With each evolution of the SR15 KAC made incremental improvements over the previous model. The most noticeable feature are the types of rails used in each stage of its evolution. Early Block I and II saw the use of a variety of quad rails, some with integrated front sights or front sight gas blocks.
The Mod 1 is arguably the most significant "evolutionary leap" for KAC carbines and rifles. The Mod 1 introduced the full ambidextrous lower receiver as well as integrated QD sockets (now discontinued), as well as the superior E3 bolt design, and the URX 3 and 3.1 lightened quad rails, which basically eliminated excess weight and unnecessary rail real estate. The Mod 1 also saw the introduction of the KAC recoil reduction system, incorporating a proprietary "intermediate-length" gas system that is unique to 14.5" and 16" KAC carbines. The Mod 1 line up also introduced the SR30 series rifles/carbines, and ball milled barrels were featured in select models for both the SR15 and SR25 line for further weight reduction.
The Mod 2 introduced an innovative "
leak proof" gas system by using an unconventional method of securing the gas tube to the gas block. The Mod 2 also ushered in a new era of lightweight rails, the URX4, in both Keymod and MLOK configurations.
At least that's how I remember it. I'm sure a KAC rep will step in sooner or later and present all the real facts. The bottom line is, if you're not into cloning or collecting, it's really not worth paying premium for discontinued KAC parts or complete guns. Although KAC does intend to do limited run "Vault" releases annually for the die hard collectors and fans. If your intent is to train and shoot the piss out of it, go with the latest and greatest that's readily available.
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