You didn't specify what gun your'e shooting it in, so, I can't speculate if you have a gun problem. If you're shooting it in an old S&W Mod.-10 "Victory Model" that has been converted from .38 S&W to .38 Spl, that could be your problem.
However, I think that a weak crimp/case neck would not completely be the problem. It could be you have some deterioated powder, or a primer problem. I had some bad Win540 (same powder except for lot #'s) back in the early 1980's - it had gotten heat deterioation in a shop that was going out of business.
Steve was right- try a different primer, maybe powder too! HP-38/Win231 (essentially the same powders- just different lot #'s) works a lot better for what you're trying to load for.
Load what Hodgdon lists as their max load for best results with HS-6; and use a Federal 100 primer. If your using a CCI primers, that is a big part of your problem. Win. -SP are also good.
I have used the listed powder charge with a 125gr Sierra JHP and JSP bullet and gotten good results.
HS-6 is not a particularly clean burning powder in standard velocity/pressure loads, but not bad either. Accuracy and velocity is usually outstanding though.
The large amount of empty space is normal in most .38spl loadings.
Try checking your brass, I have seen large variations in brass thicknesses. A particularly thin batch of brass could also contribute to your situation.
(I had a bunch of Military .38 brass from an Airforce contract that was so thick, you could not seat a lead bullet sized to .356 without lead shaving... [PGU-85 w/130gr FMJ -or something to that effect]- You can imagine the poor accuracy !!! My brother picked it up on an Airforce range back in the '70's. He said that the original loads made the Airforces Mod.15 S&W's kick like .357's, and the stuff was tack-driving accurate, so he thought the brass might be something special- It was, especially thick to take the phenomenal pressures! Far above SAAMI +P.)
So long TFL...................