As a one man GS (retired now), I can tell you that what is going on is not correct.
If he is swamped, then he needs to inform you of such things and REDUCE his intake.
Back in 2018, prior to Covid here, I had a great problem obtaining stocks of any description, informed all 4 of my clients and 2 of them went elsewhere. 12 months later I was contacted by them both and they wanted me to finish their jobs because they knew I finished the other 2 jobs in the extended time I told them I would.
Great gunsmith or not, courtesy outweighs anything else when a CUSTOMER has paid/supplied parts and is waiting without being told the truth. I have never done that, ever. Problems arise, my mill broke down 10 years ago, parts had to come from Britain, out of action for 10 months….
Cheers.