Actually, if you put armor piercing rounds in the minigun and fire at anything that has enough armor, you'll actually do less damage than standard bullets! Yay!
I didn't believe this was true but I ran some numbers
0 -> i{0} -> j{0} -> kfor i,1,30,1 ; standard rounds max(12-i,12*0.2) -> j[i] ; armor piercing rounds max((12-max(0,i-15))*0.95,12*0.2*0.95) -> k[i]endforDT norm APi j k-----------------1 11 11.42 10 11.43 9 11.44 8 11.45 7 11.46 6 11.47 5 11.46 4 11.47 3 11.48 2.4 11.49 2.4 11.410 2.4 11.411 2.4 11.412 2.4 11.413 2.4 11.414 2.4 11.415 2.4 11.416 2.4 10.4517 2.4 9.518 2.4 8.5519 2.4 7.620 2.4 6.6521 2.4 5.722 2.4 4.7523 2.4 3.824 2.4 2.8525 2.4 2.2826 2.4 2.28(repeating)
So, it's true. At DT of 25 or higher, standard ammunition in a minigun does 5% more damage than armor piercing. Realistically I don't think this is really enough to get stressed out about, since the armor piercing obviously fares much much better at any lesser amount of armor, and the difference at DT 25 or more is only 5%. Let's not forget that the minigun shoots 20 rounds per second, too, so it will do more than 40 DPS to *anything* regardless of how much armor the target has has.
Logical failure yes, gameplay balance failure not so much.