Lavan,
If the Briley chokes are not rated for steel shot then don't shoot steel through them. I am guessing that these are Briley thinwall chokes. If so then, as I remember they are probably not rated for steel. Check the sides of the tubes and see if it says steel anywhere on them.
The danger is not only that you will swell the choke tube, but that you could also bulge the area of the barrel with the threads that holds the tubes. If you do this then the old threads will not be able to hold new tubes securely.
If the chokes do not say steel on them, I would call Briley. The Briley techs. are friendly and knowlegable. I have talked to them a couple of times regarding their tube sets and they were more than helpful. Check their website at http://www.briley.com/
