You may want to try some cowboy loads. Not scientific, but when we were shooting a BFR in 45-70 we were shooting the 405 cowboy loads that are a pussy cat in the rifles and they were stout, we then tried the 300 grain bullets that are warm in the little rifles. And they were nothing out of the handgun but threw a huge flame as most of the powder was burning out of the end of the barrel.
You may want to look at loadings for the 45-70 that use fast powders like Black powder equals. Explosive vs Propellant. and combusts quickly in your 12" barel while many powders and loading are for the 20" carbine / rifle barrels.