My Cooper 17 Mach IV requires me to neck turn for clearance during firing, but it will chamber a newly formed piece of brass prior to any work. Sure sounds like it could still be the barrel, as odd as that seems.
Maybe try using some sandpaper on a case neck, just as a test to reduce the o.d.

I would call Fred again and ask him if you could return the barrel with 3 or 4 of your pieces of brass for him to examine.
muck

I have a bullberry 17 Mach IV contender barrel that when I got it was chambered as 17 Mach IV MATCH. Ordered formed brass from Dustin Brass. After resizing, trimming, etc they would not go into chamber all the way. Talked to Fred at Bullberry and he said he would change barrel to regular Mach IV at no charge since I did not want to do any neck turning. That was in Dec.--Got barrel back today and brass will still not go into barrel far enough to allow the barrel to close in frame. Looks to be off about 1/2 thickness of rim. I have full length sized in redding dies to point of die touching shell holder. Trimmed to slightly less than recommended trim length. Chaffered inside and outside of necks. I am not sure at this point whether it is barrel or brass. I thought about ordering some formed brass from Woodchuck den and see if his will work in this barrel. I don't want to buy a $100 plus forming die if it is still the barrel. I am also not wanting to send the barrel back to bullberry again for another 2 month+ wait especially if it is not his problem.
Any ideas out there on what to do. Ed

To finalize this thread: Fred at bullberry emailed me last night with a solution. It worked and now the cartridges will allow the gun to close. Here is what he said to do:

"But the bottom line is this. If new brass doesn't have a "wrinkle" at the shoulder, its not being fully sized. People don't like to do this, but its necessary to get it to the proper length. This only occurs on the initial firing. Please turn your die down another half a turn so as to make it very difficult for it to cam over center, and I'm guessing you'll get a fully sized case. Honestly this is the toughest job I have to do concening the Mach IV. Convincing folks to do this extreme sizing, or what at least, seems to be extreme."

Now I am going to fireform and then back dies off and see if it continues to work. Thanks for all the help Ed