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The Lyman shotshell loading books have some data - at least in 12 bore. I never looked at 20 bore. I started off using SR4576 and SR7625 as those were the slower powders I was using with my duck hunting shot loads. They developed good speeds with low pressures. I knew/figured round balls would produce less pressure than an equal weight of shot due to little or no scrubbing friction adding to the powder produced pressure.

Some extropolation was/is needed at times and is how I got started, like using data for those high speed, lower pressure (slower powders) shot loads of similar weight, reducing slightly then working as pressure signs that I could see or feel, indicated. It worked perfectly with no surprises.

This is what I did with my 12 bore double loads, prior to buying the Lyman #3 manual, which would have started me more easily, but I arrived pretty much there, anyway. Their idea of merley cutting off the petals didn't work satisfacterally for me, accuracy wise. I came up with the gas check cup, cup-up under the ball for centering the ball - that worked for me.

Instead of having decent data to start with, I had to work my way there & ended up slightly conservative to their data, but then, I was using a different, slightly smaller ball. They used a .690" which is actually the 'normal' 12 bore 'full choke' size of 40 points of choke(.040").

It was a good process though - loading my RB loads as if they were rifle loads & developing my loads by changing wads and powder charges, instead of projectiles and powder- similar, as it is. Having experience with loading shotshells helped, I'm sure. The new (current) Lyman book might give even better data as there are many newer projectiles being used these days, sabots, etc. It's possible other makers have data that can be used or extropolated from to help us load. If anyone comes across a source, here's the place to let us know of it - the source that is.

The interior of shotshells differs. Some are straight sided, some are tapered and wads are manufactured to bit both but aren't interchangable - sometimes. I have some steel shot wads that are quite large in diameter and will bulge some shotshells like AA's. The interior of the steel wad is quite cupped. Removing the petals leaves a square base cup for keeping pressures in check, with a rouned cup facing up to hold an undersized ball in the middle of the tube. Initial tests showed these worked well.

Today, with back bored barrels being popular, the old standard full, modified or imp cyl choke sizes could be changed to about anything. One must measure his bore to be sure. Friends of mine used cloth patches around the undersized balls, inside what would have been normal shotloads for 1 1/4oz. high speed 4756 loads. Their ball speeds weren't very high, but the moose shot from their duck blinds at 50 yards distance didn't know that & died where they were standing. I told them to try to shove the cloth patched ball out the chokes of their guns. If they could do that, they were OK. They did this. One fellow tested the 466gr. WW RB against a factory 1,500fps 1oz. WW slug from his 2 3/4" duck gun. The factory slug made a deep dent in the side of the 45gallon drum (50gal.US), barely cracking/splitting the metal at the bottom of the dent with a tich/bit of lead squirting through the crack. The hardened RB punched through one side then exited the other. It holed both sides with very little denting or movement of steel. He was impressed.

One must experiment.

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Daryl

"a gun without hammers is like a Spaniel without ears" King George V