Kutter, awesome post, and awesome work…thank you.
Extra holes are the nightmare for restorers, especially screw holes.
Here’s sort of a rundown of issues that I run into when dealing with holes, especially on barrels.
Most screws (especially gun screws) are made out of 12L14, it has lead added for increased machinability. 12L14 will not weld well, due to the lead, lots of porosity, especially with TIG. 12l14 also rusts quicker than any steel that I know of.
I once had a receiver with a couple of extra holes That I plugged and peened, then the receiver was nickel plated. The repair showed up about a year later, the seam between the base metal and plug had gotten some sort of contamination in there started rusting under the plating.
Screw holes inherently have a lot of crap in threads, and will eventually leak/leach out around the seams. It also provides an excellent source of contamination when TIG welding. If I’m plugging a screw hole, I always drill it out to remove the threads.
Most barrels made prior to 1900 were made out of mild steel fairly close to 1018/20. Many barrels, especially smokeless barrels made after 1930 were made out of 4140, different steels. Really need to plug with like materials.
Welding a plug into 4140 presents its own challenges. Most will take a piece f cold rolled steel and use that to plug a hole. Unfortunately, 4140 and 1018 are different steels, and will take bluing differently, 1018 will tend to be darker when blued. The other issue is the HAZ or Heat Affected Zone when welding. 4140 is a thru hardening steel, 1018 is a case hardening steel, not enough inherent carbon to self-harden. When welded, the 41 series steel will be hard, the 1018 steel will be soft. This HAZ, will not only show up when blued, but also when polished. Using the same grit, the softer steel will show the polishing lines more prominently than the hardened HAZ area.
I have found using like steels when plug/welding really helps, and on parts that can be.. annealed after welding to remove the HAZ. But in the case of barrels… nearly impossible.