Problematic in what way ? And what was the solution ?
Thanks
Let me address the copper issue first then the WD 40.
Copper Issue
Like most shooters, I read gun magazines then came the internet and became convinced that aggressive copper removal was necessary to maintain peak accuracy performance.
For most of my shooting career now approaching 60 years, first I used Hoppe's 9 like everyone else in my circle back in the 60 and 70's. The I switched to Shooter's Choice sometime in the early 80's because I read that it's does a better job on copper. I would learn later that it only does a marginal job on copper compared to more aggressive solutions. I was completely satisfied with its performance in that it maintained POI and I had no clean barrel flyers. Truth is, I could not detect any difference in Hoppe's 9 and Shooter's Choice on target, but I stayed with Shooter's Choice.
Due to my wife becoming ill a few years ago, I had to abandon Shooter's Choice since odor was making her sick. So, I searched for an odorless alternative. After research, I choose Bore Tech products, the two-part system, C4 and Cu+2. Both are outstanding in removing carbon and copper. I choose the two-part system because right or wrong, I absolutely believe in a bronze brush to remove carbon thus the selection of C4 for carbon since it does not react aggressively with a bronze brush.
I began getting clean barrel flyers and the need to "recondition" the bore with about 5 to 10 rounds, depending on the rifle to re-established POI. After extensive research*, testing with added frustration, I traced the culprit to Cu+2. It appeared I was removing too much copper in my rifles. Once I suspended the aggressive copper removal, things settled back to normal with just C4. Apparently, C4, functions similar Hoppe's 9 and Shooter's Choice, they remove a minimal amount of copper. Again, the truth is, I couldn't detect any difference between these with to regards to results on target. Maybe the reason is that I clean every 50 to 60 rounds.
WD-40
I used WD-40 after cleaning with Hoppe's or Shooter's Choice to neutralize any residual solvent and provide some corrosion protection. It seemed to work ok in that in didn't affect POI, but the problems became if you got that stuff in the trigger assembly in gummed things up fairly severely. So, I stopped using it several years ago and went back to a very light coat of Hoppe's gun oil (mineral oil). I dry patch before shooting.
Note
*During my research I discovered a few articles dealing with the concept of establishing "copper equilibrium", which essentially advocates not aggressively removing copper with strong copper solvents. I was very skeptical because this is contrary to every expert opinion I have read by top competitors. Nevertheless, I tried it and it worked for me. However, I am not an expert, top competitor, long range shooter, or bench rest shooter etc. I am only a varmint hunting but that does require some level of precision in the sub 1 moa range. Therefore, it is not appropriate for me in any way to refute these top shooters claims regarding copper removal. I can only comment on what I experienced.
References: gunblue490 web site, Gun Cleaning and Solvent, How to Clean Your Rifle Professionally.