perhaps it's just the local player base...or maybe just that there's been an explosion of new players this past year..
but the difference between the two groups of players has never been clearer (IMHO).
There's guys who in general prefer "mil-sim" (aka serious, aka realistic, aka stuckup, aka "elitist") events that have structure, objectives, planning, real cap, limited ammo, comms, etc. In truth, some "mil-sims" are very easy going and probably not much more than a formalized skirmish...others are very well done.
There other type are the "fun" games. No ammo limits, short-ish skirmishes, lots of head-on confrontation. They're lots of fun too. They tend to draw out newcomers and lots of them. Drop of the hat and you've got 30-50 guys (at least at the local place here) ready to go. It's an absolute shoot fest and a lot of "bunker standoff sniping" but they're fun for a no-worries type of fun shoot.
At a mil-sim...in decent temperatures a reliable GBBR would hold it's own. I'd take 12-14 mags, shoot semi and leave the suppression to the SAWs. GBBR vs. AEG at a milsim...I expect to be just fine. Indoor or outdoor. (NOTE: GBBRs do not do well if you're shooting down a ladder at the floor/room below you....you'll get a lot of gas venting

)
At a fun shoot...you're going to be at an ammo disadvantage, depending on your style of play. If you want to suppress and trade round for round all night you just won't have enough ammo. For most fun shoots guys tend to run mid-caps and you're looking at a 4:1 ammo disadvantage. But...real caps are fine, just depends on you shooting buddies and how you play.