i shoot 75gr bullets all the time from 1-9" twist barrels.
i have shot 77gr as well with no problems. the thing is not the weight it is more the length of the bullet a 75 gran JHP is the exact same length as a m855 bullet the 75s just weigh more cause it don't have a steel plug in it.
a barrel marked 1-9" is probably more accurately called a 1-8.75" anyway.
a true 1-8 twist rate will shoot any length bullet you can fit in a ar-16 mag-- and some that are too long.
the 1-7" tr the DOD went to is total overkill. the tracers were way longer and the extra twist rate was for them not standard ammo ---
of all the twist rates i think 1-8" is best overall -- however the 1-9" and 1-7" also work fine-- if you got a 1-9" or 1-7" your pretty much set to shoot most all of the heavy loads for the 223/5.56--