Hevi-Shot says .665 and larger, they said pattern can open when tighter than .665..
I use JellyHead .665 with Hevi-13 and it shoots awesome.. I don't think there will be too
darn much diff. between the .665 and .670 personally.. But if your worried get the .670 cuz I'm
sure it will do fine.. Or call them and ask for yourself, they do more about it than I do that for sure..