As pointed out above, some tubes can handle harder than lead shot, some can't. That is the same for any choke tube. There may be caveats when using harder than lead shot too- I've seen some chokes that where a maximum velocity was listed, anything higher was not recommended. Same for buffering though that advisory was in regards to non-buffered shot.
There is no blanket answer other than use lead (or bismuth or tungsten/poly) shot. Anything else you would need to refer to the tube manufacturer.
