im a lefty and i just learned to adapt, at first i was using an aug.... but after a few years i have now switched to a m4 and have found it to be just as easy left handed as it is right. the mag release i hit with my right thumb as im switching mags. on any pistol i use my left index finger to hit the mag release instead of my thumb, you cant pull the trigger and change mags at the same time anyways. i can also clip the slide release with my index finger also... keep in mind i have rather large hands but still works very well.
the only thing i have found to be tough as a lefty is the bolt action snipers. i have successfully pulled it off but i have to re position the gun to center chest and use my right hand to pull the bolt back prone, its very awkward but can be done.
i don't have a bolt action sniper myself but i know you can switch the bolt to the left side on most models by drimmeling the stop on the left side and flipping the bolt over. this is something i'd consider doing if i was going to build a sniper. I just adapt when im using a friends for fun.