What Badge said!
Not only are you using a grossly oversized projectile you have to remember that a string of shot will 'flow' down the barrel, a solid bullet will not, even if the .410 is cylinder bore it will still blow up, any choke at all and it will blow up even better (or worse).
When I was in high school a friend decided to 'make' a gun in shop class, he figured he could use a .41 magnum bullet in a .410 barrel and cobbled together a crude reciever that worked on the 'zip gun' priciple.
We convinced him to tie the thing to an old tire and fire it from a distance (we couldn't convince him to not do it). when it went off it blew peices all over, busted out the window of a car and launched the split barrel over his Dad's garage.
Closest thing to a grenade I've ever seen.
Don't do it.
RD
