I just bought a 20 gauge 870 and would like to use it deer hunting but I can afford to buy the rifled barrel. Is there a rifled choke for this gun? If there is not what choke should I use if I plan to use slugs followed by buckshot? Thanks for any info.

use a full choke it is the best for that gun. espacilly if your gonna be shooting slugs then buckshot

Try shooting with different tubes to see which one patterns best. I have found all guns are different. Some like em tight some like em open.

Improved Cylinder my friend. I wouldnt dare shoot a slug through a full or even modified choke.

Correct. Don't shoot rifled slugs through a modified or full choke barrel. It's too constricting and can have adverse affects. It may damage the choke tube and\or barrel. Improved cylinder or cylinder bore is best. Don't fire sabots unless you are using a rifled barrel either. Since you are following up with a round of buckshot, go with the improved cylinder. You can try modified, but be aware that over time it may open up the choke some and lodge the threads of the tube into the barrel. 

Interesting tidbit here as well: Remington claims that it is safe to fire their "slugger" round through any standard choke tube or barrel ( no extra full or turkey chokes! ). Their round is made of a soft conical lead that is able to swage down to narrow bore diameters. Their rounds are the exception to the rule, IMHO.

 

improved is the best for slugs. remington slugger or federal have the cheapest when on sale 2 3/4 works just fine for everything. Does not hurt to buy a box of each and see what shoots the best. All the slugs on the market have a little different response to different guns.