can I shoot a 20 ga slug through a mod choke. my son wants to deer hunt with me next year and i was wondering if i have to buy a slug barrel or if we can use what we have.

Yes you can shoot it through a modified choke, I think most people have better luck with accuracy through an improved cylinder choke. Shoot her and see how your accuracy is, if you aren't satisfied, try a different choke, if you still aren't satisfied, go for the rifled barrel.

Yes sir..I shoot an Ithica 12 guage witha modified..I get a 3inch pattern at 50 yds..I use Winchester Rifled slugs..from Walmart.

When shooting rifles or shotguns w/slugs it preferable to shoot groups instead of patterns. It should provide all the accuracy he needs-just determine what the max range of him and the gun/slug is capable of within an acceptable amount of accuracy.