3 1/4" cases need attention loading. One suggestion I would make, is to use full cases of powder in that case. When Dan worked up loads for the Sharps long cases, they ran into trouble with less than almsot full cases of powder. Ringed chambers were the norm, apparently.

Check Accurate Arms site as well as Lyman Cast and standard loading manuals for data - they both have data for the 'Sharps' cases.(even though "Sharps" never chambered a 3 1/4" case) The original .45/120 was a 2 7/8" case.

You should be able to drive 400gr. well over 2,000fps and 500's to 1,800fps in that case, using appropriate powders and still keep pressures under 35,000PSI.

I think Accurate Arms only loads smokeless to the same pressure they recorded with a full case of Black Powder and standard bullet for the ctg. being tested - ie: less than 30,00PSI in the 3 1/4" case.

As to fillers, I've personally been using them for over 30 years with narry a problem - could be due to my laoding practises, maybe luck. Some guys have rung chambers using fillers - note that if using fillers, they must be FILLERS, not just a small tuft over the powder charge - as that seems to be the reason for rung chambers. Another is squibs with partial burning - then cmoplete after a pause, ie: hangfires. The bullet is launched into the bore and stops momentarily, then the charge finally catches, and the bullet acts as an obstruction - boom! One must know what one is about, when dealing with unknowns and lack of experience in loading only exacerbates the situation.