This is not an exact comparison, but I have data that is close between 18”, 22” and 26” 308 barrels.
Basically my measurements show about a 25 fps loss/increase per inch of barrel.
At 600 yards, it all depends on what you want to do. The longer barrel gives extra velocity and I would think that is an advantage a long range.
PTR91 18" barrel
150 gr Australian Ball MF83 headstamp
30 April 2008 T = 75 °F
Ave Vel = 2599
Std Dev = 23
ES = 63
High = 2630
Low = 2567
N = 10
150 gr Hornady FMJBT 43.0 grs AA2520 wtd Lot 9595 Czech Mixed LC WLR
OAL 2.785
30 April 2008 T = 75 °F
Ave Vel = 2544
Std Dev = 32
ES = 84
High = 2574
Low = 2490
N = 5
Ruger M77 Tactical, 26” Barrel
150 gr Australian Ball MF83 headstamp
30 April 2008 T = 62 °F
Ave Vel = 2819
Std Dev = 19
ES = 64
High = 2859
Low = 2795
N = 10
150 gr Hornady FMJBT 43.5 grs AA2520 wtd Lot 9595 Czech Mixed LC WLR
OAL 2.785
30 April 2008 T = 65 °F
Ave Vel = 2756
Std Dev = 28
ES = 97
High = 2806
Low = 2709
N = 10
M1a 22” Douglas 1:10 barrel
150 gr Hornady FMJBT 43.5 grs AA2520 wtd Lot 9595 Czech Mixed LC WLR
OAL 2.785
18 May 2008 T = 70 °F
Ave Vel = 2700
Std Dev = 30
ES = 85
High = 2736
Low = 2651