I got my first 500 from my dad back when I was 16 (1990-91). I bought a 500 at a gun show (2004)that came with 2 barrels. 1 rifled which stays on that gun and the other was a acu-choke vent rib. Long story short I have had no problems with the acu-choke barrel on the gun my dad bought. I am not a gun buff but I have read and heard about a 500 A model. Maybe that is what you have and the barrels are not 500 A barrels. I could be wrong and I am sure someone will let me know but it is worth a shot. The 500 and 500a maybe interchangable. Just a thought and maybe point you into the right direction.