This is Arms Law (1980), the very first product published by Iron Crown Enterprises.

It is a percentile based combat system derived from the system the company founders used to replace D&D’s standard combat for their long-running homebrew game at the University of Virgina. It was conceived as the first in a series of rules modules that would constitute a new system called Rolemaster. Subsequent releases were Claw Law, Character Law, Spell Law and Campaign Law.

The low page count of the booklet might imply a simple system, but nope. See, the thing is that the booklet sits on top of a stack of tables printed on cardstock, about a quarter inch thick. People call it Chartmaster for a reason.

To attack, you roll a percentile, add your attack bonus and any modifiers, then subtract the target’s defense. That result is checked against a dedicated chart for the weapon used, and cross-referenced with a chart corresponding to the target’s armor, which will give a damage value that is subtracted from the target’s pool — take enough hits, fall unconscious or die. Rinse, repeat, for every single exchange of combat in the game. It’s a lot. Too much? For me, yea, but I know a lot of folks who swear by RM. Even though I don’t particularly care for the results though, I appreciate the spirit of RM to the core. Rules tinkerers of the world unite! Oh, also, the infamous critical hit and fumble tables for each type of weapon are in here. These are legendary in their deadliness. I bet I am not alone in using just the crits and fumbles in my D&D game and pitching the rest of the combat system.