TOMMYAIRSOFTER, everyone else in the thread has explained it pretty well, but as I used to live and play airsoft pretty regularly in Mississippi I figured I would just throw in my cheap .02 cents real quick. Yes, you can wear a BDU top year round without dying of a heatstroke, just know your limits and drink plenty of water. With that being said, you are going to be significantly more comfortable in a combat shirt, especially if you plan on wearing a plate carrier over it. The only time I found myself wearing a BDU top versus a combat shirt was during more "green side" style operations where I was using a chest rig and I felt the added camouflage was helpful. If I was in your position, and I had to choose between the two, I would go with the combat shirt. There is nothing stopping you from picking up a combat shirt first, and then down the road picking up a cheap used BDU jacket (If it's in m81 woodland or DCU, standard tops can be had for pretty cheap).