Apparently, there's technical data in Yury Natzvalade's
Trophies of the Red Army during the Great Patriotic War of 1941-1945, according to
this thread that unfortunately has had all of the photo links killed by Photobucket. That thread also suggest the Russian one is at the Military Historical Museum of Artillery, Engineers and Signal Corps in St. Petersburg. I'll need to see if I can find a copy of that book. The thread suggests the MG18's barrel is slightly shorter than the T-Gewehr's, in discussing muzzle velocity,