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The difference is much less than you might think, the loads shown below had a mean difference between 16" bbl and 20" bbl of only 123fps, so 60-65fps difference on between a 16" and 18" go figure...

I can't tell you the difference between different length SCARs but I can tell show a chart showing the muzzle velocity of 3 different loads fired from two different rifles on the same day under the same conditions.

Temp 72
Alt 5200

.................DSA SA58 16" bbl..........FN PBR XP 20" bbl
American Eagle .308Win 150gr FMJBT #AE308D
...........................2538.............................2669
...........................2537.............................2681
...........................2506.............................2703
...........................2538.............................2635
.....................Avg 2543.......................Avg 2663
....................SD......32.23................SD.......32.81
120ft/sec difference

Federal .308Win 150gr Soft Point #308A
...........................2588...........................2724
...........................2594...........................2741
...........................2595...........................2729
...........................2582...........................2736
...........................2605...........................2726
....................Avg 2593......................Avg 2731
....................SD........8.54................SD........7.07
138ft/sec difference

Barnaul .308Win 168gr FMJBT
...........................2424...........................2554
...........................2388...........................2515
...........................2455...........................2507
...........................2379...........................2505
...........................2399...........................2518
.....................Avg 2409.....................Avg 2520
....................SD......30.74................SD......19.92
111ft/sec difference

This means even that even budget .308 American Eagle FMJ is hypersonic past 700 yards from a 16" bbl

Sign me up for an FN SCAR-Heavy with a 16" bbl!
 

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Barrel length doesn't determine accuracy...
That being said, why do most 308 sniper rifles have 24 inch barrels? If a sniper could get the same effect out of a shorter barrel I think they would use it because it would be lighter, Right?
 

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devildoc said:
Barrel length doesn't determine accuracy...
That being said, why do most 308 sniper rifles have 24 inch barrels? If a sniper could get the same effect out of a shorter barrel I think they would use it because it would be lighter, Right?
Light is good up to the point it decreases your ability to hold it steady (target rifles tend to be heavy, plus there's the issue of variation from the first shot in a cold barrel to subsequent shots, favoring thick and therefore heavy barrels), but often a sniper is trying to "reach out and touch someone" at long range, and the extra muzzle velocity helps. A bit more reach, less wind drift, etc. If he's trying to make a marginal shot it could make the difference.

- Harold
 
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