It seems to use precharged air in four cartidge-like chambers. Those are spring loaded in the barrel.
The barrels are indexed by pushing the head of the cartridge forward, and rotating the barrels by hand. The indexing and lockup is VERY tight and precise. I'm unsure how the cartridges are charged, or how the actual mechanism of firing takes place. I've collected high end springers for almost 20 years. I know almost nothing about PCPs.
I've tried to research this airgun but found almost zero information about it. On some British airgun forums I have found some references to this gun, and others, that are essentially created and manufactured by Steve Harper. He's apparently a gifted genius of airguns and machining. Some of his designs are used by Daystate.This is an odd, rare gun for the airgun collector that appreciates high quality and rare guns. I'm a retired aerospace machinist and quality control inspector. So I can provide specific dimensions or measurements, if you need them. The manufacture and quality of this airgun are quite phenomenal. If you need additional pictures of a particular area, please ask. I've been collecting and shooting high end Springer's for over a decade. I have never been in to PCPs, so I can't speak from experience about them. I take great pride in my feedback and your satisfaction and happiness is very important to me. The item "Harper Classic Pepperbox Airgun PCP British. 22 RARE Cased 4 Shot" is in sale since Tuesday, October 6, 2020. This item is in the category "Sporting Goods\Outdoor Sports\Air Guns & Slingshots\Air Rifles". The seller is "4570thumper" and is located in Cullman, Alabama. This item can be shipped to United States.22