Well, I just ordered a tsd sd702/well mb07 and am looking for the answer to the inner barrel length. I'm not ready to say, "I can't find it", but I'm coming close.
Also, I am considering adding a mock silencer to it to gain more distance for a longer inner barrel.
UPDATE: Thinking the sd inner barrel is 431mm is 16.96847 inches. Is this long enough for a sniper rifle? It is stock and is probably not a TB either.
my cheapo/reliable sidearm: a Y&P mk23 NBB with mock suppressor/barrel extension.