PDW was a concept that emerged in the late 20th century. at that time, ceramic armor plates were not yet common, and people typically possessed simple kevlar soft body armor that could only withstand 9x19mm rounds, which these PDW ammo (5.7 and 4.6) could penetrate. however, by the 21th century, their penetration power had lost its advantage.

these PDW rounds have the same kinetic energy as 9x19mm rounds with higher muzzle velocity and slightly enhanced penetration.due to their higher muzzle velocity they are more accurate and insignificant stronger armor-piercing capabilities(but not all these PDW ammo are AP rounds). this seems to be their real advantage————and very expensive.

PDW ammo's power cannot compare to 5.56x45. in situations where excessive penetration needs to be avoided,9x19mm ammunition is more suitable (enemies requiring such ammo are usually not wearing body armor). as for weapon compactness, modern SBRs with folding stocks, such as the HK G36C, are not very significantly larger than the P90 and even MP7.

I've always thought a MP5/MP5K (or something similar) firing FN 5.7x39 rounds would be the perfect pairing—yes, we have the P90, but it's still too bulky and loading its 50-round magazines is a huge hassle.