I've seen this same thing. Sometimes when FF a wildcat I could achieve tighter precision with less effort than with the formed case. And with SAAMI/CIP chambers sometimes same thing. A few of the occasions, I discovered there was a factor(s) I wasn't managing properly when firing the second...
I would recommend you test to see what shoots best for YOU in YOUR rifle, using YOUR reloading tools and processes.
National records and Championships have been won using virgin brass in precision target competitive disciplines.
There's plenty of room for noise and error in reloading...
Can't go wrong with either one. Both are backed by excellent warranties.
Look at the reticles, see if the extra elevation travel on the MK5 is really important to you now, or potentially in the future. The MK5 windage knob is capped, that slows some people up. Little larger eyebox on the AMG...
From my notes in 2009 :)
Here's the load I settled in on. Shoots 1.75 to 2 inches at 300 yds.
122 gr H-50 BMG
3.425 Ogive COAL (4.12+ OAL)
Neck turned to .013 thickness
.002 worth of neck tension
3070 fps avg over 8 shots
ES 16 fps over 8 shots
Brux 31.25" barrel...
I don't think he did, just a plug that he's a reference to take to the bank on here.
The RWS stuff will require a 1-1.5 gr reduction in charge weights in comparison to lighter weight brass.
But the stuff is incredible tough and consistent. You will wear out several barrels with a 100 piece...
When @JE Custom posts I listen.
The 7RM RWS brass from Huntington is the toughest brass I've used in any caliber of any brand of brass.
In my experiences, 75% of case growth occurs after the FL sizing operation, and the other 25% is from firing pressures.
I would build the one your likely to reload/shoot/practice with more often.
If it's a 100 round a year gun, unless you have .05% percentile shooting abilities, you won't develop or posses the proficiency to use either chambering to it's full potential, let alone see the energy/wind drift...