The topic is updated. 
Ah double hit, thats why. So therefore you don't use the HVB (to avoid 30% less dmg)?
Usually I build AUI Snap Freeze (5b Cryo Explosion;6b Tech Combo) in combo with the HVB and elemental Ammo.
Does kill a chilled Dragoon with a TC Body-shot, too.
On the other side, you did the Solo, and I still suck
Remarks / suggestions / criticisms are always welcome. You're right. You can stack armor mitigation and cover penetration with AP rounds and piercing mods, but then, when you shoot through cover, you get the damage penalty from the mod (while there isn't any damage penalty with AP rounds alone). That's why I took the HCB instead of the HVB, as ignoring 100% of armor instead of 90% isn't really a big deal on a high damage per shot weapon like the Claymore, and AP 4 are enough for double hits on an Atlas' canopy. At the opposite, I was relunctant to loose 30% damage per shot through cover, even more that Phantoms hide frequently behind various structures.
As for the build, the 5a option in Snap Freeze is both a crowd control choice / console scrubbery adjustement. When their speed is reduced by 35%, Dragoons and Phantoms basically move at the speed you can adjust the joystick.
This is even more important that the Claymore looses a big part of its damage past a few meters, so you're constantly in CQC with this gun. I took the bet that reducing my % of missed shots and increasing my % of headshots would do more for me than a 10% debuff. It's possible that I'm wrong, I didn't make tests before doing this solo. With a mouse in hands, I would certainly take 5b.
Concerning the ammo, I agree that Snap Freeze + a Claymore shot with disruptor kills a plat Dragoon. But in many cases (when they spawn behind you for example), you have too fight them without the possibility to chill them first. I find it more reliable to be able to one-shot kill them. I want them dead, now !
In the opposite order (shoot then Snap Freeze), the Cloak breaks and I have to wait the end of the cooldown to spam SF. Sometimes disruptor stun Dragoons, but sometimes it doesn't.
Disruptor is certainly a viable choice though, it would just need a few adjustments. I would like to try it once, but shotguns
amps are always too rare.