We know that rifle barrels will shoot some bullets better than they shoot others, and even with the "proper" twist, some barrels will do things that you wouldn't expect, sometimes.
The standard German 8mm round used the 196gr spritzer, so that would be the what the barrel was made to shoot.
here are some possible reasons for the results you got..
#1) You started with a machine gun barrel. Generally speaking, machine gun barrels, even new, are not noted for having (or needing) the same accuracy as rifle barrels. In fact, rifle accuracy in a machine gun is considered by the military to be a detriment. They want a "cone of fire", not a small group. The way this is usually done is "sloppy" specs in the gun although not always. The US M2 .50 cal Browning went the other way, building the "dispersal" into the ammo specs.
#2), you "turned it down, and stuck it on an action". This could very well have an impact on how the barrel shoots, and with what. Somehow, I don't think the barrel harmonics now are anywhere near what they would be in an MG 34.
So, to answer your questions, the MG ammo was heavier (than 150gr) ALL their standard ball was heavier than 150gr. As to twist rate, I would think it would be the same as the standard rifle, BUT, the book I need to check that has gone walkabout, so right now, I can't say. When I find it, I'll let you know..