Anthony Williams, small arms theorist and co-author of the book Assault Rifle with Maxim Popenker, wrote an article in February of 2006 on the development of the Personal Defense Weapon and its ammunition, which he amended in late 2013. He includes in the article two photos which together represent one of the most complete collections of PDW cartridges I know of:
Mr. Williams left off some rounds that could arguably be considered PDW rounds if only by convenience, such as the 5.45×39 (used in the PDW-like AKS-74U), but in terms of rounds designed specifically for PDWs and PDW-like weapons, these two images are shockingly complete. Rounds like the 5.7×28 and 4.6×30 are from a collector’s perspective very easy to obtain, while rounds like the .22 MARS and .22 APG are attainable but uncommon. The real jewels of the collection are the Russian AP 9×19 and 9x21mm rounds and the 5.8x21mm Chinese PDW. These rounds are almost never seen for sale in the west, and are almost impossible to get due to import restrictions (in the case of the Russian rounds, because they are armor piercing, and in the case of the Chinese due to the ammunition importation ban in place for that country). Other notable rounds include the .22 SCAMP, designed by Colt in the late 1960s for a very slick select-fire semiauto/3 round burst handgun that would have replaced the 1911 in service, the 6.5×25 CBJ, a Swedish round firing tungsten-cored saboted ammunition with extremely high penetration for the size, and the 4.38x30mm/.17 Libra, an extremely obscure round from the Czech Republic designed for the ČZW-438 compact rifle. Interestingly, the .22 SCAMP and .17 Libra both share the unusual parentage of being based on the .22 Hornet case, albeit modified to the rimless configuration. Rounds that didn’t make it into the lineup that are still worth mentioning would be the 5.56×30 MINSAS, an Indian PDW round that is very similar to the 5.56mm MARS, the 5.7mm Johnson Spitfire, a necked down .30 Carbine designed by Melvin Johnson as a retrofit for .30 caliber M1 Carbines to improve their performance, and the 5.56×22 and x30 GIAT, two rounds from a French program of the 1980s and ’90s that sought to produce a personal defense weapon.
The future of PDWs is uncertain. As long as carbines in calibers comparable to 5.56mm are standard issue, it’s unlikely that PDW rounds and weapons to fire them will very widely proliferate, as shorter barreled versions of standard rifles can easily fill that niche. However, in the event that a larger rifle caliber is adopted and replaces 5.56mm in NATO or the 5.45mm in Russia, the PDW cartridge may well see renewed interest.