Hi Stuart
The ABS plastic Marushin MP40 will definately NOT be suitable for conversion to fire normal Blank ammunition.
Early Zinc Alloy Marushin / Nakata MP40 models are not suitable either. Attempts to fire Blank Rds in Zinc or Plastic replicas are likely to explode causing destruction of the model and possible fatal injury to the firer or anyone nearby.
It's possible to convert MGC68 MP40 cap fire modelguns to Blank Firing after very extensive strengthening and rebuilding to meet UK laws' requirements.
There are also purpose built Blank Firing MP38 and MP40s available, expect to pay £1200 on average for a good one.
Bear in mind too that Blank Firing guns can only be used at Reenacting events or similar. They're far too loud to use indoors for example which is why cap-firing models have an advantage.
If your fed-up of searching for cartridges, you could always buy a cartridge-catcher pouch that straps over the ejection port
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.