The original M16 and M16A1 rifles were - if memory serves - !:12. That resulted in a 55 grain bullet going around 3200 feet per second with military rifle and ammon.
The earliest ARs, pre-M16, were 1:14. They didn't even stabilize 55s all that well, but it contributed to the "explosive" wounding reported by Army advisors in Vietnam in the early '60s.