http://www.youtube.com/watch?v=AGwkHktkTxU

Now keep in mind that ARMA 3 plays in 2035, so the issues the Colt M4 has should be fixed in the future guns they use if the HK416 already could do that 2008.

Pretty much any gun which has enough drain holes and doesn't use direct impingement gas system can do that, so even the US Army's improved FNHerstal M4 with a gas piston system, probably won't have that problem anymore and that should be issued by 2018.

While this could be added as an option, it is definitely not a high priority issue, as it only affects a small minority of players.
And about the free advertisement, if you ask me BI should focus on making it a better game and not on advertising it, so again low priority.