I have that combo of a Hevi-13 choke and an 835. Densest pattern--not many observations--has come from Winchester 3.5", #6 LBs. Only time I bothered to measure, I counted 250 in the 10" center and 139+ (a few were off the paper) in the 10"-20".  Nothing else has topped 200 in the core.  For whatever reason, the LBs shoot several inches higher than any other shell I've ever run through that combo.