Free: Contests & Raffles.
I just got myself a dedicated dog gun. It's got a 16 inch barrel with a 13 inch hand guard. People will tell you 18's shoot better, but are harder to wield in the woods. 16's are "less accurate" but more maneuverable. I honestly don't think there is enough merit or difference to either argument to warrant any stress. I like the look of the 18's but would rather a bit more ease when hiking, as I enjoy doing a bit of that I think you will be fine with whatever you do.
Hardly any difference. If you want to do it right, go 18" with a rifle length gas system, 13.5" or longer handguard. It's a 18" with the performance of a twenty. How wrong can the MK12 be. Can't wait for the "Google Foo" types to get all spun up on a rifle length 18".
Some more food for thought...So I have the 16". My buddy has a 20" and a 12.5" with suuuuper flash hider on the end. After finding ammo the guns like, we shoot groupings nearly identical. The only one that lacks a bit is the 12.5, but it is not free floating, and has cheap glass on it. However, while the groupings are similar, I have a lightweight barrel on my gun, and he has heavy barrel on his 20", the weight is drastically different. you can easily notice just lifting them. I would say the length isn't as big of an issue as the weight. I'm sure some other guys with more experience will chime in though.Long story short, don't sweat it. And the longer foregrip is sexy. Do it!!!