I have only worked on one of my GU's. The other one, the cylinder is still soaking in Kroil to get the nipples out.
The one that I worked on, no caps would fit properly and the mainspring was blowing back even with light 12-15 grain loads. The nipple threads are the same as original Colt's so I replaced the nipples with Track of The Wolf POC-I nipples, they were still too long so I filed the cone portion down. I also had to file the ring on the recoil shield to make it smaller in diameter, to allow the cylinder to rotate freely without the #11 caps scraping. Then I replaced the mainspring, or rather a friend replaced the mainspring with an old one he had from his parts box, I don't know what kind it was, I had to file it a bit shorter.
This got the gun running and it is accurate enough within 10 yards. At 25 yards it will still stay on paper but the groups are pretty wide. I have not checked the chamber to bore relationship nor the timing. There is almost no forcing cone and no crown on the muzzle. I think the arbor might be fine, or if it is short it's only by a few thousandths. Eventually I will do, or have all of this stuff done for both guns by a professional gunsmith. I may even have them reblued and the color case hardening redone because I really like these guns.