Guest blog from our friends at Criterion Barrels, Inc.
A typical AR-10 or other .308 AR pattern rifle will normally come standard with a SAAMI specified .308 Win chamber. While this configuration is designed to accommodate a wide range of different ammunition options effectively, it is less than ideal for a precision .308 AR rifle build. A wider throat diameter (.310) and shorter freebore length (.090) will allow the rifle to accommodate chamber pressures typically found with factory loads, but the wider inside diameter of the throat can cause complications with bullet and bore alignment during feeding, limiting accuracy potential with high performance ammunition.
Criterion Barrels Inc. typically utilizes a .223 Wylde design on their AR-15 barrel models. The benefit to this design involves a slightly tighter throat angle than its 5.56 NATO counterpart, with a longer freebore (when compared to the .223 Remington) to counteract the additional chamber pressure introduced by the modification to the throat design.
Applying this principle to the .308 AR rifle platform, Criterion elected to use the .308 Win M118 Tactical (308 M118 LR) chamber as a basis for their reamer design. This configuration offers a slightly thinner throat (.3085 diameter) with a longer .117 freebore to effectively regulate chamber pressure and account for variance in bullet design.
While this chamber design can safely and reliably function with any number of standard factory ammunition options, by slightly modifying the traditional throat dimensions this chamber can very effectively accommodate the ogive of the Sierra 168 HPBT #2200 and 175 gr. HPBT #2275 MatchKing bullets loaded near or at magazine length.
By tailoring their chamber to optimally function with some of the most prevalent match bullets on the market, Criterion Barrels Inc. is able to offer a .308 AR barrel design that offers consistent sub-MOA performance.