The inner barrel shouldn't fall out without the comp in place. There are notches cut into the rear end of the inner barrel that the hopup chamber locks on to.
The end of the outer barrel was what I was worried about, since most airsoft GBB outer barrels have an inner diametre much greater than the inner barrel's outer diametre and relies on a holed wall at the business end of the outer barrel to center the inner barrel. Apparently the M190 does that with the comp instead of the outer barrel.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."