TexasCHLforum.com

Most of the discomfort is from the gun, not the holster. When carrying IWB the holster is between the pants and briefs and maybe a sweatshield touches the skin... but the whole of the grip an often part of the slide contacts the skin.

An undershirt is (for me) always more comfortable. 10X more in the summer. But it's not always practical. I do notice that IWB without an undershirt is much more comfortable in the cooler weather when you don't sweat. I'm fine without an undershirt if not sweating (the gun sticking to damp skin is where my discomfort comes from)

And then you have material... leather is softer to the skin, has softer corners and fewer things to scratch/irritate you. While Kydex generally has sharper corners and other hardware exposed, well made kydex designs can be very rounded and easy on the skin.

I rarely wore an undershirt before I started carrying but eventually experimented with different syles and materials and found some really comfy ones.