For me, the words "Full-Size", "IWB" and "Comfortable" cannot be used in the same sentence. Hiding a full sized M&P takes some work, and making it comfortable just compounds the task.

I can get away with carrying a mid-sized (4-inch barrel) M&P45 during the winter months when I can dress appropriately. Going with a pancake style holster makes it comfortable enough to wear all day long.

I really like this high riding belt scabbard by Kramer:



It tucks in the butt of the gun very well. With a slightly larger than normal un-tucked camp style shirt, it easily goes un-noticed.