First thing to do, clean the heck out of your chamber. Then use some Snap Caps to see about feeding and extraction. If your rifle extracts snap caps just fine, then I doubt it is the extractor or ejector.
Second thing to do, use different ammo to see if that fixes the problem. It may have absolutely nothing to do with the powder charge or bullet weight of the ammo you are using and possibly everything to do with rim geometry. Or it could be the that the ammo wasn't made properly and is too short in the shoulder a little bit which causes more headspace. I don't know, and can't know without inspecting your rifle and the ammo you shot.
Third thing, come back and tell us if different ammo and a clean chamber worked or not.
Machine guns are awesome until you have to carry one.