Hi All
I am considering getting a 17HMR and wish to use it from a vehicle a fair amount so a shorter length is better than longer.
Also i would like to fit a sound moderator as well so i need your thoughts on the minimum barrel lenght required to obtain the advertised velocity for the 17HMR
thanks in advance

24 to 26" to obtain facorty advertised velosity

Depends on the tolerances of the barrel. I had a Ruger 77/17 with 20" barrel that would shoot in the mid to high 2500 range. I replaced that barrel with a 20 or 21" Clark barrel with a match chamber that shot in the high 2400 to low 2500 fps range. The fired cases from that chamber were noticably enlarged in the shoulder region though it was super accurate.
My sons 20" CZ FS shoots in the high 2500 fps range. My CZ Special with 25" barrel breaks 2600 fps more often than not. FWIW.

In answer to your question I would feel very comfortable with a 20" barrel giving factory velocities.

In my testing of a bunch of 17HMR's, Marlins, Savages, Remingtons, NEF's, the norm is for all of them to be ABOVE the advertised 2550fps with the 19-21" barrels and this is from hunde=reds of rounds fired over the chronograph on a lot of different days and conditions!
Seems to me that BullBerry did a test on a match barrel where they shot the barrel at 22", removed an inch, shot it at 21" and did this all the way down to 10". What they found was on optimum length of right around 19" BUT the surprising thing was that they lost very little as they stepped all the way down to 10"! From my own experience with my personal 17's, my Marlin VS will average around 2630 with the 17 grain Hornady loading and Remington 17 will be almost identical. CCI usually is a bit faster but accurracy sucks!!! In my 10" barreled Striker, I get 2310fps with the Hornady 17 grain loading and 2200 with the 20 grain Hornady loading. Sticking a long barrel on a 17HMR would seem to be a waste of metal!!! GHD