I just heard back from Robert Schatton... and he DOES offer the following barrels for 1/144 German Submarine:
2 cm : 1,1 E
3,7 cm : 1,3 E
Brass, drilled, with conical hollow muzzle brake.
However, no 8,8cm barrel.
He also offers in Alu drilled 2 different Periscopes for 8,50 E.