I just enrolled in a class to get my CCL and have started researching ammo. In this research, I have come across quite a few posts where people mention that they carry the same ammo with a different bullet weight in the winter. Mostly it's been 155/165gr in the summer and 180gr in the winter. Needless to say, this got me thinking. Is this a matter of in the summer people wear fewer layers of clothing and thus there is less of a barrier for the bullet to travel through for self defense? Where as in the winter, people will have an undershirt, shirt, fleece and a jacket and the more layers create more of a loss in energy of the bullet before it hits vital organs? If that's the case, perhaps you can help me in my specific situation. I will be carrying a G23 or a G27 most of the time (if funds allow I may get another caliber in a subcompact size), so .40s&w is my caliber of choice. I live up in the mountains at 9000 feet, so during the winter (at work, in my town, around my house) it will be below freezing most of the time and many people will have layers of clothes on. However, since the town I live in has a population of ~750 people, I have to drive down to the closest town (16 miles away) that has a Walmart and other big stores, to do my grocery shopping. This town is at 4000 feet and during the winter, it doesn't get all that cold during the day (it's in the desert). While I guess I could have a mag of 155gr, a mag of 165gr, and/or a mag of 180gr and swap them out depending on the temperature, this would just be a hassle. I seem to prefer the 180gr in all of my practice ammo, though I've only shot a couple hundred 165gr (the recoil is just a bit too much for me to shoot it quickly yet accurately). So, do you think I need to worry about changing out ammo based on the temperature? Or should I just find one bullet weight that I can accurately shoot and stick with it year round to ensure I never accidentally grab the 165gr when I meant to get the 180gr and am thrown off my focus when the recoil is much more than I'm expecting?