FWIW - How much pressure can a .410 take...

[Old Ironsights]

Regardless of the "I'm an Idiot" videos of these guys running .444s & .454s through an old Rossi break open:

http://www.youtube.com/watch?v=BwKydDPY8Xo&feature=BFa&list=UUWJHDMgKWWvOsdyRF3HPVEw&lf=plcp

http://www.youtube.com/watch?v=o0MYG_-XCY0&feature=relmfu

In reality, what is the limit of what a strong .410 can handle?

It can only handle what the Brass can handle.

Here is what happens when you begin to approach .444 pressures in a .410 hull in a .444 capable gun:

(NOTE: THIS WAS AN INTENTIONAL "PUSH IT TO FAILURE" TEST & NOT AN :"OOPS"... though the extractor did need to be replaced afterward....)

http://i5.photobucket.com/albums/y180/MrMisanthrope/IMG_6794.jpg
http://i5.photobucket.com/albums/y180/MrMisanthrope/IMG_6793.jpg
http://i5.photobucket.com/albums/y180/MrMisanthrope/IMG_6792.jpg

This is the result of 1.6cc ( 21.6 gr) of 2400 behind a 210gr slug, a cardboard wad and a "stump" wad. Velocity of a round where the case doesn't blow out? 1864fps. :shock:

Velocity when the case blows out (taking the Marlin extractor with it... [smilie=b: ) 1647fps. :|

Back to 1.3cc/17.5gr @ 1550fps. (ya gotta load these with a Lee Loader...) :veryconfu

[eye shot]

I load .410 plastic hulls with 300-355gr. .44 cal. slugs to fire only in my converted .444M. The .444M will stand more pressure than a .410 plastic shotshell will. The shells you show have been subject to high pressure, but the brass-steel didn't blow out. That gouge was from the extractor when you tried to eject the stuck hull. I've only have had that happen with Federal hulls. I load slow burn rate powders at 3xxx what your are and can reload them. Caution do not use .410/.444 in a .410 shotgun!

[Old Ironsights]

Except that there was no extractor to gouge it. The shell had to be rodded out because the extractor snapped off at the ring on firing.

I would like to use .444 brass, but the ID is just too big and I'd rather not ream out the .410 to .429