Semi automatic rifles in reality are typically noticeably heavier than their bolt action counterparts due to all the extra parts needed to allow automatic cycling. They're also somewhat less accurate than bolt actions, due to more loose fitting parts needed for reliable cycling.
So if the balancing is even needed. Just make them start swaying sooner than bolt actions would, when shouldering the weapon without support.
So if the balancing is even needed. Just make them start swaying sooner than bolt actions would, when shouldering the weapon without support.