You can really use anything you want. In fact there's a video of a guy doing an IPSC or 3 gun style course with a 249.
That said it's all up to you in the end. Really, underneath all our guns is a system that propels 6mm BB's with exterior styling around it (no such thing as different calibres other than 6mm and the odd 8mm rifle). You could get anything from an MP5K to a 20" "varminter style" M16 and it's all basically the same fundamentally.
That said if you're looking for short and light and like a plain jane gun something like a 10.5" CQBR M4 would fit the bill, so would an AK74UN underfolder or side folder or a MP5A5 with collapsible telescoping stock.
Also, if you want lighter and don't really care about "realism" then an option would be to get a plastic bodied rifle. Either the King Arms plastic body version or a G&G Combat Machine (though my preference if you're wanting plastic is probably the G&G CM). The G&G CM16 with the 10.5" barrel should be able to be found for around $225-ish. It's up to you to look around yourself since you're not AV'ed though and as you know no links outside the classifieds/AV restricted section.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE