I have the Hatsan Flashpup, both the wood and the synthetic (S). I also have the Kral PB in Walnut. And the Artemis p12. All guns are in 6.35/ .25 cal.
HATSAN FLASH PUP: the Synthetic one has issues with a crooked scope rail, and the plastic stock is less stable than the wood version. That combination means that the barrel- cylinder- rail assembly in the S tends to move, causing the POI to shift. I solved this by shimming the scope mounts and fixing the cylinder into the barrel band with a set screw on either side. I also used threadlocker. Power for both is 50+ Joule. Accuracy is very good at 50+ m with jsb exact King and Baracuda Grizzly slugs. I put an Audrius reg (59 eur) into one and an altaros (80eur) into the other. Excellent consistency in both (20 shots before falling off the reg), but the Audrius has a larger plenum. This enables a lower (125 bar) set point and thus milder hammer spring tension and easier cocking. The cocking side bolt on the synthetic is larger but cocking is more "industrial"; it needed 300 shots to become more smooth. Both bolts needed fastening with threadlocker. I put a hogan decimeater moderator onto both (50 eur at Richtkijker.nl). The 2-stage triggers are adjustable and sweet after some experimentation. This Hatsan uncharacteristically is the lightest gun of the 3 brands. The build quality is also the most "nonchalant", but I really like the side mounted pressure gauge and the degassing port. The S also has an adjustable buttpad. Brothers in Arms has amazingly detailed tuning videos.
KRAL PB: beautiful walnut stock requiring sanding and linseed oil for the beauty to shine. I opened up the ports to 4mm. 60j power and, unregulated, nearly as consistent&accurate as the regulated Flashpup. Very sweet adjustable 2 stage trigger, hardened seer that I polished slightly. Higher shotcount than the Flashpup; larger cylinder and thus a heavier gun. Very nice build quality and very easy to work on. The hammer spring action benefited from putting a 3cm long / 6mm id pvc pipe over the thin spring guide. This is the most refined gun of the 3, and I silenced it like the flash pup. Smooth side lever action, and low noise except for a slight ping in the absence of a reg. This gun requires high mounts plus risers.
ARTEMIS P12: this one-shot is the most powerful and accurate of the three. It craves Baracuda (match) domed pellets. I regulated it with a 59eur Audrius set at 130 bar, enlarged the delrin tp to 5mm and kept the power at 60j by turning out the hammer spring adjuster. 40 consistent shots. The reg eliminated the loud ping. The muzzle report benefited from moving the shroud forward to insert 2 6cm hair curlers wrapped in felt, and separated by 1 baffle from an 8usd Chinese oil filter (aliexpress). This gun has high quality components: titanium 330 cc cylinder and a good barrel and trigger group. I love the screw-on fill probe cover. Some call the stock bland. Well I would rather call it solid and ergonomic. It gives the gun businesslike character and makes an attractive minimalist statement. The bad news: the hammer retention pin on the cocking probe was gritty and broke. I replaced it by a much larger hardened and polished allen screw; this now makes for very smooth cocking. This necessitated an enlargement of the inside of the pellet probe housing, otherwise the larger head of the new pin would not fit. The 2stage trigger is now very sweet after I messed with it. I put heat shrink tubing (used to insulate electricity wire) onto the trigger transmission rod to stabilize it in its guide; this eliminated the play and increased predictability. I have only 400 eur into this gun and it is my favorite over my FXs. (It's human to start loving your torturer ...). Would not want to meet it in a dark alley.
LONG STORY SHORT: If you want accuracy and consistency and a well-made refined looking bullpup out of the box, go for the Kral. If you want a lighter gun, consistent 6.35/.25 /power without too much noise, and a sweet trigger, go for the Flash pup but be ready for some mild tinkering including a regulator. And if you want high power & accuracy, plus quality components in a package with character, then go for the P12 but be ready for some metal work and learning by doing.
By the way, did I mention that all aforesaid guns require thorough barrel cleaning and may benefit from some (easy to do) recrowning attention ... Yes, I have learned that PCPs, especially the cheaper ones, are not for salon shooters ;-)
