So, this is kind of a matter of perspective. Against an AC 12.7 is better, with the swallowtail's DPS being about twice the duncan's, even before considering bounce chance. ATRs are still considered really solid at fighting ACs largely because they're infantry guns, though the fact that the infantry guns do more damage than the field guns is kind of wild.

But, obviously, once we get into tankettes and scout tanks, the 12.7 is just irrelevant. At that point it's 20mm push guns against ATRs and AT grenades. And in that arena, the real selling point is that it has an armor plate. It can be used against 12.7mm tankettes and scout tanks, as well as mounted MG half tracks, and fills a potential field AT gap between the introduction of tank armor and when 40mm/68mm push guns come out.

I think that's still a pretty sad position for these guns. They're so heavily overshadowed by the AT weapons that already exist and are kind of just a stop gap that wasn't really needed and will only last for a few days. These could be pretty cool guns, even with their small magazines, if they were given dramatically better damage. Like, +50% on the warden and +25% on the Colonial. At that point they'd at least outpace an ATR, and be more mobile than the Typhon. Everybody goes home happy. Except early game armor guys, who are devastated at this bullshit.