Hello Jope,
You should be fine with the Lothar walter to stabilise the 650 grains military bullets, but I think you will struggle with the 750 gr amax.
As you are in Finland, I suggest that you send an email to Dr Kolbe of Border Barrels in Scotland. He can make any twist barrel you want , even in 50 cal.
His web site is
www.border-barrels.com
Tell him what sre your requirement exactly (stabilising a 750gr amax at subsonic velocities) and He should point you toward the right direction for the twist rate.
Regards.
ND