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Well,you are getting closer on your terminology.
Headspace does not have anything to do with ammunition.It is all about the firearm.It is about the depth the chamber is cut,or the wear/setback of the firearm.
When the gunsmith chambers the barrel,the chamber is cut to a depth that will accept a "go" headspace gage,and not accept a "no go" headspace gage.
There is a longer gage called a "field" that is used to determine a firearm is worn and should be taken out of service.
Ammunition is manufactured to tolerances to function in chambers cut to these specs.
When we reload,we resize the brass,and,with bottleneck cases,what we do setting our sizing die controls the case shoulder location,which is the ammunition's factor in HEAD CLEARANCE. Head clearance allows the bolt to close and lock without interference.Too much head clearance will result in excessive brass stretch.As brass stretches,it gets thinner.The thinning will be localized in a ring inside the case just forward of the case head.Look up "stretch ring" and "paper clip test".
If you back your resizing die out so you do not bump the shoulder,imagine this:
Put a banana in your hand and squeeze it down.The banana gets longer!Same with brass.And,this lengthening is not controlled.We want some shoulder bump to reform the shoulder and put it precisely in the right place.
If you carefully creep your sizing die down,not 1/4 turn at a time,but try 5 minutes on the clock face,you can size your brass to fall between the high and low step on your Wilson gage.If you do that,you will have sized your brass to factory load tolerances,and your ammo should work well.
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