I own a "Spruce Gun" and may be able to help. I realize this is an OLD thread, but feel free to email me at tomgraham01@msn.com if you still need help.
I would like to know more about your gun including verifying that yours has the US and flaming bomb symbol stamped at the top front of the receiver.
There were 1800 of these shipped to the Pacific NW in 1917 for use by the guards of the Spruce forests around the PNW which were being used for gliders, etc. during WWI.
There are less than 200 of these known to have survived and Rick Hill has been trying to update the list as new survivors are found. Yours may be one of the missing ones!!
Regards,

Tom