Wow, good one too be sure.
On the 870 the barrel has no bearing on the mag tube feeding. The pce that keeps the magazined shells from releasing into the reciever is spring steel that is just pressed into the side of the reciever.

My guess is one of 2 situations relating to this component of your gun.

1. the spring steel shell stopper is bent or worn.
2. powder residue and grime have built up behind the spring steel and flexed it out of shape.

Both of these situations "could" release a shell with a heavy recoil. You will get a more aggresive recoil from slugs as opposed to bird shot. A good test for this would be to put the bird barrel on (with a modified or improved cyl bore) and shoot a couple of your slugs through it. My bet is you will have the same proublem.
Dont know your competency with firearm dissasembly??? I would first try (or have a cometent gunsmith) removing the spring steel stop and clean everything real good. WARNING this piece comes out very easy, go's back in real hard!!!!!!!

ok ill try that thanks for the help.

Yeah ADKBEAR is right. There is something wrong with your shell latch. Sounds to me like it's just barely holding the shell in the magazine. To verify this I would put a spent shell in the tube and leave the chamber empty. Then tap the butt of the stock on a hard floor and see if it gets released into the carrier latch. Is this a new 870?

no its about 15 years old but its clean inside and out