I had a P15 and a PP700 pistol, both as accurate as anything else I have. I’ve also fettled a couple of P15’s for the lads up at the club.
All have been very consistent over the chrono, most have needed the power turned down a tad (11.98 on most with 8.4 / 15 gr slugs).
The barrels on all of the rifles I’ve set up have needed removing from the action and a thorough cleaning before they’ll shoot anything resembling a group. Conventional airgun/lead solvents/degreaser and a pull through won’t touch the stuff inside the barrel, it has to be properly cleaned.
I use a nylon brush with the proper tough solvents I use on my powder burners (KG/009/bore shine etc), then further soaked patches until it’s clean, then clean patches, then a pull through before refitting to the action.
After this the barrels I’ve experienced don’t need any different treatment to any other Airgun barrel and I’ve never had one that hasn’t responded greatly to the above treatment.
Hope that helps a bit, cheers, Craig