I own guns that have been through a fire. In fact two of them were Marlins. One was a 39A and the other was 336 in .35 Remington. They did not come in contact with the flames. However they were subjected to fairly intense heat. The only mechanical problems I found was that it affected all the springs in the guns. Mainly in the action itself. Remember, this is what happened to my guns. Depending on the amount of heat. It could easily affect the strength of the steel in the firing pin, hammer as well as the barrel itself.
As mentioned above. If you know a good smith, I would have them take a look at it before firing the rifle. When you are talking about high pressures. You really don't want to take any chances.
Jim