Seriously, P-27 is one of the cheapest plinkers even within Gamo's standards. I own one and was quite disappointed after the impulse buy because the pistol has fixed sights and they were way off even at 10 feet. But then I installed an el cheapo laser on it and now the gun at least hits approximately where intended. As for pellets, I shoot Geco 7.0 gr flatheads in all my air pistols. It's a German product good enough to be used at lest at my skill level up to the old competition air pistols of which I have a small collection. Rumor says that Geco is same stuff as RWS Hobby, which should be available in US.
I'm sure any decent quality, 7.0 - 7.5 grain lead pellet will work well enough in your P-27, as long as the projectiles are not too long to prevent the magazine disks from rotating. If you shoot paper with flatheads and holes tend to tear, you may try domed or even pointed pellets, unless of course the latter ones are not too long. Flatheads need more velocity than the P-27 can provide before they start to drill nice clean holes.
And even if Gamo says the P-27 is dual fuel, I would never ever shoot steel BBs through a rifled airgun barrel.
