Tuning Need help with Taipan Veteran

I don't own a Taipan Vet, but I wonder if that steel ball goes in the hole and the screw holds it in? The picture you provided looks like there is a seat for the ball to be forced against. You may have to get a bottom tap and clean the stripped threads and maybe replace the grub screw if those threads are damaged. If the threads are damaged enough, you may need to have the fill port body replaced.

Could be. It would be interesting to see that screw. If it's metal-on-metal fit, no seal, it would have to be a darn good fit. 
 
Here are pictures of the screw, the screw is not damaged.

685A277E-27DD-4DE3-B181-992AA85C97D9.1620615407.jpeg
5232F02C-ED4E-47BA-9817-9E0A23498D23.1620615408.jpeg
4D273D49-0AC5-4613-A9AB-CE25214ADC98.1620615409.jpeg

 
After looking at a parts diagram, I think jwrabbit might be right. Detent ball for fill port dust cover. I dont see a spring in there, but it also doesn't show that grub screw either. How did you get that cover off?

The dust cover should slide off when you remove the 2 mm grub screw (not the one the OP is discussing, but the one in the rotating dust cover). 
 
After looking at a parts diagram, I think jwrabbit might be right. Detent ball for fill port dust cover. I dont see a spring in there, but it also doesn't show that grub screw either. How did you get that cover off?

The dust cover should slide off when you remove the 2 mm grub screw (not the one the OP is discussing, but the one in the rotating dust cover).

Exactly. There's another bearing that the magazines interact with on the receiver.

There's no bearing related to the dust cover.
 
After looking at a parts diagram, I think jwrabbit might be right. Detent ball for fill port dust cover. I dont see a spring in there, but it also doesn't show that grub screw either. How did you get that cover off?

The dust cover should slide off when you remove the 2 mm grub screw (not the one the OP is discussing, but the one in the rotating dust cover).

This is correct, the dust cover came off when I removed the grub screw.
 
Hey, awesome news you've located the home for the steel ball! So, now you need to figure out the diameter and thread pitch of the grub/set screw. A GOOD hardware store should be able to help as they usually have a guage to measure fasteners there. Then get a thread tap to clean the threads up. I'd use a BOTTOM tap, but a hole tap might do it. Be sure to apply some SILICONE based grease to the tap to collect any loose chips from the thread cleaning process. Maybe a little dab of the grease on the hole too, to prevent chips from entering gun. If you didn't get too carried away when attempting to install the screw, you may have only damaged the first thread. Use a cotton swab to clear out the grease and threads when done. Again, be sure to inspect the grub screw threads. While at hardware store, you can buy a nut the screw fits into to run the screw through to be sure there are no rolled threads on it. Best of luck with the TP vet. 
 
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