Then comes price. Ruger introduced a piston system AR—the SR15 —that retails for over $2,000. I found an M4 from one of the more popular piston-gun makers listed on for $1,925.

Designed to compete with budget AR-15, the rifle features a highly similar layout to the AR, but an AK-like operating system that uses a long-stroke gas piston and a charging handle integrated on ...