I thought about using the .410 Magtech case with solids in my .45/.410 Contender barrel but did not for three reasons: the .444 case works so good, the .444 is designed as a very sturdy case for its relatively high pressure which meets and, in fact, exceeds my desire to load in a handgun, and I do not know how much pressure the .410 Magtech case can take, since the shotgun design pressure is only about 13,000 psi or so.
Having said that, I just measured a new, unfired .410 Magtech case as follows, in inches:
Base Diameter in front of rim: 0.472
Rim Diameter: 0.521
Length: 2.348
Diameter at neck: 0.459
I have loaded the brass Magtech .410 with 13 grains of Lil Gun and a half ounce of No 8 shot inside of a Remington plastic wad column, and she works extremely well!
As noted earlier, I am also very pleased with the .444 case in the .450 Mongo (.45/.444). I am shooting the .450 Mongo in a 10-inch .45/.410 Contender which has a beautifully carved handle from the 1970s, but the handle works better with light recoil versus heavy. However, it fits my hand very well and looks very good, so I'm sticking with it even with the Mighty Mongo. I stopped my load development at this point at 18.5 grains of Lil Gun with a 332-grain Lyman cast bullet and polyester filler, achieving 960 fps. There are no signs of pressure, and I would assume the gun would take a lot more pressure and velocity, it's just that I won't. Even at my stopping point, this is potent medicine for deer and hogs, especially since I am shooting iron sights and won't be trying long distances anyway.
Incidently, besides showing no pressure signs, the .444 case actually has expanded only in the first half of its length. The rear half is still exactly the same diameter as unfired .444 cases, even after four reloads. It makes the case look a bit strange, larger front half than the rear half, but the bullet is held securely in place by the chamber-sized front portion, and accuracy has been fine. I don't think I want to feel the recoil necessary in the unscoped Contender to finish forming the rear of the stout .444 case. I'm happy with the way it is.
Good luck with your project. I think the .45/.410 barrel is the ultimate survival weapon, capable of taking about anything you would care to try for, from hummingbirds to, well, how big you wanna go?