Thanks again for all of your help and suggestions.
I think I got it done, but I have a couple more questions. I couldn't find an aluminum rod that was 7/16" so I found a steel rod and used it instead. It worked pretty well I think. The first plug I made I used a wooden mallet to tap the rod to flatten the round ball, and it didn't flatten much, though it was enough that I couldn't see light passing the ball while it was in the barrel. It seemed hard to measure so I thought I would do it again, but this time I tapped the rod with a regular hammer. The plug this time came out much better as far as surface area to measure, and is the one in the pictures. I pretty much followed the instructions that I had found and quoted above.
Here's the rod cut in the two pieces:
And here is the round ball and the finished plug that came out:
You can see about where the round ball started, size wise:
Now here is my question. There are a couple different measurements on the plug that I made. The largest is .510, and the most common smallest is .503, so what I'm wondering is, does this mean the bullet size that I would need to order would be the .503?
I did also see some .502 and a half, and .503 and a half, but the most common was the .503, which of course that could be the digital calipers I'm using, but... I'm not 100% sure. More thoughts and or advice?
Thanks again for your time.
Here's the three together. Regular ball, first try at making a plug and second try at making a plug: