Had a blast playtesting some combat rules last night with Oliver and Russel, thanks guys for your input. Your experience supplied excellent feedback! Doing some post session analysis I think I missed up on some things. One of them was using tracer bullets to aim machine gun fire or using bullet impacts to do the same. That was not considered during the usage of the M249.
Towards the end of the game the teams were making a quick exit from a town on a stolen pickup. They were driving through streets, markets and alleyways using their night vision goggles and shooting anything that got in their way opening fire with their M249. On various occasions they strafed and strafed. I realize now that the usage of tracer bullets should have improved the gunners precision after each strafe, right? So he lights them up, misses, then comes back again with better aim. The moving truck they were on added some complexity to the shot, but the tracers should have given a good feedback on how to adjust aim.
There is also the aspect of visible weapon impact. During one of the turns they end up in a plaza with an enemy occupied building. Soldiers begin to pour out and take positions on a balcony. The first M249 sweep misses and comes low from their target. Regardless of tracer usage, at that short range and with night vision goggles the impact of those bullets should have been clearly visible against the wall (dust and wood flying off), allowing the character to adjust up on the second sweep and get a bonus.
So rules will be modified to consider fire impact adjustments. Sustained MG fire with tracers or some way to clearly see were the bullets are hitting (impacts that cause visible debris: dust, glass, water) will add bonuses on the next sweep roll.