I was thinking about putting together a tactical rifle chambered in a 308 winchester. Question is, what have you guys found to be the shortest possible barrel for producing .5 moa groups out to around 300 to 400 yards? Would a 21 or 22 inch barrel work, or would it have to be longer?
Your short barrel length won't have much to do with keeping 1/2 MOA groups. Actually, in theory, a shorter barrel "should" be more accurate, due to being "short & stiff". Look at the BR crowd. They shoot short tubes & turn in some incredible groups. You will give up a little in the velocity department out at 400 yards, but if the velocity levels meet your requirements, the accuracy of the short barrel should be fine.
If you use an Obermeyer or Rock barrel and maybe a Broughton 5C barrel, a 20 or 22" tube will be just fine. I have a 20" Rock on a lightweight 308 (but still heavy barrel contour) that gives me 2650 fps with a 175 MK over 45 gr of Varget. Makes it to 1000 no sweat. Personally 22" is about the ideal, however.
I don't think that getting a shorter barrel would affect accuracy adversly , I actualy think that it would help , giving the facts that a 20" barrel that is the same diameter as a 26" will be stiffer
I think that the only problem you would have would be the velocity loss and that realy isin't a problem unless your trying to shoot way out. Even without one of the faster barrels like Rock's I'm sure that with handloads you could get the 175gr MK's to 2500fps.
And if you are only gonna be shooting out to say 400yds , personaly I'd shoot one of the lighter bullets like a 155 to shorten the time of flight , I doubt that you realy have to wory about high BC heavy bullets at less than 500yds. BUT I could be wrong