Glad to hear about the mag primer. Just for grins and giggles I ran your three loads through Quick Load. All this is going to do is give you some approximations. Things like cylinder gap, crimps, and different primers keep this from being anything, but a guess-tomation.
240 XTP w/16.5 of H110 gives 10451 PSI, 825 FPS, Burns 51% and 11% Ballistic Efficiency.MP4606
240 cast w/7.5 Unique 14958 PSI, 914 FPS, Burns 99% and 27% BE. Muzzle PRS 4168 PSI
255 cast w/8.0 Power Pistol 12215 PSI, 830 FPS, Burns 85% and 20% BE, Muzzle Pressure 4226PSI
As you well know nothing takes the place of a good chronograph and a pressure barrel. But now you have an idea, what is going on with your loads. I did load some 255 cast with 7.0 gr of Unique and it was good too. Here is what Quick load said.
255 cast w/7.0 Unique 14613 PSI, 861 FPS, Burns 99% and 28% BE That proved to be a good load also. I really like the cast Performance bullet. Either load will do you good.
What happens with these different powders is the burn rate primarily causing the greatest differences. The H110 being the slowest burning is still burning towards the muzzle. Look at your pressures that is why I posted them. All of the powder does not burn. H110 is better suited for larger cases, The 44 mag on up. Now you say, Hey! there are holes in your theory Look at the Power Pistol load! Unique is faster on the burn rate and the faster powders usually end up being a little snappier. The medium burning powders usually give a much better performance and are easier on the shooter. HS-6 is one that many people are raving over with the 480 Ruger, recoil is less than H110. I have used a 255 gr cast with the 44 mag and 10gr. of Unique and it is milder than a load of 240XTP and 10 grs of Tite Group. That titegroup is fast burning and real sanppy in the recoil department
Last is your sizing die screwed all the way to the shell holder? and the other thing is, do the shell holder and die match in Brand? I have had this problem with pistol ammo when I loaded some hot. Also I have had 223 shell holders not be correct to size the brass all the way, especially with small base dies. Then again this might not be the problem at all.
It might be that the chambers are tight on the FA and might be restricting the bullet some.
If you can check to see what the chambers measure it might tell you. They could be .4295 or .4305 I do not know without checking them with a set of Pin Gauges. I had some 360 gra be a little difficult in one of my 45s.
Pin gauges come in Plus and minus so depending on what is used will determine what the reading. I have machine shop friends that helped me. And some Gunshops have them. Once I had a gun checked both chamber and use a Field rod on it. The guy did it for free.