i had a similar problem. it seems the older 500-s had the ammo tube loc-tited in. if you do a search on google you should be able to find a step-by step. it involves using a bolt(i dont remember the size?) and a nut on nut technique to twist the tube out. you need to be careful because if you do it too hard you will bend the tube . when i got the bolt and 2 -nuts on it my tube came right off. i didnt loc-tite mine back on this time. wish i knew the size bolt you need but you should be able to figure it out. if you dont know how to tension a nut against a nut to back out the tube just ask a mechanic friend they will know. its not hard. good luck!