pzh_155mm_m109 is the previously-standard German self-propelled M109 Howitzer, being replaced by pzh_155mm_m2000.
schalldaempfer is the German language term for silencer or muzzleblast suppressor.
Meppen is the German Army Ordnance proving ground in northern Germany, with facilities for testfiring everything from small arms to self-propelled 155mm howitzers, plus a few other noisy toys. It's the German equivalent of our Aberdeen Proving Ground, and they can touch off their big stuff without upsetting their neighbors.