To get 45grs FF in standard lenth 56-50 cases I use cut down 32ga shot shells,a compresion die and a NEI bullet that does not sit a long way in the case,I find this 45gr load to be a big improvment over 40grs FF in Starline cases,which is the maximum powder I can get in these solid head cases using the same method.Bullet selection is also very important for acuracy in the original Spencers depending on modle ,for instance I belive ndnchf has nailed the best load for the Springfield converted Spencer Burncide rifle with 45grs of FF and the Rapine 350T bullet,however this cartridge is to long in the case to fit original unalterd Spencer arms,also the twist rate in the Springfiel conversion is 1 in 42 inches so the shorter 350T bullet and 45grs FF is a perfect match,if using a short bullet in the faster twist original unalterd 56-50 Spencers the powder charge has to be droped to 40grsFF or slitley less,however the faster twist in the unalterd originals improves with a longer bullet ,the BACO 375gr Spencer bullet or the NEI 395gr bullet have proven to give me the best acuracy by a fair margin(I use the the NEI bullet with 45grs FF for the best acuracy in my rifle)