I have no idea what an A shirt is. I googled these and did not get a straight answer.
My own full sized High Noon OWB leather holster protects my full sized CZ 97B 45ACP completely. All contact is metal to leather, except for the grip of course.
I wear this like an IWB holster by putting it on its own gun belt and strapping it on over my skivis and T-shirt and under my trousers and untucked shirt (over-shirt). I can also tuck the shirt if I want, and wear a sweater over the shirt. Normally I wear a jacket over this as well.
Ergo my trousers conceal the lower portion of the holster, and my over-shirt or sweater conceal the upper portion. This results in perfect concealment, and yet I can still draw almost as fast as from open-carry.
So for those wanting an IWB holster I recommend an OWB one and then wearing it under your trousers on a separate gun belt.
For summers, a sleeveless cut-off sweatshirt works fine and is plenty cool, with perfect concealment.
For winters, I layer my clothing of course, with easy access to the pistol at the 3 oclock position IWB with the OWB holster.