The 870 Express is fine -- if you polish the cylinder and/or shoot brass-base cases. Polished (15min) it shoots the steel stuff
as well as anything else. (I've got both an old Wingmaster and new Express.... and you ain't lived `til you've jammed
an autoloader w/ a steel-based case. NOTHING has the ejection/jamm-clearing leverage of a pump shotgun)
As to bottom-eject pumps, the Browning BPS and the Ithaca-37 set the standard.
As to single-loading, drop the shell into the ejection port and/or push it into the magazine tube and rack the slide. Nothing could be simpler