The projectile is the weapon

so we start our description of Fûz's Thumper with caliber and cartridge.

Jeff Cooper originally recommended .44 Auto Mag, made by cutting down, blowing out straight and reaming a 7.62mm (.308 Winch) cartridge case to acccept a .429" bullet. This was back in the day when AutoMags were big, all-stainless, custom rotating-bolt pistols of a unique design. Clint Eastwood's Dirty Harry had an AutoMag as a SHTF gun in Sudden Impact. We can recall no other movies featuring this rare pistol.

You can form your own cases and load your own ammo---in fact you have to. Reportedly the guns themselves were finicky and would feed reliably with only a few loadings. A 180-grain bullet at almost 1800 fps from the muzzle would be devastating.

The AutoMag is now a collector's item, but the .45 Winchester Magnum seems to be around to stay. A firm briefly offered an M1 Carbine rebarreled and otherwise modified to fire this cartridge, but we don't hear from them anymore.

That cartridge would suffice, we suppose, but we add some criteria:

• we'd rather have a shoulder to headspace against
  
• we want a hard crimp to make the chamber pressure more consistent from round to round, and make the ammunition more tolerant of the recoil banging it around as it climbs the magazine towards the chamber.
  

We want a case with a shoulder. It doesn't have to be much. We'll give a little on bullet diameter to get one.

So we propose a case of 33mm length, just like the .44 AutoMag, with a tiny shoulder and outside neck turned if needed so it takes the .410" bullet. Load the same weight to the same velocity, with a jacketed truncated cone bullet. This is no new invention, it's the .41 Jurras Magnum Pistol cartridge, for which we can find no URLable load data.

This cartridge, if OAL is managed, will fit in the M1 Carbine's magazines.

Not enough, you say? Put the same treatment to the .45 Win Mag case instead. Then you have the .41 Avenger. It will have greater propellant capacity, and tolerate higher chamber pressures, but it will be more expensive to manufacture and supply because it relies on a less-commonly available component. Fûz's Thumper will not require special feeding if we can avoid it.