I fired 24rds today and the scope mount doesn't appear to be moving. I'm still not confident in it but it seems to be holding in place. I was going to try the no-drill Arctic Fox mount but I loaned it to a mate to try on his .303-25 and haven't gotten it back yet.
I increased the charge to 30gn of AR2206H behind four bullets, Bruce's 87gn (2.650"), Speer's 87gn Hotcore (2.750"), and Hornady's 110gn FTX (2.900") and 117gn RNSP (2.840"), and seated them a little deeper to increase pressure. I can't remove the bolt with the scope installed so boresighting consisted of trying to generally point the rifle at a fence post and seeing how far off the scope was It got me on paper though, and I gave it a rough zero with Bertram's 87gn, then went back to 50m and rested off the back of the ute to shoot through the chronograph. I fired four 5rd groups to get velocities. All four groups gave me a wide flier. The 117gn RN was particularly good giving me four into 22mm, and the very long boat-tailed-polymer-tipped FTX did stabilise just fine. The brass was a mix of once-fired and brand new so I didn't expect precision groups, I just wanted to fireform some brass and get some velocity data. Velocity averages were 2396fps for Bruce's, 2306fps for the Speer, 2321fps for the 117gn RN, and 2394fps for the 110gn FTX. I expected the RN to be slower as it's 35% heavier than the 87's, and it doesn't go very deep into the case. The FTX is 25% heavier but does go deep into the case so pressure would be higher. The 87's were a pain in the magazine as usual but the longer/fatter 110gn and 117gn were better.
I haven't measured up the brass yet but I'm sure these are fairly light loads. I'll bump them up a little hotter and load more of all four for some proper testing for accuracy. Right now it's zeroed 45mm high at 50m with the 117gn RN, giving it a 175m zero, just in case anything comes up.