Well there are really only two option left, Boltguns and Heavy Weapons.
I am not going to talk in any detail about the Boltgun, because any Space Marine player worth his salt has their own opinions about the standard Space Marine issued fire arm.
The Heavy Weapons that scouts have in their tool box are only two choices. The first is the Missile Launcher. This is a great choice because it gives you two different modes to fire. Kark and Frak, I like to give one of my scouts this weapon if they are in a Sniper Team with Sgt Telion, it can be the on hit that is needed to take out the tank. When using Sgt Telions special rule "voice of experience" your Heavy Weapon can fire with a BS 6 so when you have to make it count. That is really the only time that I spend the pionts on the Missle Launcher.
The second is the Heavy Bolter, this one also has two modes of fire for the scouts, the standard and a one shot hellfire shell that uses a large blast template. I am not a big fan of the heavy bolter to be used with the scouts. In my mind I think of them as a mobile force, and the heavy blter just seems like it would slow them down. That being said I do field it with the scouts I have use it with two sqauds of five, each with 4 Bolter Guns and 1 Heavy Bolter. What I did with them was influtrate both of the squads on to the same objective and and have them just shoot, shoot, and shoot. It worked out well because I was able to hole that objective, because it gave me two shots with the hell fire shell. One form each of the squads, by turn 5 I had one scout left and he was scoring. I don't know if this would work for every one, but it worked for me in that one game.
So those are the basic scout weapon layouts.