I have handled FN 49’s, I assume that it has the same free floating firing pin arrangement that FAL’s have.
Actually, the FN-49 does not have a free-floating firing pin. The original design for the bolt was a one-piece firing pin, a firing pin return spring, and a couple different types of firing pin block that would prevent the firing pin from traveling forward unless the bolt was in battery (though some of the Egyptian-contract 8mm guns lack the FP block). Later, it was found that the one-piece firing pin was prone to breakage and would cause the forward section to stick and the gun to go full-auto. Because of this, the firing pin design was changed to a two-piece design although the FP block only acts on the rear section. The front section of the firing pin is quite light and is held rearward until the trigger is pulled by the firing pin return spring.
I've had issues with this rifle slam-firing before but thought I had fixed them. Before, the gun would intermittently fire 2-3 rounds with a single pull of the trigger, but never out-of-battery. I replaced the older one-piece firing pin with a newer two-piece one and replaced the firing pin return spring with an extra-heavy one. This seemed to fix the issue when I used Federal Power-Shok ammo (the type I normally use in this rifle because I usually find it the cheapest). The incident in question was the first time that I'd used Winchester ammo in several years (I did run a box through the gun not long after I bought it but had feeding issues at the time).
The Gander Mountain gunsmith didn't really insinuate that the incident was my fault as he did concede that the ammo may be the issue. His thought was that perhaps some bit of material got into the primer pocket under the primer during the manufacture of the ammo and that may have caused the round to fire before the bolt was fully in battery.
The only other situation that I can think of is that perhaps my gas system should have been readjusted for the Winchester ammo. As I mentioned earlier, I normally shoot Federal ammo in this rifle and thus had last adjusted the gas system for that. I suppose it is possible that the Winchester ammo may have been hotter and thus caused the action to open before the pressure had dropped sufficiently. I find this unlikely, however, because from what I've read, such a problem usually results in the case sticking in the chamber and the bolt either failing to cycle or ripping the rim off of the case.