Ok I'll bite.. First of all this pice has to be dish washer safe! No wood and no parts that a hot dish washer soap, rinse, and blow dry will create can have any effect, so cleaning is so simple a child can do it and can't fail.
All the moving parts shall be coated in teflon, so no liquid lubes are needed.
Since it is limited to BP and synthetic BP powders it must shoot a ball diameter of no less than 38 calibur and no larger than 50 cal.
The diameter isn't or doesn't have to be the bullet diameter, so it could be a sabot bullet lesser than bore diameter.
Since it is dedicated to self defence at with in reasonable limits means 21 feet and less, it needs nealy no sights and mine would have none.
Instead it would have laser grips, but totally water proof.
To get effective loads the clyinder on my which is a wheel gun would be bases on the .41 cal snake charmer, so it would have a long cylinder for no less than 50 grains capcity, and would handle with ease FFFFg (4F) powders for the most pressure possible. The metalurgy would exceed that of the Casuyl (sp).454. Fully loaded at the cylinder it would hold 5 ready to go charges.
In addition there would be a wide spectrum of projectile choices, from shot to mulitple solid wafers besides sabot and bore sized slugs and round ball.
Inside the grip would be a few mechanisms, that loaded at least 5 charges after the cylinder was shot empty, and also cap these 5 extra charges so once the weapon was loaded, it would self load 5 more times, with out doing anything more than pulling the trigger to fire.
This inner grip area would seal 110% safe proof, with no chance of a spark getting in the grip.
These mechanisims would be made of high denstiy fully machinable plastics, as levers, linkage, springs, and screws that move the powder, projectile, and caps in to the clyinder.
The filler caps would self retain like a chain saw gas cap, so when cleaning in the dish washer all you would do is unscrew the cap and let it dangle.
To load it you would drop in the rounds, the powder and the caps and cock the piece 5 times, and then added enough to make 5 more full rounds.
After the first shot the empty bore would self load in stages just like making pistol ammo on a press loader, except there is no primer to punch out.
The barrel would be 1.5 inchs long and have aggressive fast twist rifling.
Recoil would not exceed that of a common .357 MAX, and better would mimic the new .327 just out.
In the grip slabs would be a very sharp double edged mini knife, with a bottle cap flipper for kicks and grins since this isn't gonna be real, and on the other grip you get a instant match stick that is nuclear. Hey why not?
Probably the frame is made of titainium to keep it light.
It would pass both kali and Mass law and have no safety.
It will not explode if you run it over with a train.
The grips would be heat sensitive and know if the shooter was under duress, and a little gyro embbeded in the frame would steady real shakey hands.
The heat device would have a safe off mode for target work.
It would have ability to test skin for higher than legal levels of liquid libations, but still be smart enough to function under duress.
Ok so now everyone can have a good hearty laff on ol' Mac