The simple thing is buy a bag of cases and stick one in the gun.
Not wanting to start a squabble, but I looked at a couple of sources and both listed the dimensions of the 410 as larger than the 9.3 at every important dimension. The 410 is shown as .470 base, .535 rim. .060 rim thickness and 3 inch length. The 9.3x74R is shown as .469 base, .525 rim, .055 rim thickness and 2.94 inch length. The rifle rim is about .01 smaller than the 410 and .005 thinner. The base is .001 less. There should be no case work left except forming. I also found a quote of 82.3 grains of water for the 9.3 capacity but this is meaningless unless you know the fill point quoted. Add to that the fact that the case volume will go up a lot after forming and get a little longer to near the 3" mark.
I am making a rough guess that the 410 cas has an ID close to .410. Measuring a wad should give this number(I don't have any).
The 9.3 is .469 base and probably has around .015 walls for most of it's length. Net ID would be roughly .440. Also close to what I measured on the Magtech. 9.3 is very slightly shorter, but basewad in 410 likely cancels this out. From this, the volume difference should be right at 15% more than the 410, according to my rough calcs. If you really want to stretch it, use punched wads and an overshot wad glued in and you can really get more capacity. The overshot you will use will give you quite a boost. Given the fact that the 410 is star crimped and the brass case can be fully loaded, I suspect the difference will be closer to 25%. Or, as we say, a 28 gauge load. (Most 28 gauges weigh less than the same 410).
The big unknown is the pressure. That's a small bore and the guns run at pressures above most shotguns. Start raising shot charges by big jumps and trying to keep velocity reasonable could put you into rifle pressures.
