The KS-1 was the first rifle in the KS model line. It has a heavy profile dimpled 13.7” barrel, a URX6 handguard, reduced profile ambi charging handle, and a fully ambidextrous lower. All KAC rifles with less than 16” barrels have been ported to run optimally with a 2H buffer and standard carbine action spring. The 13.7, 14.5, and 16” share the same gas port location, but obviously with different port sizes. They were designed in conjunction with our low backpressure suppressors, so if you want to use a legacy or high backpressure suppressors you will need to adjust spring rate and buffer weight for optimal performance when suppressed, but the dual-ejector E3.2 bolt does give a nice range of carrier velocity while still getting good ejection.