Concerns over extra holes in No4 Mk1 body

[vintage hunter]

When plug welding holes it's not recommended to use copper as a backing plate because this will cause cracks in the weld. It causes cracks because copper particles will get mixed in with the filler material and copper won't bond to steel on its own. The correct thing to use is yellow brass or a piece of ceramic like the shield cup off a heli-arc torch. There's a lot more that could be said concerning plug welding and firearms but I won't get into it right n. I will say that the use of mig welders is not the best method.

[vintage hunter]

Bearclaw, I've been a welder for over 25 yrs, this is tricky bussiness and I don't want you to fubar your reciever so a couple more tips may help. The left side of a no.4 reciever is about .300'' thick, give or take, and you just ain't gonna get full penetration with a mig because the wire will touch the side of the hole before burning all the way through and only partly plug it leaving a crater inside. When plug welding holes there's all kinds of variables but these are just too small and too deep to be 100% plug welded on a firearm. Then there's 4 holes to be filled all in a line and if you don't clamp the reciever down to a rigid surface BEFORE welding and leave it clamped till it's cooled there's the chance it could ''draw'' it to the left and leave it looking like a banana. Still don't like the mig option for this particular fix so if it were mine and i was bound and determined to get rid of the holes I would fit a threaded plug, flush inside & a few thou high outside, then quickly fuse over them with a tig rig then polish them smooth. ATB, VH