Regen mob is kinda confusing when you come hunting it.
There's a theory that the loot return is based on the damage you deal on it, and is CAP at x3 its hp. To me, it seems very true on ambu.
You can use very powerful gun setup to kill it fast, or just a lower powerful gun to kill it slowler but gain more loot. It's based on my auctual experience. Because with the p5a+104, it seems that you need more peds per ambu so you will loose for sure. But for the long run, you get back all you spent. Quite confusing, isn't it.
You may laught if you know that I use Kesmek Vass, a lvl 18 shortblade to hunt ambu for skilling. It has 51/51 attack and 26~52 damage only. The killing speed of course is very slow, even 1.5 times slower than P5A+104. In the end, the loot return vs cost is still the same as using bigger gun for long run.