I visit a war museum before and saw this weird heavy machine gun which had a lever for setting various fire rates. It could go as low as 100 rpm. I wanted to take it apart to find out how it works...
Some guns you can do that. The British version of the FN MAG (the Generalé), is fitted with a gas regulation system so you could alter the cyclic rate of fire from 600 to 800 rpm with a few clicks. This is called balancing the gun, and needed to be done so it fired properly. It's one of my favourite things about the GPMG, as they all have different balance settings, making each gun individual with how much attention they need to get them to sing properly.
The air mounted versions of the M2s rate of fire is electrically controlled using solenoids.
I've never heard of a method of manually slowing the bolt down on the forward stroke. The stoppages that would cause. There'd need to be some kind of brake shoe/pad, pressing through the body onto the carrier as it reciprocates.
Most guns are limited rate of fire-wise by useable accuracy and overheating. So even though an MG-42 is capable (apparently) of nearly 1200rpm cyclic, a gunner would never fire more than 100-200 rounds per minute, and can be as low as 25rpm in most cases and is down to burst sizes and breathing cycles for accuracy.
I think using a separate self-loading rifle is better for field battles.
Yeah, absolutely. In the land of the pistol chambered SMG, the rifle is king. But you'd struggle to fit an SMLE, a full length M-16, or even a 16" M4 derivative into a main battle tank. And the only thing worse than a pistol chambered SMG in a field, is the only other thing you'll fit in an MBT next to your seat.
A pistol.