40-2 1/2 load - where to start ?

I'm new to bullet reloading - have loaded shotgun for years. I got a rifle barrel liner for my 1882 Remington 10ga SxS and on it is stamped 10ga 40 - 2 1/2. A friend who is a gunsmith had me fire form some 30/40 Graigs. We slugged the barrel - .408 - so he had me order a 400gr Lyman mould that was .410 and gave me some Lino-type [ sp ? ] - I got booits that weigh 415grs. Looking at a some old data for the 40-70 Sharps I found - 330gr boolit , 23gs of 4198 had a MV of 1250 with no pressure given. The powder fills the case just under half way. With the boolit in the case, there's about a .400 gap between the two. I also have some large Fed rifle primers. I would think I'd have to reduce the powder charge by about 30% after looking at some data for 45/70 using 4198 when they went from 400gr boolt to a 330gr boolit and reduced the charge by that amount. Or would I be better off reworking the mould to throw a 330 boolit ? Should I use a filler over the powder ? I try to be careful about pressures seeing how I shot damascus barrel SxS's and keep my reloads low. Any help where am I going wrong - everywhere ? Paul

I guess my question - how does one work up a load without published data ? Or , only data for a boolit much lighter than what I have.

Be very conservative with loading for this. Some will fill cases with water and wieght then compare wieght to similar cases. On 40- 2.5 you could probably work from 40 - 65 data ( 40 2.1) ad work up slowly from there. Keep an close eye on primers extraction and other issues. Working up slowly is important. Cartridges of the world may be of some help here also. May not give alot of data but will give factory date and velocites to work for. Yse of a ghronograph can be a big plus. Wildcattters have been doing this for years and its a slow process. Originally this round was a black powder round and you might consider using real black powder.

paul, have you looked at the possibility of developing a load using Trail Boss?

I've worked up many loads without published data. Pinging real people for ides on things they've done that is close helps significantly (this is the best site for that because you have to trust your source), then start checking similar cartridges for synergies. The advice to investigate and consider/ponder load data for 40-65 IMO is spot on. Find the water weight of both cases, extrapolate for desired pressure max (I assume low), pull off another 10%, doublecheck you aren;t doing something odd like filling the case only 50%, and off you go to experiment.