A 16" bull barrel is going to be heavier, no doubt. It will work though, but if you are buying a new upper specifically for 3-Gun, look at what the better shooters are using (not necessarily the top guys, they have sponsors to please and could beat most of us with a 24" varmint gun).  The choices are typically 16-20" in lighter contours. If you plan to shoot nationally (specifically out west where the ranges are bigger), you will see more 18-20" guns. Matches tend to have more short range targets in the eastern US, so 16" guns are more popular.

We build custom competition uppers in 16, 17, 18 and 20" (Also a 14.75" for guys that have a special upper for hoser-only matches).  The shooters we supply that are consistently in the top 10 are all shooting 17" uppers as their primary choice. Long enough to have decent velocity out to 400 yards and short enough for tight courses.