I am not a gunsmith, but my first question is handloads or factory shells? Handloads might be high primers. Factory shells, then check the sear and hammer connection (worn or broken sear or hammer claws), the sear spring and the firing pin spring. Obviously the gun should go out of service pending an overhaul.
Was reloaded ammo, another gun being used as well with same ammo, zero malfunction.
How would a high or not completely seated primer cause it to fire? Wouldn't the bolt face just dive the primer in further or the bolt just not go into battery? With a hull loaded many times over, the primer presses in pretty easily.
Is there a gunsmith that specializes in these Browning Auto 5s?