Daryl's theory regarding your loose bullet to case fit sounds logical to me.

What dies are you using?

A .375 will slide through the neck of my fired 9.5X57 handloads, as will projectiles from original DWM531, but either fits tightly when loaded with my RCBS dies which leave a slight taper crimp and set the cannelure of Hornady 3715 at the case mouth when seated to proper depth in a properly trimmed case.

A lousy scan of; 1) Reeds Ammunition 9.5X57, 2) my handload (unfired) from .35 Whelen through RCBS dies with Hornady 3715, 3) original (1924 dated) DWM 531, 4) my (ex Whelen) handload, once fired

Quality Cartridge makes new, properly headstamped, 9.5X57 cases which can be found at a decent price:

Grafs

MidwayUSA

Hornady 3715, unfortunately, are an endangered species. The .375 270 RN hasn't been offered by Hornady for several years, yet there are commercially available .375 Ruger that sure look like they are stuffed with them.

I laid in a supply of 'factory second' .375 RN from Midway a couple of years back for a very good price. As stated in the customer reviews, there doesn't seem to be anything 'second' about them and they appear to be Hornady 3715. The last time I saw them offered the price was significantly higher than I had paid: Midway seconds

Woodleighs in .375 270 RN, when they can be found, generally run about that price. Expensive Mannlicher food, but properly sized.

The Reed's 9.5X57 is properly made of Norma brass that has been drawn straight, then formed and filled with Hornady 3715 which he advised me had some of about two years ago.

* edit - I've just looked at Reeds 'site and he now offers the 9.5X57 in 235 grain soft point and 270 grain spire point in addition to the 270 RN. As you likely know, your Schoenauer magazine wants the 270 RN and depends on it for proper alignment while feeding. Reed's

When forming your own cartridges, make them to the precise dimensions of the Eley drawing below and they will function flawlessly in your MS (provided your MS is to specs).