Your guns, if older, probably have a fixed choke made into the barrel. Near the receiver end of the barrel, there may be some lettering that will let you know how the barrel is choked.
A quick google search such as this, will give you a clue on how to find it.
If you choose to shoot slugs through your gun, different brands and types may shoot better or worse, so you may want to shoot a couple different types to see what works the best. Out of my 12 ga 870, a 2 3/4 is more accurate than the 3 inch. Sabots for some reason do not shoot well at all. The kids youth 20 gauge shoots slugs best of all.
Browning Barrel Choke Symbols
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