All of these shirts were worn as an undergarment. I use the Glaco V-Hawk in its various iterations for every gun I carry(various sizes of 1911). I have an El Paso Saddlery dual mag pouch on the weak side.
Various brands standard cotton A-shirt. The veritable wife beater. Form fitting, has plenty ease of movement. Absorbs moisture well, little if any evaporation or cooling. These can bunch under the holster, and have the potential to gall. I did not experience this, some discreet tucking and tugging was needed. You can get these in virtually any color you want. Very affordable. Great for non summer, non high heat and humidity use. This is my cool weather go to.
Hanes Cool-Dri. Purchased for me by my kids. 100% polyester. True to size. Very easy to move around in. I would recommend a size smaller to fit better the rolls and bulges around the waist, and to reduce the chances of bunching. Feels slicker than snot on brass doorknob. Wicks moisture quickly, very cooling, moisture evaporates quickly, even through the outer shirt/cover garment. If they weren’t Hanes, I would be buying these. Come in long and short sleeve versions. 4 to a pack. Very affordable.
Carhartt Cotton Force, moisture wicking shirt. Cotton/Poly blend. True to size. Wicks moisture well. Little if any evaporation, even in light to moderate winds. I think I would likely get too hot with this shirt as an undergarment. Sweat/heat rash might could be a problem too. On the high end of affordable. The missus found this one at a close-out store for $19. I don’t recommend this as an undershirt.
Duluth Trading Co. Don’t remember what it is called. Bought it in the underwear section. 93% nylon, 7% spandex. I bought a size smaller. A meshy kind of fabric. Transfers moisture very quickly. Evaporation is fast. Even through the outer garment. No bunching was experienced. You might forget you have it on. Spendy at $25 per. Highly recommended.