Jason: I'm not so sure it really would be safe. I believe your Enfield in .410 was not converted but manufactured in that caliber for police work in Shanghai (I think, I think....). It certainly was not designed as a sporting arm..... That being said I'm not sure of two things (1) that you would be able to safely chamber a 45 Colt cartridge and (2) that the pressures would be safe for that rifle.
But, all is not dismal. You mentioned she shoots slugs pretty well, so consider staying with the same caliber. There is new .410 ammo out on the market today, thanks in large part to the success of dual caliber revolvers like the Judge, that take both the 45 Colt and the .410 has benefitted from it. I have a nifty little .410 NEF that shoots the Sellier and Bellot and the Brenneke slugs both very well. There is also new ammo containing different size shot loads for defensive .410s but I can provide that the lowly little .410 has accounted for lots and lots of Chipmunks and Squirrels around here. I doubt the rifle would recoil much at all.
