M4 / M16 / AR15

Rifles & Machine guns – 5.56mm and 7.62 caliber – Fail-Safe

UTM 5.56mm Man-Marker and Target Bullet Round Conversions are uniquely designed to prevent the discharge of traditional 5.56mm/.223 “live” ammunition (5.56 NATO, 5.56 x 45mm, and .223 Remington). Fail-Safe is achieved by utilizing an offset firing pin design, which will only work with the rim fire primer used in the UTM Cartridge, but not with a “live” center fire cartridge. In the event a 5.56mm “live” cartridge is chambered and the trigger is pulled, the Conversions offset firing pin will strike outside the primer pocket of the 5.56mm “live” round. This makes it impossible to detonate the “live” cartridge in the UTM converted weapon.

M4 / M16 / AR15

NATO Stock Number

  • MMR 1005-99-848-2082
  • TBR  1005-99-262-1452

 MMR Rifle Bolt

All UTM 5.56mm rifle conversions employ a Fail-Safe measure where the firing pin is offset from center. Not only is the firing pin strike outside the strike area of a “Live” cartridge center fire primer, it does not strike the primer at all, hence the weapon fails to fire the host weapons “Live” caliber ammunition.


UTM’s Rifle and Carbine conversions and conversion kits are designed and engineered for the upmost in safety, in-service reliability and maintainability. The conversions and kits are easy to install, typically a simple exchange of the manufacturer’s Bolt and Carrier Assembly. UTM conversions adapt the host weapon to fire UTM ammunition with the feel and function of “live” ammunition. With the exception of the HK-416 and SIG 516, which require their own conversion, the UTM standard rifle bolt conversion will function in any right hand ejecting, direct impingement or gas piston M4/M16/AR-15.

TBR Rifle Bolt

Shown is a UTM M16/M4 TBR Rifle Conversion. Notice the offset firing pin. The firing pin in the UTM 5.56mm TBR conversion is positioned outside of the red circle indicating the primer area of a “Live” cartridge, hence the weapon fails to fire.

As an additional safety feature, TBR ammunition has a recessed primer to prevent firing pin contact from a “Live” or MMR bolt carrier assembly.