Editing M16 rifle (section)

===Barrel===
Early model M16 barrels had a [[rifling]] twist of four grooves, right-hand [[twist rate|twist]], one turn in 14&nbsp;inches (1:355.6&nbsp;mm or 64 calibers) bore—as it was the same rifling as used by the [[.222 Remington]] sporting cartridge. After finding out that under unfavorable conditions, military bullets could yaw in flight at long ranges, the rifling was soon altered. Later M16 models and the M16A1 had an improved rifling with six grooves, right-hand twist, one turn in 12&nbsp;inches (1:304.8&nbsp;mm or 54.8 calibers) for increased accuracy and was optimized to adequately stabilize the M193 ball and M196 tracer bullets. M16A2 and current models are optimized for firing the heavier NATO SS109 ball and long L110 tracer bullets and have six grooves, right-hand twist, one turn in 7 in (1:177.8&nbsp;mm or 32 calibers).{{sfnp|Colt, ''M16 5.56mm Rifle''|2003}}{{sfnmp|1a1=Cooke|1y=2005|1p=1|2a1=Hickerson|2y=1991|2p=9|3a1=Hogg|3a2=Weeks|3y=1985|3pp=195-196|4a1=Simpson|4y=2011|4p=1}}

M193 ball and M196 tracer bullets may be fired in a rifle with a one turn in 7 in (1:177.8&nbsp;mm or 32 calibers) twist barrel. NATO SS109 ball and L110 tracer bullets should only be used in emergency situations at ranges under {{convert|90|m|yd|0|abbr=on}} with a one turn in 12&nbsp;inches (1:304.8&nbsp;mm or 54.8 calibers) twist, as this twist is insufficient to stabilize these projectiles.{{sfnp|Cooke|2005|p=1}}
Weapons designed to adequately stabilize both the M193 or SS109 projectiles (like civilian market clones) usually have a six-groove, right-hand twist, one turn in 9&nbsp;inches (1:228.6&nbsp;mm or 41.1 calibers) or one turn in 8&nbsp;inches (1:203.2&nbsp;mm or 36.5 calibers) bore, although other and 1:7&nbsp;inches twist rates are available as well.