The 6mm AR is a centerfire wildcat cartridge initially designed by Robert Whitley^[1] for long-range performance in an AR-15 rifle.

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6mm AR
Type	Centerfire rifle
Place of origin	United States
Production history
Designer	Robert Whitley
Specifications
Parent case	6.5 Grendel
Case type	Rimless, bottleneck
Bullet diameter	.243 in (6.2 mm)
Neck diameter	.271 in (6.9 mm)
Shoulder diameter	.4323 in (10.98 mm)
Base diameter	.4433 in (11.26 mm)
Rim diameter	.4449 in (11.30 mm)
Rim thickness	.0591 in (1.50 mm)
Case length	1.525 in (38.7 mm)
Overall length	2.260 in (57.4 mm)
Case capacity	36 gr H2O (2.3 cm3)
Rifling twist	1-8"
Primer type	Small rifle
Ballistic performance
Bullet mass/type Velocity Energy 105 gr (7 g) Berger VLD 2,750 ft/s (840 m/s) 1,763 ft⋅lbf (2,390 J)
Source(s): http://www.6mmbr.com/gunweek068.doc

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The cartridge uses a 6.5 Grendel case that has been necked-down to accept a 6.2 mm (.243 in) bullet.^{[citation needed]}

The 6mm AR takes advantage of the wide variety of 6.2 mm (.243 in) caliber bullets. Slim, long bullets with high ballistic coefficient are ideal for energy retention at long ranges.^[2]

It is similar to the 6PDK which also uses a 6.2 mm bullet and a similar muzzle energy and case capacity to the 6mm AR and 6.5 Grendel, in a casing somewhat popular^[3] for the AR-15, a necked-down 6.8mm Remington SPC casing.^{[citation needed]}

This variant uses an Ackley Improved fire formed case with less tapering and a 40-degree higher shoulder permitting a larger powder capacity. It is claimed by its maker to be capable of driving a 6.5 g (100 gr) or heavier grain projectile at 880 m/s (2,900 ft/s) (satisfying the "6mm Optimum").^{[citation needed]}

These wildcats also push forward the shoulder of the same necked Grendel case similar to the Turbo 40, yielding more powder capacity. 6.5 g (100 gr) or heavier grain VLD boat-tail bullets have to be seated deeply within the case neck, however, rendering some of these volume gains illusory but there is an increase in volume even with the longer bullets nonetheless.^{[citation needed]}

Similar to the 6mm Turbo 40 and 6mm FatRat, the 6mm Grinch has a blown-forward shoulder on par with the 6mm Dasher to eke maximum powder capacity from the necked Grendel case.^{[citation needed]}

• 6mm ARC
• .223 Remington
• List of rifle cartridges
• Glossary of firearms terminology