You may have looked at this, but MagTech makes 2-1/2" cases that they claim are for 2-3/4" chcambers. Not sure what that means but I have had some and they seem to be serviceable. As with all the brass shells, they have a larger internal volume than a standard hull and require larger wads.
I have for a while purchased Magtech shells from thegunworks.com. They always seemed to have the best stock and best prices for them.
Their name is, I believe, The Muzzleloader Emporium. The gentleman who started and ran it passed away a couple of years ago but his wife and staff are continuing the business.
I just checked and they have MagTech 410 in stock for $48/box. That is higher than the other gauges but so is everything else .410.
Making 3" from a rifle cartridge is do-able, but for proof of concept it would be easier to buy a box and some wads and try them. They carry a lot of wads, and if they don't have them TOTW has everything from .410 to .450 in stock. (Circle Fly) The recommendation for this hull is .430 for the internal wads and .45o for overshot. With cut wads, the higher internal volume and no an overshot wad right at the mouth, you should easily get a 3" load in the shorter shell. If they work out, a longer rifle conversion could be made, but you can try it without much work. One interesting comment on the Magtech 410 is that, if memory serves, there is no 2 thickness base on them. Due to small diameter, I am remembering that the base is all one level at full thickness. This is a big deal because it negates the objection most people have about the wad encroaching on the flashhole and should be stronger due to the thicker head. I will look to see if I have any of these left to check this.
