This test was run by a company named MDT Sporting Goods. They recorded their test and it'll be linked at the end of the post.
The company’s engineers built a bolt action rifle with a six-foot long barrel to see what effect barrel length has on the projectile’s velocity for a given cartridge (.308 Win...) by gradually cutting the barrel down. Who makes six-foot long barrel blanks? Nobody, the barrel of this experimental rifle was made by screwing together two barrels! MDT claim that with an overall length of 88 inches, their project gun is the world’s longest rifle.
If you couldn’t watch the video, the chart below shows how the experiment went. The MDT engineers used Federal Gold Medal Match ammunition loaded with 175grain Sierra MatchKing bullets that have an advertised muzzle velocity of 2600fps from a 24″ barrel. Contrary to what many people would predict, the 6-foot long barrel did not slow down the projectile because of the friction – the muzzle velocity was 2785fps. In the first 20 inches of cutting the barrel, there was not much velocity drop – only 20fps per 10 inches. At around 30″ to 34″, every inch cut off of the barrel resulted in about 15fps of velocity drop. They cut the barrel all the way down to 19″ at which point the muzzle velocity was 2567fps.
I think the main image has some solid meme potential
Testing video: [7:47] https://youtu.be/XCqa2umL8ME?si=
If Sephiroth had a rifle
Monster Hunter was on to something with its Gunlances.