For
7mm-08, or .308, you will be just fine chopping the barrel to 18", but 20" is optimal. Tac-ops, who builds arguably the best sniper rifles in the world, considers 18" the optimum length for an urban environment. http://www.tacticaloperations.com/SWATbarrel/ 30-06, with its slower burning powders, would be better served by a 22" barrel if you are planning on longer shots.