machine guns

[Grumbler]

To be "dug in" you need to spend an entire turn in a hex without expending an MP's (movement points). As far as getting shot up, look at the units size. Some MG units have 1 MG and 3 men, they are a size zero units, which mens they are harder to spot and hit. The MG units with several guns and 8 or more men are a size one unit, which is the normal size for any infantry unit. They should take casualties at the same rate as any other infantry unit of that size.
If it seems like your MG units are getting shot up more then the other infantry units, then it might be because they are getting shot at more. They are more of a threat. Or it could be that you are on a run of bad luck. Laugh if you will but that does happen.
When 3.5 first came out, the effectiveness of small arms fire was reduced. I played my first game against someone who had hijacked a transport train full of RR Jeeps. I couldn't kill them for nothin. I had 3 BTR-80's ( with 1 heavy and 2 light MG's gang bang ( that is where you pull up next to an enemy unit and empty your guns into it over and over with several units) a Jeep. It drove off laughing. The Paris Hilton of Jeeps. I thought the game was hosed.
Since then jeeps have mostly burned after a few squirts of MG fire. So
It turned out I was just on a run of rolling snake eyes, and once past that, things got back to normal.