Slk: Your 510-512 grain weights are not unreasonable. Lyman moulds are made to throw the designated weight using a #2 alloy, which you can look up in any lyman cast bullet manual. Most casters don't have access to or don't make the effort to have access to the exact ingredients to get the perfect weight bullets. The Lyman 405 gr mould throws anywhere from 405-418 gr depending upon the alloy or with pure lead. I've cast bullets for my vintage Winchester and modern Marlin .45-70's and 90's for over 30yrs and have in that time used about every form of scrap lead under the sun-- fishing net weights, wheel weights, wine bottle wraps, battery lead, you name it, much of it alloyed with babbit material containing lead, tin, antimony, and sometimes nickel or copper. They all go out the barrel with devastating effect and have killed bears, moose, deer, and caribou. Wait 25 days after casting to size your cast bullets; they'll be about done shrinking by then. Size them too soon and they'll shrink more and rattle down the bore and print all over the paper. At low velocities there's no real need to hard cast for the .45-70. Pure lead bullets actually work better on game as they expand more. Even pure lead bullets will go clean through both shoulders of a large moose at 100yds, or even 200 yds. I've seen it in my hunting. When you slug a barrel make sure to use pure lead and lube it v. lightly by hand. Drive it through with a wooden dowel or brass or aluminum welding rod. Wood works very well. Make sure to mic your sized bullets to make sure the sizer is on.... they don't all size to what they say they will. SPG lube from Buffalo Arms is supposed to be the best for the big cast bullets in the big old guns according to a friend who knows far more than I ever will about the subject. Lube can make a huge difference in accuracy I'm told. I use IMR3031 for all my .45-70, 90 loading. Cast or jacketed bullets.
