years ago I tried a .640 and my main pattern danced all around figured it was choked too tight. now I shoot #5's with a .665 briley straight rifled choke. Winchester supremes (black shell). I have a 3.5 and a 3 inch 870 that are super tight patterns. I just got a SX3 NWTF and it has a .655 choke in it, I bought a .665 briley for it too. I haven't patterned it yet but I think a .655 choke would be a little tight for #5 shot. Briley says if it doesn't work out I can return it within 30 days. Also recently I heard that .660 to .670 were best size chokes for #5 lead shot