At this point, we’ve talked about 25 different intermediate and full power calibers as part of a series comparing different types of modern small arms ammunition. However, one subject not yet thoroughly covered is rounds for personal defense weapons (PDWs). These weapons are designed to be smaller and less obtrusive than full size rifles, while being more capable than pistols or submachine guns, particularly with respect to armor penetration.
Technically, we’ve already covered five PDW calibers as part of the previous series; two of which were designed from the outset for the role. Those five were the 5.56x45mm NATO, 5.45x39mm Soviet, and 6.8x43mm SPC, three rounds designed as assault rifle rounds but commonly put into service with short-barreled carbines functioning as PDWs, and the .300 AAC Blackout and 6x35mm KAC/TSWG, which were designed from the outset for short-barreled PDWs.
However, all of those rounds can function as either PDW ammunition or assault rifle ammunition, and this series will focus instead on rounds that are too low performance to be considered assault rifle ammunition. In many cases, traditional pistol calibers like 9x19mm can be considered PDW rounds as well, and those may be covered later.
The first round we’ll be talking about is the Belgian 5.7x28mm FN, a .22 caliber high velocity round designed originally for the P90 submachine gun. Later, the projectile was shortened to fit within the grip of a pistol, which became the Five-seveN handgun.
Below, we compare two loads of the 5.7x28mm to the old 9x19mm M882 standard. Note that I have omitted the wind drift chart, as the differences in wind drift between calibers of this type at ranges 300 m and below really won’t matter (even the very different 9x19mm and 5.7x28mm have almost identical wind drift characteristics at 300 m):
(NOTE: There is an error in the graphs below. The 40gr V-Max of the SS197SR is listed as having an 0.084 G7 BC; the actual number is 0.100 G7. The data in these graphs is correct, it’s just the label that is wrong.)
You can see how the 9x19mm has a substantial energy retention advantage due to most of its flight being in the lower-drag subsonic flight regime. 5.7x28mm, on the other hand, sheds velocity very quickly, even though its bullet is much better shaped for supersonic flight; by 300 m the SS190 has just over 100 J of energy left, barely enough to make a lethal wound. This drag problem is one of the most significant obstacles facing these pistol-sized small-caliber, high-velocity rounds.
Besides armor penetration, perhaps the biggest advantage of the 5.7x28mm FN is the low recoil. We can see what a dramatic advantage this caliber has in two screenshots from kwk.us’s recoil calculator, where it is compared to the 9x19mm M882 again:
A little more than a quarter of the recoil energy ain’t bad at all, that’s for sure.
Lastly, the 5.7x28mm and other small calibers of this type give a substantial advantage in the ammunition’s weight. While 9mm M882 weighs 12.6 grams (more than 5.56mm!), 5.7x28mm clocks in at a very modest 6.5 grams per shot.