JP Enterprises, known for the LMOS (Low Mass Operating System) and SCS (Silent Capture Spring) has announced the fusion of the two systems into the VMOS (Variable Mass Operating System). In short, the VMOS is a bolt-carrier group that combines the basic shape of the LMOS with the adjustable weight system of the LMOS on a single bolt carrier.

The VMOS is initially released for large frame (LR-308 / DPMS “G1”) pattern firearms. The small frame (AR-15) is expected to launch early 2017. Retail for the large frame is set at $319.95 with the small frame expected to be slightly less.

I, for one, welcome the large frame AR release first. Simply put, .308 weapons are far more tempermental in their function than their intermediate cartridge cousins. While adjustable gas blocks are a solid solution to the problem, often adding mass to the large calibers will help with function as it increases the dwell time.

There’s no such thing as “one size fits all” in AR operating systems,” states JP founder John Paul. “Tuning your rifle is the only way to realize its potential and provide the performance you want. That’s why we pioneered the adjustable gas block concept more than two decades ago. Tuning can be the difference between reliable function and reliable frustration.”

The VMOS BCG adds in a few additional welcome features. The bolt “rails” are both polished and increased in size and the bolt carrier itself is QPQ nitrocarburized for corrosion, wear, and lubricity. Unfortunately, forward assist functionality is limited to the forward-most part of travel and looks to be compatible only with JP’s receivers.