The 5 failed to fire cases had been hit at least 5 times in 4 deferent rifles. after the 5 attempts at firing I I busted the primers on the 6th attempt (after removing and inspecting the primers for all that deformity, and broken pieces, and then there was the anvil). Properly supported? Again, I am the one that only wants air between my case and chamber, I do not want a lot of air so I size cases to fit/; I understand that is confusing but when I start with a case there is a big chance the case will not allow the bolt to close when I start. The art and science of sizing is to know when to stop reducing the length of the case from the shoulder to the case head.
One of the most accurate rifles I own has a short hook up meaning to bust a primer the firing pin has to get into a hurry in a very short distance to bust the primer, I know, I know, I can change that but I would not risk giving up the accuracy for adding time and distance to turn the rifle into another rifle with a killer firing pin. I paid $120.00 for the rifle and then I paid more than that for the mount rings and scope.
F. Guffey
If one wants to make the best possible fit one has two options: Neck size only and at least FL but make it so slightly to be snug in that chamber.
Some folks suggest the case SAAMI gauges but the only true reference is the chamber itself. So one might want to start with fire formed brass
or otherwise take the bolt extractor and with everything very clean find the best possible fit to close reliable.
BR shooters ask precision builders to headspace after their custom loads with the actual brass and not the gauges.
If one has a flexible headspace like provided with adjustable nut then that might be something one does this way between seaons.
Most people do not need to do this but what I am saying the starting point is to know ones chamber.
Because a rifle says 223R on the side gives only a very limited amount of information about the chamber. Like on this one there are at least 12 different reamers
a maker might be chambering with most of them backwards compatible with the 223.
As always the most critical meausure to start with is the headspace from the datum of the shoulder to the base of a clean chamber and case.
In the case of the op is the actual rim. and because of the straight wall there is no backup plan like lets say a 300 win mag. (virtual 2nd headspace)
The thing with trouble shooting remotely via thread is very hard because even with great pictures there are so many things we cannot see and feel.
But I would say calipers and micrometers are the best way to assure we do not assume anything. And we should not assume anything.