Well, I finally got a chance to test some of Black Hills 75gr Heavy Match load that is loaded with Hornady's 75gr. BTHP. Unfortunately, I didn't have any experienced match shooters to help me this time, so the results are probably only marginally useful.
Conditions: 65F, about 800' elevation, 36% humidity, wind 10-15mph with gusts to 30.
Shooting was done from sitting at a bench, no sandbags or other support, open sights (stock), 16" 1/9 chrome lined Bushmaster barrel. Targets were 8.5x11 sheets of paper (several sheets for 75gr @200 and 300).
First conclusion: Under these conditions, the 1/9 16" barrel will stabilize the 75gr out to 300yds. Accuracy with this rifle/shooter/ammo was less than impressive; but when the rounds did hit the paper, there was no keyholing or anything else that would indicate the kind of instability you see with 62gr in a 1/12 twist.
Second conclusion: For this shooter/rifle, the 75gr is probably unacceptable as accuracy was inadequate at 300yds and pretty marginal at 200yds. To give you an idea of how much was the shooter and how much was the ammo, I compared the following groups:
200yds with reloaded LC brass, 55gr
best group: 6"
worst group 12"
200yds with Black Hills 75gr BTHP
best group: 19"
worst group: unable to measure but wider than 20"x29" carboard backing
No group comparisons at 300yds as we ran out of ammo, targets and patience trying to get a group with the 75gr. Few rounds that did find target left nice round holes you would expect from a properly stabilized round.
Part of this may have been rifle preference as it didn't care much for the 68gr Hornady BTHP at shorter ranges either. At longer ranges though, the 68gr grouped in the same range as my best/worst 55gr. groups.