Mossberg M42M bolt cover.

[GerryRA]

The Mossberg M42M .22 cal rifle has a cover attached to the bolt that closes the port in the receiver when the bolt is locked.

The cover is pretty substantial for just a ‘dust cover’. Was it intended to protect a left handed shooter from gas and brass particles in case of a ruptured case?

[dave]

As a long time Moss. 22 collector and member of the collectores assoc., I have never heard it refered to as anything but a dust/dirt cover. However in the late 30's (1939?) when the feature was introduced it was described in Moss. adds as a "safety cover". "Besides being an important safety feature it also keeps dirt from getting into the action---". Exactly what it was to keep you safe from is never addressed. Personally I would rather have gas and brass particles blow out the side rather then down the bolt directly into my eye.
Also in the late 30's Moss made several of their models in a left hand version. They did not sell well and are quite rare, so I do not think the bolt cover was aimed at left shooters. The cover was just another sales gimmik I think.

[dave]

You are certainly welcome, sir!