1d10 Damage?
 

I'm making custom weapons for the spell 'Coin Shot', which lets me use my money as ammunition, with more expensive coins doing more damage, from a copper doing 1d4 to a platinum dealing 1d10.
I'm using a blowgun and Adjusting the crap out of them for each coin, but I'm stuck on the platinum coin, since after I hit 1d8 damage on the damage die size, it rolls over to 2d6.
How do I fix this, is there some way to force it to 1d10?

Modify a heavy crossbow.