"Prior to the 70's, there were really no high-power, high capacity detachable magazine semiauto rifles on the market."

Really?

Colt started selling the semi-auto AR-15 to "civilians" in 1964. .223 Remington, 20-round detachable magazine.

Full-page ad on page 19 of this November 1964 magazine. http://gunsmagazine.com/1964issues/G1164.pdf