One area of weapons development that has seen a lot of new, but generally unnoticed, developments in the past decade is the PDW (Personal Defense Weapon, or pistol replacement). One recent example is the Rock River Arms PPS (Performance Piston System) Pistol. This is a 5.56mm weapon that uses a new internal design. It has a short gas piston, with the recoil spring and guide rod over the 21cm (8 inch) barrel. The entire weapon (which does not have a stock) is 44.5cm (17.5 inches) long and weighs 2.2 kg (5 pounds). There are single shot and full automatic versions, each costing $1,335.
If you want a smaller PDW, you often have to go with smaller ammunition. Thus a recent Chinese PDW design uses a 4.6x30mm round. The new weapon is a variant of the Type 79 PFW (which uses the old Russian 7.62x25mm round). The new weapon weighs 1.6 kg (3.6 pounds) and has a 30 round, dual stack, magazine. Rate of fire is single, 400 or 600 rounds per minute. It has a Picatinny rail on top for scopes and sights. With the telescopic stock closed, the weapon is 34cm (13.4 inches) long, and 54cm (21.2 inches) with the stock extended. The weapon is gas operated with a closed bolt.
The bullets are about half the weight of 5.56mm ones, and the 4.6mm round is lethal at short ranges, especially in bursts. The bullet travels at high speed (over 700 meters a second). The 4.6mm round will not penetrate walls or ceilings as well as 9mm or 5.56mm bullets, meaning less chance of unintended casualties.
The Personal Defense Weapon never really succeeded, on a large scale, as a pistol replacement. The military, especially in the United States, simply developed an M-16 with a shorter barrel and stock (the M-4). Many troops who previously only had a pistol, now had an M-4. These were cheaper than PDWs, and not much larger.
But security professionals, especially bodyguards, are a growing market for the easily concealed PDWs. Some police departments use them.