FN SLP 18" help!

[Spray_N_Prey]

Ok bought this SG used (barely) it looks brand new. It was going to be my home protection gun and the 22" mark I was the competition gun.

Well it started out jamming a round half way out of the tube, I had to get a screwdriver to get it free. After that it was running fine for about the first box or so. I did run about 6 3" magnum rounds through the red (light) piston before I remembered to chance it out. It then ran fine with the other piston and I wanted to use up some 1oz crap I had laying around so I put the red piston back in and it wouldn't cycle crap. It even wouldn't cycle the 1 1/8 ounce loads after that. So I put the heavy piston back in and it still wouldn't cycle 1 1/8 loads but would cycle 00 buckshot.

Do these shotguns get that dirtied up that fast or did I damage the piston or do I need to look at sending it back to FN?

Also could someone give me some pointers on how much to lube and where.

Thanks,

Shawn G.