Couple things that bothered me...

[garhkal]

Yup, if they had toned it down a little it'd been more realistic.

I disagree. We saw back in early SG1 how a good tracker can discern info, like what master Bri'tak did with that woman prisoner in maternal instinct.

[Arga]

I guess the reason why they never shot the head is to keep the violence rating of the show low. And also because it is probably much easier to simulate gun wounds in an area of the body which is covered.

if it was a known fact that heads were a possible target, maybe they would wear helmets too, when they go on away mission...

I guess basic military usage is uncompatible with actors, effects and audience.

[Voxyn]

I thought they didn't wear helmets because they wanted to look less threatening.

Since most of the societies they meet are stuck somewhere in the Medieval era they probably wouldn't recognise the P90s as weapons until they were fired.
But they would definately recognise helmets and would feel threatened by these strangers wondering everywhere asking questions.

[Arga]

ah! very good in-universe explanation!