I believe full combs were to lessen the felt recoil to one's cheek via a larger less knife like surface area. Trap shooting is a fairly rapid fire event with only short breaks between stations. ATA events are 100 targets and usually shooters shoot 200 to 300 targets plus practice on a given day. At least that is how it is done now.
Here is a picture of two V's, one with a full comb one more standard, both 1 1/2 frames.
I do know I would benefit from some cast off on the full comb gun. One effect is I find it harder to look straight down the rib.
