The issue:
I didn't think this was actually intentional or a bug, since Flan's shields were created pre-1.9 when actual shields were added to Minecraft, but the Riot Shield from the MW and Zombie content packs fails to have any real use in Singleplayer due to it lacking the ability to block melee attacks.

Suggestions for improvement:
Now that actual shields have been added to Minecraft, it would be nice if Flan's shields behaved like Minecraft's shields: blocking melee attacks and arrows, in addition to bullets. In context of the Modern Warfare pack, this idea isn't too far fetched since in the actual COD Modern Warfare the Riot Shield could block melee attacks as well.

Also, for map creators, it would be nice if mobs like zombies holding riot shields could block bullets as well, adding variety to enemies.