The HK P46 (first called the UCP - Ultimate Combat Pistol) was/is a 4.6x30mm handgun firing the same family of ammo as developed for the HK MP7A1 PDW. The P46 designation was from the German Army who wanted the gun to replace the 9mm P8 (USP9 full size). Samples were developed and tested during HK Desert testing in Prescott AZ in the early 2000's. During that week numerous prototypes performed very well - accurate, low recoil, 20-rd magazine, reliable, BEST trigger on any HK pistol! Various customers there shot the gun - hard - with no issues. I am convinced still today if HK had decided to complete the project and build the gun it would be in use by the German Army and at least one US SOF unit.
HK designers opted not to complete the work based on concerns with long-term service life and obstructed bore concerns. The 4.6x30mm round if far more powerful than the 5.7mm FN P90/Five Seven pistol round and does not rely on case shoulder set-back for function as do the FN guns. That is the reason HK opted to use a gas operated rotary bolt system in the MP7 versus the blow-back system in the P90. Last time I asked the program was dead for technical reasons.
The German Army envisioned two P46's - the full size variant you see in the photos posted here and a compact "Shithouse" variant - one small enough that it could be carried/worn anywhere to include when the operator goes to the latrine to do his business.
As to the comment about the terminal effects of the 28-40 grain 4.6x30mm projectile, that issue will be open for endless debate. Remember that THE primary design goal of the FN 5.7mm and HK 4.6mm PDW rounds was to penetrate the Russian SPETNAZ/NATO CRISAT panel (Ti and Kevlar) at 50 meters. Both rounds do that - the 5.7mm round at 150 meters, the HK 4.6mm round at 300 meters! If you cannot penetrate the body armor you cannot defeat the guy behind it thus the need for high velocity small caliber penetrating rounds. To make it easy to use for poorly trained support personnel (the primary customer) they had to keep the recoil impulse down which means a small high velocity round was best (also makes full auto fire easier and allows more stowed rounds in a compact weapon) unlike something in say .45 ACP or .40 S&W.
I can tell you for a fact that 4.6mm MP7A1's are killing plenty of terrorists so we know in actual practice the round does work when employed properly by well trained gun fighters. That is ALWAYS the key to terminal ballistics – effective hits on target.