I'm not sure if this is just a glitch for me, but all scout rifles will have one less round in the magazine than their descriptor says. So instead of 18 or 15 rounds, I have 17 and 14 rounds respectively.
It would be nice to see this clarified.
I have the same issue. I received a scout rifle in the mail when I reached Vanguard Rank 3. It lists the magazine as 14, but in-game it only holds 13 rounds.
I found this in the help section, though I didn't see a date on it:
MY SCOUT RIFLE MAGAZINE SIZE DOES NOT MATCH THE ADVERTISED SIZE IN THE WEAPON DETAILS TOOLTIP.
We are currently investigating a fix where scout rifle magazines are one number off from the advertised magazine size. We thank you for your patience as we work on a fix.