PSA: Bottom Metal Hardware for these rifles.
All rifles in this group utilize the Gen II SENTINEL DBM bottom hardware.
This hardware incorporates the new'ish Captv-Mag feature that limits magazine movement once the magazine is latched.
Captv-Mag is two integrally machined in control points on the top back of the magwell box that intercept the top back edges of the AICS type magazines. They are designed to float about 0.020" off the bottom of the Rem and Rem clone receivers while still blocking the magazine from over-traveling upward.
This feature ensures that the user will not inadvertently cause a malfunction or bind the bolt when pressing the magazine against a barricade, window sill, truck hood, rock, etc. Ditto for resting your rifle on the magazine when shooting offhand/positional or on another support.
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This feature has been part of KMW's Mk1*, Mk2 and M5 bottom metal for 3 years now with excellent results. It is also integrally machined into the Zealot chassis.
STOCK BOLT RETENTION
You will notice a small internal groove on the I.D. of the stock bolt holes. The bottom metal should have 10mm internal snap rings in place within these grooves. This feature allows the rifle to be taken down in the field or in the shop with a standard T-handled hex wrench without the chance of losing your stock bolts.
The retention rings look like this when in place.
The hex wrench/key can access the stock bolts with these retention rings still in place.
This has been a standard feature on SENTINEL bottom metal since 2007.