Everyone should be a rifleman first... something else second.
Why this matters is that a rifle is always a solid backup weapon to go to. Hey look the saw is down, pull out the m4. or what have you.
If you know you're going machine gunner, a great choice is to go with a super light weight aeg to compliment it. Something like an mp5 or a small m4. Light, easy to carry, you can dump lots of gear and carry a light load (mp5 you can carry an entire mag loadout on a belt and have room to spare) An m4 make sense because you can carry mags that will still work in the 249 if the box mags are empty.
Why might a light weight gun/rig be important?
When you're lugging around 20 pounds of gun for those extended games, you're going to run the risk of overheating, or just plain tiring yourself out. Being able to shed most of your gear, hand off the saw to someone for an hour or two maybe, and run a light weight gun for a break means a LOT. Sure you might not think that it's a lot to carry, but it grinds you down over the course of a day, add extreme heat or humidity... or rain... it will suck.
Also for any up close engagement or indoor, a 249 will not shoot semi auto for those games that have semi only rules for safety.
Even if you dont' think you'll be a hard charger, having a regular aeg doesn't mean you will or won't do an assault role. You do whatever role the situation will dictate.
Go play some games where you can rent a gun first or borrow. And ask politely to hold other player's stuff to see what you like. You may not like an m4 after holding it, you may not like the 249 after holding it, you may love it.. you'll never know until you do.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.