The DPMS AR bolt carrier group is available in three versions, each with a very highly specified friction and wear reducing external treatment. In each case the bolt, carrier, gas key, cam pin and firing pin are all treated. Lubricity of the AR-15 bolt carrier and components is increased, friction and heat reduced, component life increased and cleaning made much easier. These AR-15 bolt carriers are designed for reliable performance in mil-spec semiauto AR-15 receivers. They feature shrouded firing pins.
Chrome is a traditional material for AR bolt carrier groups and is attractive and relatively easy to clean compared to phosphate. Early military ARs often had chromed bolts and the military only went to phosphated for concealment.
Diamond is a relatively economoical surface treatment that has an ion thick layer of carbon bonded to the surface of the the components. It has the advantage that the color and texture are similar to a phosphated/Parkerized bolt.
Titanium Nitride coating is arguably the best surface treatment available. However, it is the most expensive and you end up with a bronze/gold colored bolt that not everyone will want on their bolt carrier. TiN bolt carriers and other components should be used with caution as, if not bearing against similarly treated components they can cause wear, particularly to hammers.
Check the price of DPMS bolt carriers at Brownells, a name you can trust.
You may find this video of the Trijicon Accupoint scope of interest.
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