The discussion begins with a basic explanation of bullet velocity and why it matters. Bullet velocity is typically measured at or near the muzzle, often within about 20 feet, and then at various distances downrange as the projectile slows. As velocity decreases, so does the amount of energy transferred into the target. Understanding this relationship is important for new shooters and anyone building a rifle, because velocity influences trajectory, terminal performance, and how effective a given cartridge will be at different ranges and against different types of targets.
The focus shifts to the 5.56 NATO M855 green tip cartridge, the most common U.S. military round and a standard NATO load. The cartridge was originally designed around a 24 inch barrel, which is why companies like Hornady often publish velocity data from that length. Most civilian shooters, however, use shorter barrels. A 16 inch barrel is described as the typical minimum rifle length, illustrated by a Knight’s Armament rifle equipped with a Vortex Gen 3 1-10 optic and a B5 Systems stock. Shorter configurations, such as a Mark 1-18 with a 10.3 inch barrel and a PWS Mark 111 with an 11.8 inch barrel, are introduced to set up a comparison of how barrel length affects velocity.
The video explains how a cartridge functions once the trigger is pulled. The firing pin strikes the primer, igniting the powder and creating expanding gases that push the projectile down the barrel. For 5.56, full powder burn is described as occurring around 20 inches of barrel. Beyond that point, remaining gases vent out the muzzle as the loud report. A suppressor mounted at the muzzle captures and slows these hot gases, reducing the sharpness of the sound. Shorter barrels leave more unburnt gas exiting the muzzle, which generally makes them louder and reduces velocity. Longer barrels allow more complete powder burn, typically resulting in higher velocity and slightly less perceived blast, while still remaining loud overall.
Specific velocity figures for M855 62 grain green tip are compared across barrel lengths. Out of a roughly 10.6 inch barrel, similar to the 10.3 inch Mark 1-18, M855 is cited at about 2,600 feet per second. Increasing barrel length to around 11.8 inches, as on the PWS Mark 111, raises velocity to approximately 2,777 feet per second. A 16 to 16.5 inch rifle barrel produces about 2,927 feet per second at the muzzle. Moving to a 20 inch barrel, such as on the Fostech Lightning, pushes velocity to roughly 3,025 feet per second. The faster the bullet travels, the flatter its trajectory and the more energy it delivers on target, which becomes especially important for hunting and longer-range shooting.
The conversation turns to how velocity interacts with armor and steel targets. Once 5.56 bullets exceed about 3,000 feet per second, they can begin to defeat many steel armor plates. A warning is given not to shoot 55 grain M193 from a 20 inch barrel at steel targets, as the high velocity will quickly pit and damage them. The M855 green tip’s steel penetrator is clarified as being intended for penetration of soft tissue, roughly 15 to 20 inches, rather than guaranteed armor defeat. Previous tests against soft armor, steel armor, and body armor from brands like Premier Body Armor and Guard Dog showed that 55 and 62 grain 5.56 were stopped when fired from barrels no longer than 16 inches. It is suggested that a 20 inch barrel might change those results. The distinction between steel-tipped ammunition such as M855A1 and traditional steel core loads is briefly noted.
Barrel length selection is tied to intended use. For home defense, a short 5.56 platform like the Mark 1-18 with a 10.3 inch barrel is favored, paired with heavier projectiles to reduce overpenetration and limit the risk of rounds passing through walls and into neighboring structures. For hunting and long-range shooting, a 20 inch barrel is preferred to maximize velocity and energy transfer at distances around 500 yards. A U.S. Marine Corps M16A4 with a 20 inch barrel firing M855 is cited as being effective on known-distance ranges out to 500 yards. In contrast, shooters using the M4 with a 14.5 inch barrel notice more difficulty and greater deviation at the same distance, reinforcing the advantage of longer barrels for extended-range engagements.
The video closes by acknowledging that excessive velocity can be undesirable for some roles, which is why shorter setups like the Mark 1-18 remain popular despite the availability of longer barrels. The speaker notes still owning 16 inch and longer rifles but emphasizes that a 20 inch barrel with 55 grain M193 is particularly hard on steel targets due to very high impact speeds. Ammunition manufacturers such as Hornady often test 5.56 loads in 24 inch barrels to capture maximum powder burn and velocity, which can differ from real-world carbine performance. The audience is invited to consider preferred barrel lengths and to discuss gas system choices—mid-length, carbine, pistol, rifle, and specialized systems like the Mark 12 pattern—based on specific roles and performance needs.
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