Anything above .30gr is tricky - actually going above .28gr reliably was tricky. The .30gr BB I sell and that anyone else sells is actually not pure ABS plastic - it has additives to bring up the weight.
My problem is the additive we use tops out at .30gr so we have to find another additive that offers greater weight and density options that does not interfere with the molding process or the polishing process. So far thats been very difficult to find. I did test one batch of test .34's sent to me and didn't like them - the surface pitted during the polish process. This is why I suspect the graphite BBs are popular as a heavier BB. They use a colored BB and then add graphite powder to cover up the pitting, and then market it as some kind of BB enhancement, when it fact all it does it gum up your hopup and if you wash the BB, you're again left with pitting.
No easy answers on this one folks - plus there is the ever present volume issue - people are just not buying heavy weight BBs in quantity. This year .20 and .25 are way ahead in sales compared to .28 and .30. Perhaps I've sold 50 bags of .30 product so far - so the incentive to manufacture and carry heavy weight product really isn't there. I've got about 5 years of .30 product in stock based on that sales volume.
P.S. Aside from the obvious danger of shooting ball bearings, ball bearings will ruin the inner barrel by scoring it as its fired. Not recommended.