I am completing a custom build and am curious is anyone have reason to suggest one length being better than the other. The specific purpose will be long range shooting. I understand barrel make up, chrome lined and all but am specifically looking at optimal lenght. My understanding is that you only lose about 30 fps on the 18" and 20" provides an optimal burn off. The 18" also could provide a bit more rigid barrel and more stable platform. I am not sure if there will be a noticeable difference and am curious for thoughts. I am considering Noveske, Black Hole, DPMS, and a couple others but am open to suggestions on make as well. Last, any personal experience with Vortex or PFI scopes?