shooting heavy pellets out of a nitro piston break barrel

I think the main philosophy here has been well covered. The gun isn't really in danger of being damaged by shooting heavy pellets, but you'll need to experiment with pellets till you find one the gun likes. My strongest springer is a Benjamin Trail NP XL in .22 and I was thinking I could just shoot an excessively heavy pellet like the 25 grain JSB deer slug. 
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The results were hilariously pathetic. 
A lot of my plinking is done around my pond, so the point of impact is blatantly obvious from any viewing distance as the pellet flash boils the water sending a plume of steam up from the splash point.
What I'm about to tell you is so ridiculous you would be perfectly justified in calling me a liar. I was still very new to the gun and experimenting with pellets when I chambered the first one of these boat anchors. I fired at my Ø12" steel target which is at 65 yards and it splashed no less than 3ft high and to the right. I fired again and splash low to the right FEET away from the first impact. Thinking those were flukes I fire again and this time the pellet splashed down behind a tree trunk. Yes I couldn't see in a straight line from my shooting position to the point of impact. My jaw dropped. I'm still dumbfounded every time I think about it and periodically have tried those pellets to verify my sanity.

Now I don't know exactly what causes this crazy flight. I can only assume that it's because the pellet is in the barrel long enough that the recoil upset the pellet as it was exiting the barrel. That rifle has a considerable amount of recoil.

I've found that the Crossman brand domes work pretty well at 14 grains, but even so I can see them spiraling down range in my slow motion video footage. I've tried some GAMO Aluminum pellets that appeared to fly very straight, but they're supersonic for the first 30 yards or so leaving the barrel and I don't want to irritate my neighbors. There is also a much more pronounced vertical spread at the longer ranges I like to shoot.