BMR Notes, a lesson in blow-back actions.
This page is intended to provide answers to questions you may have and aid you in making your purchase decisions.
The BMR is a blow-back action. This type of action involves a balance of many factors
such as cartridge power, case geometry and bolt mass.
In the AR system, the bolt, the buffer and half of the spring weight constitute the bolt mass.
Altering the mass of the buffer allows for tuning the action for a particular cartridge.
Our 22WMR upper functions quite reliably using ammo with 40gr or heavier bullets.
Bullets lighter than 40gr, especially 30gr, will probably require some adjustment for satisfactory results.
Our 17HMR system may also benefit from tweeking for a specific brand of ammo.
A17 ammo is not recommended at this time.
A2 Configurations (full length buttstocks)
In the above configurations, the topmost is standard with the CAR recoil spring for 22WMR and 17HMR ammo.
The lower configuration uses the standard A2 buffer with a travel limiter.
Use this configuration if you experience case head doming or excessive unburnt powder in the action.
The travel limiter is necessary to prevent the bolt handle from bottoming in the receiver slot.
A4 Configurations (collapsable buttstocks)
Similar to the A2 configurations, the uppermost is standard and the lower should be used
if you experience case head doming or excessive powder in the action.