I have a RKS gain twist to 11.5" coming soon, and also have a Boomer mould which is like the PJ 235 gr. spitzer.

Question:  What cartridge would work best? It will be breech seated.

I have been contemplating a .32-40, but could a smaller cartridge drive that bullet fast enough for good stability and accuracy?

TIA for any suggestions.

Paul

Paul
Nothing wrong with 32-40, I have the same barrel and use a 236 grain bullet from a Hoch mold Also a basic R Long style.
I chambered mine in 32RKS and have been very happy with the results. You can get all the velocity needed with this case but I do not think I would go with anything smaller.
Dales

Pressure, pressure, pressure

If you don't mind the trials and tribulations associated with the small capacity cases it might be a good choice for you. If you would prefer to not want to experiance any of them the 32-40 would indeed be your best choice. The Pros and Cons of each have been discussed in great detail several times on this site. A quick search might bring those detailed conversations back up for your benefit. In short the small capacity cases use to rule our line of competitve shooters and the 32-40 was quite rare. The 32-40 now rules the line and a the small capacity cases are quite rare. I can honestly say that the 32-40 does not take a back seat to any of them and in most cases is indeed far superior by my having substantial competitive experiance with them both over the past sixteen years. All of my personal competitive accomplishments have been achieved with my 32-40 including a National Title and most recently a new ASSRA record along with numerous others. The small capacity cases provided me with very few and they were all short lived. They were the 33RKS Schuetzen, 32 Miller Short and the 28 Darr chambered for the 357 Max. straight tapered to 28. The 28 was re-chambered to the 25-35 blown out to 28 and the other two were sold. All three encountered pressure issues that kept me from reaching the velocities I required for the very best in accuracy. This is not and has not been ecountered with the 32-40 or the 28-35SS I can push them both well beyond the velocities I require for both with no pressure issues encountered along the way.

JLouis

Paul 38-55 brass is readily available and only requires sizing down, I bought a Lee 32-40 sizing die body only from Lee for 9.00 dollars plus shipping just a few years back. If memory serves me right it was not shown on their Web site and simply required a phone call.

I hope this might be some how helpful.
JLouis

Paul how were the cases lost? If by collapsing a filler such as cream of wheat sand being more ideal should help to solve it. I also think your idea of using a 30-30 case blown out to 32 would also be a good one. If having a custom reamer made make sure there is enough out diameter at the mouth to enable you to insert a bullet so you can make up a seater case. My 33RKS Schuetzen reamer was not designed to be able to do so and required an adjustable plugged case being pushed in by use of a rear sight base. Not being able to insert and seat the bullet central to bore was one of its down falls the other of course being pressure.

JLouis

For sizing down.
For blowing out a case I have used a regular charge of powder, filled the rest of the case with cream of wheat and then placed a wad on top. 

JLouis

Jubilado, I think those longitudinal "grooves" from forming are partially due to excess lube. Maybe. Why didn't you try the Imperial if you were losing so many- It may be the trick here, even it it wasn't elsewhere. 
And, please tell us what the negative results were while using slightly short cases. I have a .32-40 That I switch back and forth on length for thirty five years now, and can't see any difference in the loads or the results. No idea how many rounds in the ninety years prior to me. My cheap bore light doesn't show any harm. Just curious, ya know.
I agree with John- and others- that the .32-40 is a terrific choice.