In reality, you can't fit a sniper rifle or rocket launcher in your jet's cockpit with you. It's simply not possible. So why not emulate this in-game?
Certain seats such as gunner, pilot and commander slots should not allow the carrying of Rocket launchers, Sniper rifles, and for some certain slots, perhaps even long-barreled rifles.
Passenger seats would allow units to carry these, as they can be put between the legs, on laps, etc.
And certain spaces (such as MRAP gunners and drivers) could allow players to carry long-barreled rifles
But some slots, especially pilot and gunner seats in aircraft (and perhaps armor as well) should not allow anything larger than a carbine to be carried comfortably within them, as there isn't space for such an item in the cockpit.
If a player wishes to bring a rocket launcher along on their helicopter transport mission, the inventory is the realistic place to store it.