How Much Does Barrel Length Affect 5.56 Bullet Velocity?

Test Setup and Shortest 5.56 Barrel Velocities

The video opens with a structured 5.56 NATO velocity test using PMC XTAC 55-grain ammunition, rated at 3,120 feet per second on the box. The host explains that this factory rating typically comes from a 24-inch test barrel, so slightly lower numbers are expected from the longest rifle in the lineup, a 20-inch Colt M16. The test spans barrel lengths from a compact 7.5-inch Springfield Saint pistol up through 10.3, 11.8, 12.5, 14.5, 16, 18, and finally 20 inches, with five shots fired per gun and averaged. Initial expectations include roughly a 1,000 fps spread between the shortest and longest barrels. Chronograph testing begins with the 7.5-inch Springfield Saint, which averages 2,183 fps, showing how much velocity is sacrificed in ultra-short AR pistols. Moving to the 10.3-inch Daniel Defense Mk18, the average jumps to 2,513 fps, demonstrating a several-hundred fps gain from just under three extra inches of barrel and hinting at the steep early velocity curve.

Mid-Length AR Pistols: 11.8 and 12.5 Inch Velocity Gains

The test continues into the mid-length AR pistol territory, starting with the 11.8-inch PWS MK111. This setup delivers consistent muzzle velocities around 2,644 fps, clearly outpacing the 10.3-inch Mk18 and showing that each additional inch in this range still yields meaningful gains. Next, the 12.5-inch IWI Zion pistol is chronographed. Its average comes in around 2,789 fps, a substantial increase over the 7.5 and 10.3-inch barrels and roughly 150 fps faster than both the 10.3 and 11.8-inch configurations. These results highlight that 5.56 performance improves dramatically as barrel length moves into the 12-inch range. The host uses the 10.3, 11.8, and 12.5-inch data to argue that, for AR pistols, a 12.5-inch barrel represents a strong balance between compactness and effective velocity, unless the absolute smallest package is required. This sets the stage for comparing these mid-length pistols to traditional carbine-length barrels.

14.5 vs 12.5 Inch Barrels and 16 Inch Gas System Comparison

Attention shifts to a 14.5-inch Daniel Defense M4A1 with a pinned and welded muzzle device to meet the legal 16-inch overall length. Surprisingly, its measured velocities are very close to those of the 12.5-inch IWI Zion, with some runs showing almost no practical difference. In later retests, the 14.5-inch even averages slightly slower than the 12.5-inch, contradicting expectations that a longer barrel always produces higher velocity. The video then introduces a 16-inch SIG MCX, which finally shows a clear step up in speed over the shorter barrels. A direct comparison is made with a 16-inch LWRCI direct-impingement rifle. Both 16-inch platforms, despite different operating systems, produce nearly identical average velocities, around 2,865 fps for the MCX and 2,872 fps for the DI gun. This suggests that, at least with this ammunition and setup, piston versus direct impingement has minimal impact on muzzle velocity compared to barrel length itself.

18 and 20 Inch Barrels and the 3,000 fps Armor Threshold

The test progresses to an 18-inch Daniel Defense Mk12, which finally pushes the 55-grain PMC XTAC into the 3,000 fps range. The host emphasizes that around 3,000 fps is often cited as an important threshold for defeating certain types of body armor and other hard barriers, making this velocity milestone significant for practical performance. The lineup concludes with a 20-inch Colt M16 featuring a 1:12 twist, representing the original design context for 55-grain 5.56 ammunition. Individual shots from the 20-inch barrel register between roughly 3,000 and 3,100 fps, with an average of 3,052 fps. While this does not quite reach the 3,120 fps printed on the ammunition box, the discrepancy is attributed to the factory rating being taken from a longer 24-inch test barrel. These results show how extending barrel length from 16 to 20 inches yields modest but real gains, pushing the cartridge closer to its intended performance envelope.

Full Velocity Recap and Counterintuitive 12.5 vs 14.5 Results

After completing the chronograph work, the host recaps average velocities across the tested barrel lengths. The 18-inch Mk12 produces about 2,998 fps, just shy of the 3,052 fps average from the 20-inch M16. The two 16-inch rifles, one direct impingement and one different operating system, cluster tightly around 2,872 and 2,865 fps, reinforcing that gas system type had little effect on speed in this test. The 14.5-inch carbine shows averages around 2,793 and 2,763 fps in two separate runs, while the 12.5-inch IWI Zion posts an average of 2,789 fps, actually outpacing the second 14.5-inch run. This counterintuitive result challenges the simple assumption that longer always equals faster. The recap underscores that while the general trend is more barrel length equals more velocity, real-world factors can cause overlaps and anomalies, especially in the mid-length range where 12.5 and 14.5-inch barrels performed nearly identically with this specific ammunition.

Gas Port, Dwell Time Speculation and Overall Velocity Spread

The closing discussion explores possible reasons why a 12.5-inch barrel could match or slightly exceed a 14.5-inch barrel’s velocity. The host speculates that larger gas ports on certain carbines might bleed off more gas to cycle the action, potentially reducing the energy available to accelerate the bullet. Dwell time, the distance and time between the gas block and muzzle, is also considered as a factor that could influence how much pressure remains behind the projectile before it exits. While acknowledging a lack of formal engineering credentials, the video suggests that gas system tuning, port size, and barrel configuration may meaningfully affect velocity beyond simple barrel length measurements. The overall spread is highlighted: from about 3,052 fps in the 20-inch M16 down to 2,183 fps in the 7.5-inch Springfield Saint, nearly a 900 fps difference. The host briefly references a separate test on whether silencers increase muzzle velocity, contrasting real data with video game portrayals of suppressors.