Heaviest .25 Pellet for Leshiy?

H&N Grizzlys are best bang for your buck when sized properly and shoot reasonably well out to 200 yards, but if you can get ratsniperslugs in 31gr or 36gr, they're the most accurate slug I've tried in my Leshiy. Sub-moa on a calm day @ 100. I prefer the 31gr as the 36gr shoots a bit slow (around 700fps), so it's more wind sensitive due to longer flight time. NSA slugs didn't work in my gun, but all barrels are different so might be a good idea to test them also.
 
I’ve shot the 34gr out of the 250 and 350mm barrel.

That seems like a good top end weight.

H&N Grizzlys are best bang for your buck when sized properly and shoot reasonably well out to 200 yards, but if you can get ratsniperslugs in 31gr or 36gr, they're the most accurate slug I've tried in my Leshiy. Sub-moa on a calm day @ 100. I prefer the 31gr as the 36gr shoots a bit slow (around 700fps), so it's more wind sensitive due to longer flight time. NSA slugs didn't work in my gun, but all barrels are different so might be a good idea to test them also.

I think you're absolutely right about them all being different. What are you guys using for pellet sizers? I'm waiting for my new digital calipers to arrive so I can figure out whether or not I can just buy something sized to what I'd need. I might get a block precision machined with some different sizes as well, just to test out with different pellets / sizes. (Every "off the shelf" pellet that I've tried so far in .25 seems large).

I'm guessing the long range YT videos are yours @jNc? Long range shooting is my main goal.