65% is awfully close to full choke.

That's tighter than I personally want a 20, but if you really want it to shoot even tighter you might try hand loads with some of that Italian nickel plated shot that BPI sells.

I'd try something like Herco, and a wad like the SP-20 that has a long shot cup.

Some guys say that the base pellet hardness is more important than the plating, but like most of this it's subject to debate.

Good luck.

"The price of good shotgunnery is constant practice" - Fred Kimble