6.5 Grendel barrel Length?????

[Klem]

My take on this topic is that bullets are either accelerating or decelerating, which means they are never at constant velocity. Bullets accelerate down a barrel until friction overcomes the decreasing pressure of burning powder, then they decelerate. In 223's and 308's that's about the 30" mark. In a 300Blackout it's the 16" mark due to the relatively large volume of the 30cal barrel relative to the small amount of powder burnt. The Blackout was never designed to be a 'pistol' or 'SBR', those are simply arbitrary lengths and requirements placed on firearm owners by the US authorities. The Blackout was designed to be both a supersonic and subsonic calibre for the AR15 platform and 16" just happens to be the point where there is no reasonable benefit from having a longer barrel. Given the Blackout is such an average supersonic performer most 300BLK advocates concentrate on its subsonic capability, which is unique and reasonable. With subsonic you don't need 16" of Blackout barrel. It's actually easier to keep bullets subsonic but still with enough pressure to cycle to the action if you use a shorter barrel. That's why Blackout AR's tend to be short.

Back to accelerating and decelerating...The time/velocity curve is an inverted 'U'-shaped curve. This means acceleration is greatest at the start and lessens to a peak then the opposite occurs as friction and gravity take over. Rather than have the most efficient length of barrel, 30" we choose shorter barrels for weight and practicality. That means we are deliberately handy capping ourselves in velocity. But given velocity gains and losses are marginal towards the peak of the curve then the practical aspects of a compact rifle end up more important. In that scenario the bullet accelerates to the muzzle and decelerates from that point on.

Quickload displays time/pressure and distance/pressure curves for various scenarios. I am attaching a Blackout to show how you get negligible gains in velocity after about 12" of barrel. Plus QL predicts how much powder is burnt for various barrel lengths and how much muzzle pressure (which is synonymous with sound). The shorter the barrel the more likely there will be unburnt powder out the muzzle. If not all powder is burnt then there will be flames and sound and less consistent muzzle velocities. QL is great for doing what-if's with powder and barrel lengths.

Barrel length/Pressure/Velocity curve for a 30" Blackout barrel. Note that after 12" you get almost no incremental benefit from a longer barrel.
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Footnote: I have hunted with both Blackout and Grendel. My experience is that in supersonic hunting the Blackout is almost a basket case compared to the far superior Grendel. I have never tried making the Grendel subsonic so I can only imagine the heavier Blackout would be superior, and with less problems getting the action to cycle. Subsonic Blackout hunting is like shooting animals with an accurate .45 handgun - same weight and velocity of bullet. They die slowly and this means for the same weight and size of rifle you are handy capping yourself. Subsonic hunting is more akin to bow hunting than rifle shooting.