Erma gallager clean out screw

[Lithuanian artillery]

I am working on an Erma gallager. Does anyone know the thread size of the clean out screw? I think the Smith is 3mm and they are notorious for breaking off( being so small). I have had a few ignition problems with it and the fix I did on the Smith was to retap the screw to 5/16 NF and grind a slope on it. I notice a prior post to use a larger nipple hole, and maybe a larger hole in the cartridge head is in order. Any thoughts?
Thanks
Andy