I like the ballistics by the inch site , very interesting. Also interesting to see that the velocity from the 16" barrel is slightly higher than from the 18" barrel ?
That is called friction. Once the force of acceleration of expanded gas is exceeded by friction, the bullet will slow down. You will also notice that with most pistol calibers u quit getting significant velocity gaines after 12". Wished they had the resources to do testing with rifle calibers so we could see the results there, we do know it will be longer length for sure.