.502 through an airforce texan lss 50cal with donnyfl emperor?

That's a question for Donny Du at DonnyFL. Call him up and ask. Or send him an email. He's very nice and very responsive. But typically the inner diameter of a suppressor will be slightly larger than the slug/caliber it's affixed to. If you're shooting a .25 rifle, the inner diameter of the suppressor would typically be .260 to .265. This enables tolerance to prevent harmonic issues or clipping. But the typical rule of thumb will be to get 1 size up on the suppressor. If you're shooting a .25 caliber, then buy a .30 suppressor. I'm assuming that the Emperor in .50 cal will probably be .550 inner diameter. But that's a Donny Du question.

Also, keep in mind that the threaded end cap on the Emperor for the big bores is M18 x 1. So you'll need an adapter, or you'll need a machinist to cut those threads into the end of your barrel (which is more costly, but better IMHO). But make sure it's done by a reputable machinist. Spend the extra money! It's worth it.