Start lifting weights, princess. You have a year to kill anyway.
Seriously, do you really want to paint yourself into the weakest-link-on-the-team, always-out-ranged/out-ammo-capacitied/out-firepowered, last-to-get-picked corner because you didn't want to have to make the effort?
Maybe you don't mind settling, but if the above paragraph sounds ugly to you, you may want to rethink your approach. Just like anything else, there's a learning curve to using/maneuvering a big gun. I'm short myself, fullsize M16s and G36s look like anti-aircraft guns on me. That's never stopped me. My M4A1 was just a fraction too long to be "just right" so I converted it to Commando (733/933) length (this was before TM put out the 733). There's currently a lot of off-the-shelf Commando/CQB-length M4s.
As for weight, it's psychological. Unless you're getting some big wood and metal job, you're looking at around 3kg. M16A2 will be a bit more, M733 a bit less, but you're always in that neighborhood. You won't be running a marathon with it anyway, this isn't paintball; other airsofters are going out with a ton of gear on (gun, sidearms, lowcaps, radio, camelback, body armor, etc), if you go out light (gun, chestrig, a few mags) you'll be light enough to keep pace. Might be a workout at first, but it'll get you in shape. Players that go out with a full realistic load with body armor and a helmet, on a hot summer day, worked themselves up to that (or have real world experience); nobody "starts" off like that, unless you're trying to get heatstroke.
So if you want a shorter gun, fine, there's options. But IMO, you shouldn't limit yourself to tiny ones. Check out an AKS47, M733 or one of the other shorty M4 variants, P90 if you dig that styling (I've seen some upgraded ones hold up well outdoors), G36s (maybe a G36K more than a G36c), L85A2 carbine (more barrel length for less overall size), etc.
And get a sling, if you're afraid it'll tire you out. It helps.
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