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Here's the Scout Marine entry in the 4th edition codex where you can see that the BP/CCW loadout is standard, with bolters and shotguns being free alternatives and sniper rifles as a special option. And since they're considered light infantry by Astartes standards they're frequently expected to operate in tight quarters where a full Marine, especially in power armor, probably won't fit or will otherwise find his combat effectiveness hampered due to his size relative to his surroundings. The Salamanders trilogy has a flashback scene by Chaplain Elysius to his time as a Scout where they were playing cat-and-mouse in the sewers with a few Genestealers so they could tag one with a tracking round and find its nest for the full battle-brothers to then cleanse and purge. Not the sort of job you can send Tacticals on and get good results given how big and noisy they are, on top of being unable to go down narrow passageways.
So, bolt pistol and CCW for light, speedy, and quiet movement, especially over broken terrain, and there's a lot of things that even a Scout can handle in close combat, especially if they've managed to get the drop on them.