I'll throw another suggestion... What ammo is misfiring, all brands or one in particular? I've known gun and primer combos to not get along.
Example, my buddy bought a box of Fiocchi primers for reloading. His Browning would barely ding the primer and had misfires galore. He gave me the loaded ammo and balance of the primers and I never had a problem out of my Win M12.
Now, there is likely some wear in his gun. But we've tried Fiocchi primers from different lots from time to time and always misfires.
Remember, shoot straight and shoot often.
Good... Bad... I'm the guy with the gun.