is quite deep but was surprised at how deep it is.

The bore is in very good condition. I wouldn't rate it perfect, but pretty good with no real bad spots noticed as I was putting the slug through. I generally slug a barrel by pushing the slug all the way through. Once it gets started, if the amount of effort changes, there are problems with the rifling in that area.

My initial thoughts were to use bullets sized to 515. This too large or maybe 512?

Thanks for the reply. The bullets I'm considering using are from the same molds that I cast for my 1858 Smith. They run from 515 to 520 and one is the Lyman 515 mold that some 50/70 shooters like. One that casts at 517 is a modification of a 50/70 design that we put a much sharper nose on to move the center of gravity forward a bit. So far, my Smith doesn't like that bullet, but I'm hoping my 12.7-50/70 will. Here's a link to that bullet from NOE-

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I've got two 12,7. And one thing for sure you can count on is that there are no two alike in size of bore or any rifling dimensions. They are all close but in those days standardization was not like it is today. I too was not sure what to use when I got my RB's even after slugging. What I did was to get Lee's .515 dia. mold in 450 and in 500gr. I also got a Lyman Plains mold to try and I've since purchased another mold during a group buy from MP that is about .500 in dia. I've found that the secret to this and most of the BP cartridges is a very soft alloy and to rely on the slug up ability of the BP to do it's job and if you use a soft enough alloy it will do what it does best to slug it up and fill the bore. The next most important factor of the 12,7 is the case size and length. What are you using for cases? What have you decided to use for lube, a grease cookie or just groove lube? How are you going to get a consistent compression on the powder, or are you going to just compress when you insert the bullet. Are you crimping? If so, what are you applying the crimp with? A roll crimp or a taper crimp? I would start out with a bullet sized to around .510 and a alloy of 30-1 and then depending on the result on your paper target I would move up in diameter if groups are spread to see if they will tighten up as you go up in diameter, if not you can go down in dia. I have a couple of Lee sizing push through dies to play with the size to see how they shoot. I think I got a .501, .510 and I think I also have a .512 from Lyman.

Hiwall55 wrote on Jun 21^st, 2020 at 2:10pm:


I have seen some folks shooting these Swedish rifles with bullets under bore size and getting good accuracy.

After slugging my bore, I could be convinced it would work as the depth of the grooves is on the order of .030+. That's a lot of material to move. If a bullet is say .510, and bore size .515 and groove depth is .030, the top of the lands is at .485, then where would all that bullet material go as the bullet is forced into the rifling? My theory is that the vast amount of bullet material that has to move for the lands has to go somewhere and .005 more in the grooves is an easy fill to seal the bore.

Hiwall55 wrote on Jun 21^st, 2020 at 2:10pm:


Because if you are just starting out working with a new rifle in this caliber a good place sometimes to start is with the same size bullet that the government or military arsenal used in the original cartridge design, and in this case that is: Bullet diameter      .497 inches\12.615 mm. Likely it was of an alloy soft enough to slug up. One must remember not all of these rifles has the exact same measurements, plus other owners may have had work done to their rifles by gunsmiths of long ago to modify chambers or throats that are unknown today, and also since I have other fifty cal rifles and I was going to be ordering a custom crimping die from Lee to crimp my new bullet mold from MP for my Remington 12,7 I figured I'd order a few other sizing dies to help spread the freight charges over and it would give me more options if needed in getting these two rifles tuned up.

Trailboss wrote on Jun 22^nd, 2020 at 8:07am:


Going to order Starline. No current intentions of muddying the waters with another type at the moment. Project is on temporary hold due to big project at work.