First, while others like fillers for various reasons, I don't. I think they add variables, depending on the filler, not addressed in the manuals and most of those variables increase pressures and may have other affects. When fillers are used they MUST be under compression in order to give SOME assurance they or the powder charge won't migrate. There may be others but so far as I can remember at the moment, only PUFFLON is intended as a filler. Many have use other items like TP (which obviously has problems with consistent compressibility), card wads (which add mass and friction), cream of wheat or other cereals and so forth. I think that Ross Seyfried used compressible foam ear plugs (IIRC). It seems to me that one can have far more consistent pressures by loading to over 65% loading density.

Some will likely disagree with me but I think that the ONLY thing TP has going for it is availability and is the most inconsistent of all the commonly used fillers. Just look at the supermarket TP shelf for all the variations available. Yet, users never specify even which brand much less which specific product is used. I think it makes a big difference. Yes, I've tried it and found that there is no interchangeability.

As to the use of IMR or H 4198, I think it is perfectly safe, both with regards to pressures and loading density. Fillers are not required IF one uses the recommended charge weights. Because the charge weight for 4198 is 40% of the BP charge, it is easy to calculate and produces velocities very close to those produced by BP. HOWEVER, I have found that in the lower limits of the charge weight window the powder density MIGHT reduce exposure of the charge to the primer flash resulting in incomplete combustion. This will give a squib. I have found that the CCI 250 is better for this application as compared to the CCI 200. It may seem a small difference but there is a great difference in reliability. Also, increasing the charge slightly increases loading density and reduces the possibility of squibs because, I think, the powder charge is more consistently exposed to the primer flash.

The manuals have consistently recommended charge weights of 4198 below the 40% calculation for the 1876 cartridges (I am particularly interested in the .45-75 and .50-95 cartridges) and I don't know why as there is plenty of independent data that supports the 40% rule. For the .45-75 that puts the charge weight at 28-30 gr. (for 4198) not the 24-26 gr. in the various manuals. In my experience there is a big difference in performance between the two charge weight ranges. Also, all the squibs I had were with powder charges of 24 to 25 gr. and CCI 200 primers. EITHER switching to CCI 250 primers OR increasing the powder charge and the squibs ended. BOTH increasing the powder charge AND using CCI 250 primers has absolutely eliminated squibs and the charges burn more cleanly.