Area of mesh refusing to smooth properly

[Wrangler249]

I'm relatively new to modelling, so please forgive me if this is a stupid/simple error that I am getting stuck with.

I'm making an assault rifle, and held off from smoothing it until I had the basics down to my liking. When I smoothed the sides of the model, I noticed that there was a certain area that refused to cooperate and thus looks shit.

Here, see for yourselves:



Please don't critique the model itself, I'm only interested in finding out what I've done wrong so I can learn from it.

Thanks

[polycruncher]

http://www.free3dtutorials.com/modeling/3d-max/smoothing-groups.php

This may help.

[crazycarl]

weld/target weld those vertices together.