I've been wandering about the same question and found a lot to read at http://varmintal.com/17hmr.htm
including a graph of velovcity for differant barrel lengths and these figgures:
Bullberry Barrel Length vs. Velocity DATA
22 inch = 2517 fps, 103 fps spread
21 inch = 2523 fps, 66 fps spread
20 inch = 2515 fps, 86 fps spread
19 inch = 2538 fps, 83 fps spread
18 inch = 2522 fps, 41 fps spread
17 inch = 2505 fps, 43 fps spread
16 inch = 2492 fps, 53 fps spread
15 inch = 2477 fps, 27 fps spread
14 inch = 2432 fps, 70 fps spread
13 inch = 2419 fps, 32 fps spread