Just saw the Facebook post about them working on new 357 Sig barrels.  

Quarter Circle 10 Facebook

As one of the early adopters of the Oly pistol caliber uppers, I was interested in the 357 Sig when Oly announced it as a chambering way back when.  

After calling to order, I was told they would build me one, but they were having problems because the brass came out looking like 40 S&W, that is, the case was blowing out to straight walls from the residual pressure.  

Wonder if anyone has made a straight blowback in this caliber and if you have experienced a problem in this regard?

People have in the past, and experienced the same problem. Blowback and bottleneck cases don't get along. Ask the PS90 reloaders how long their brass lasts.
Now take that barrel and make it DI or delayed bb and you will have a nice shooting gun.