I got the collar today and installed the express sight (the one for the 275 Rigby) on my 450/400.
The collar does not come out easy. I was fooling around for a couple hours with various punches and angled bits. I finally hit on a solution which was to use small wire cutters. I was able to cut right through the collar. I cut it again and took a section out. The relieved the pressure and it worked out easy. Old quality American cutter. I think we used to call them dykes. The new collar was a tight fit and I ended up slightly relieving the edge all around to get it lined up before tapping it home.
I slid the sight in, seated the collar down hard and then applied as much pressure on the screw as I dare.
That should be it. I dont see reusing the old collar because it is engraved pretty good when you remove the fold down sight.
The new (express) site was a little sloppy in the v groove and I ended up cutting a 10/1000 stainless shim for the space under the sight. This is before I drove the collar down. I am sure this is not needed. I just wanted to minimize and daylight and not have to drive the collar below flush. The shim should be trimmed under size so as not to show when done.
I am sure this is an easy job for some. I was sweating bullets trying to get it done and not mess up the rib. Expect a tight fit and be prepared to fuss over it some.
