I don't understand why some of you are saying a fixed stock would balance better than a collapsible one... a fixed stock is hollow plastic... I doubt it weighs much, if any, more than something like an Vltor.
The thing that affects balance a lot more than what stock you have is what weight of barrel you have. A big fat bull barrel or HBAR will be front-heavy no matter what stock you have. And just because it's a 20" barrel doesn't mean it is heavy... a GI-profile 20" barrel, like what BCM sells, is not heavy at all. It weighs less than some of those 16" barrels that are really fat under the handguards, like what DPMS puts on a lot of their rifles.
In fact, if you want a 20" for 3-gun, I recommend a gov't-profile barrel rather than an HBAR, because it will balance and handle better, and it won't be as hard to lug around. It will still be plenty accurate for that type of shooting, especially if you put a free float handguard on it.
And whoever said you can't get a cheek weld on a collapsible stock is totally wrong. A collapsible stock has the same height as a fixed stock, and puts your eye at the exact same level when your cheek is properly rested on it. It is no harder or less comfortable to rest your cheek on.