i think the substantially-more-armed-ness (can you tell it's near the end of my work day and my brain is cooked?) of u.s. cops has to do with the substantially greater number of guns in the hands of the general public here. the one chicago suburb that i know used to have shotguns in every police car hasn't for at least a decade, maybe longer. rifles/shotguns in regular cops' vehicles is, i'd guess, a more rural thing than an urban thing--urban areas are much more likely to have a SWAT team (special weapons and tactics) or something similar, where a limited group of officers are trained in closer-to-military weapons and tactics.