I recently had an 'out of battery slamfire' in my, two piece firing pin, FN49. I had made 8mm Mauser ammo from milsurp 30-06 cases. The dimensions of the cases (30-06 and 7.92x57mm S) are identical at the base and only need to be cut down to the correct length, sized and trimmed. The problem with making your own ammo for the FN49 is that it is a very robust firing mechanism that will ignite a sensitive primer before the round is seated in the chamber. I was using CCI 200 Large Rifle Primers which are reasonably sensitive for bolt action rifles and, I have found out, too sensitive for semi auto rifles. Even though I've used these primers for years in my FALs, and WASR AK47, I'm going to stop and use CCI 34 Large, 41 Small, or milsurp in the semi auto rifles.
The 'oob' slamfire caused the receiver and bolt carrier to become a bad fit! I searched (Google) and found a gunsmith in Missouri who could fix the rifle. His name is Guy Snelen at AMG International in Humansville, Missouri. He repaired it, test fired it, and sent it home to me. If anyone needs a good gunsmith, get in touch with Guy.
The up-beat thing about the gunsmith (Guy Snelen) is that I now Know a good gunsmith who charges a good price. I have met others who I felt overcharged for the work done.
I also learned a lot more about the FN49 and the ammo used in it. Use 196 grain military for safest, best firing results.