i have a 1-9 twist bm h-bar how heavy of proj. can i shoot?

69 grains will certainly stabilize.  If it is a true 1-9, not 1-9.5 or so, it will also stabilize 75 to 77 grain bullets.  Only way to know for sure is to try some.  If they go into the target straight, they're good to go.

Some barrels end up being a little slower or faster than the advertised twist due to mfg tolerances.

Short answer:  It depends.

I have a 1:9 barrel and 68 Gr  Hornady boat-tail hollow points fly straight and don't  seem to have any hint of keyhole at 500yds.  

Geek answer: Stability will depend on more than weight, but the specific way that's distributed over the length of the bullet you select, to wit, it's shape.  There's not a lot of variation in that, though for quality bullets.  Also, your barrel's particular imperfections and tolerances will have some influence.

Shoot lots, see what works best for you.