SMAW can be good or bad though. I attempted to do the same thing that you were describing and a sniper hit the missile causing it to explode in my face resulting in a headshot. http://www.youtube.com/watch?v=_XzumsREhbs&feature=plcp
There is no "best gun" in MW3. Yes, there are some that perform better than others, but in the right hands, they all work. ARs work better on some maps than other guns, SMGs are best on some maps, LMGs ... all of them have their niche. SGs are best used with Overkill, IMO, but they still work great - in the right situation.
N*gga pls PP90M1 Rapid Fire w/ extended mags
- or -
ACR R.D.S. HB Sensor/Grenade launcher
.44Magnum w/ TK / Akimbo
Extreme Conditioning Pro
Assassin Perk Pro
Steady Aim Pro
Support Strike Package
UAV,SAM TURRET,Escort Airdrop
Dead Mans Hand Death streak
This is a good combo to level up with, for them noobs.