It's been a long while since I've played. I seem to remember the barn is a deathtrap, hardly any protection.
One thing maybe worth doing is to try to suppress multiple buildings at once with your MGs, so the new occupiers are not immediately under accurate fire. Suppression may not stop the enemy from firing altogether but it will mean their fire is less effective (eg less accurate), that is what I believe is the case anyway.
Grenades have a max range. Currently no 'throw grenade' command, wish there was.
Rifle Grenade launchers are different, and they can explode inside a building and be effective.