I posted this in the target shooting forum, but I thought it might get more answers here:
I was going to use a 30" barrel for a 6.5-284, but I read an article in Rifle Shooter about velocity and barrel length and now I'm not so sure. In testing, longer barrels did not produce particularly great increases in velocity. Longer barrels do flex more and reduce accuracy (slightly). Given that weight is a consideration in my class of shooting (9kg or ~20lb rifle with scope) I'm now considering a 26" or 28" barrel with a heavier profile.
Comments?

Try posting it at www.benchrest.com in their forums, serious target shooting is all those boys do. I am sure you'd have an answer from top shooters straight away.

I don't know what shooting discipline you shoot in but, the shooters using the 6.5 X 284 in 600 and 1000 yard F class competition and conventional long range competition all use 28 to 30" barrels on their 6.5 X 284's. I have seen several 32" barrels on the firing line.

Shoot Safe, Shoot Straight......RiverRat

I agree about the comment that you should go to www.benchrest.com and seek their opinions. However, let me tell you of what I know and some conjecture.

The issue of barrel dimensions in relationship to grouping potential (precision) is related mostly to stiffness(in the context of this question). As barrels get longer, they need to be disproportionately fatter to maintain stiffness. There are some tricks to this, such as fluting the barrel, but the basic facts are impossible to avoid. As a barrel gets longer, it gets droopier and needs more fatness to stay stiff. Yes, I am still talking of barrels.

You should be able to garner a few extra fps with a longer barrel, especially in this caliber. My 6.5x.284 is built for heavy bullets and shoots a slow burning powder. Such rifles get more out of extra barrel length, but there is a point at which the penalty exacted for the added velocity is too great. If you intend to shoot 1000 yards, you do not need an excessively long barrel, and the high ballistic coefficient of 6.5mm bullets is a real plus and the initial velocity is quite high for that caliber.

What you want is a muzzle velocity that will ensure that the bullet does not drop back through the speed of sound at or before your intended range, if posible. Work backwards from that with a computer or set of ballistic tables and see what it takes. It may be that you can acheive your needed velocity goal with a 28" barrel (that is what mine is). After all, a 175 gr bullet in a .308 Win can acheive the needed velocity without any special problems, so a 6.5x.284 should be a snap.

Hope that helps. Be safe.