Limits on Match 1 in 9

I have an opportunity to buy a DPMS 20" match .223 rifle with a 1 in 9 twist rate. I have been thinking about shooting some longer distances of 300 - 500/600 meters (not necessarily for competition (yet)) and am wondering what is the maximum weight of a bullet that I could use in this barrel. I have read the 1 in 9 will stabilize a bullet up to about 73 grains, but is the twist rate fast enough for a 75 or 77 grain match bullet? What is the difference in velocity and accuracy potential between for example a 69 grain match and a 75/77 grain match if the latter is too heavy for a 1 in 9 barrel. I am really trying to decide if I should buy this gun or keep looking for a rifle with a 1 in 8 twist. I would appreciate if the responses would not try to convince me to switch to a different caliber, as I am rather fond of the .223 and want to see what I can do with it right now.