That's an excellent answer, but are there any instances of NCR troopers being issued with AP rounds in the game? Without them, all the Service Rifles in the world aren't going to dent a deathclaw's hide.
Actually 20% damage leaks through any amount of DT. Each shot with the 20-damage Service Rifle is reduced to 4 damage, and a deathclaw (250hp) would require 63 hits to kill. 8 troopers could fire 64 times in 8 seconds. Troopers could still get the job done, it would just take more men and better tactics.
I don't see any plausible reason why the NCR wouldn't issue armor piercing ammunition on such an exceptional and dangerous mission. The stuff is widely available.