This might help you with bottle neck rifle cases.
Headspace in YOUR rifle won't be exactly on specification, just a fact of life.
Just keep that in mind, nothing is 'Exact', accept that an life goes on...
The 'Headspace' in any given .223/5.56 is determined from a closed bolt face to the 'Datum Line' or Datum Point' on the case,
Where the case is *Supposed* to contact the chamber (at the case shoulder when everything goes correctly).
This should help you visualize the situation,
Click the Photobucket link below, the image *May* appear for the Datum Line.
Photobucket has changed it's policy about linking directly in, so I'm looking for a new (free) host if anyone has any ideas...
*IF* you are reloading for one specific rifle, you simply screw the die down in the press a little at a time until the cases fit & function once reloaded.
This is the 'Trial & Error' method, it's slow, and since there isn't any specific measurement taken so it's not repeatable.
If the sizing die is moved/removed you have to do the trial and error all over again.
With a case gauge, you get upper and lower limits, a 'Go/NoGo' range via a groove cut into the case head end of the gauge.
Case sticking up past the higher lip, the die needs to be lowered.
The case dropping past the lower lip, the die needs to raised up a little.
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The second measurement the case gauge makes super easy is the Datum Line to case mouth length..
This is where case length trimming comes in...
This type of gauge has a 'Go/NoGo' groove cut into the case mouth end.
Cases bloat when fired, they expand to the chamber walls, that bloat actually makes the case SHORTER.
When you push case walls back in where they belong, that brass is pushed back up.
After a few firing, the case stretches enough it may have to be trimmed.
The case gauge will tell you very quickly when that happens.
The third is the case body. All cases bloat, that bloat needs to be pushed back in where it belongs.
AR style rifles have an UNSUPPORTED lower chamber, and AR fired cases like to bloat down low and a lot of tighter AR chambers will have issues.
This issue has made 'Small Base' dies real popular since they reach a little further down the case sides...
The side case profile cut into the case gauge will be a little 'Loose', but will get your case bodies fitting into everything but the tightest chambers.
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When you get a little more experienced, you can use a caliper with the case gauge to get pretty well precise measurements of you MEASURED Datum length.
You simply measure an UNSIZED case, dropped into the gauge, from the bottom (mouth end) of the gauge to the head of the case.
This gives you the EXPANDED case length, your 'Base Line' measurement.
Then you simply size the case MINIMALLY, a little at a time, until the case fits back into the chamber.
When it fits, that is your MEASURED, REPEATABLE measurement, repeatable because the gauge & caliper will allow you to adjust the die to repeat the die positioning.
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This gauge also works as a FINAL INSPECTION quick gauge.
Drop loaded rounds into this gauge and it will tell you instantly if the shoulder bucked when bullet was inserted or crimp procedure failed.
The round simply won't drop into the gauge down to the 'Go/NoGo' groove area.
If feeding reliability is an issue, this is a really big deal!
This is simply the easiest way to do that final inspection I've ever found.
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One addition I made to my case gauge is a 'Cartridge Over All Length' adapter.
It's simply a little 'L' bracket and hose clamp... about $3.
I simply put a properly length measured loaded round into the case gauge,
Position the 'L' bracket on the side of the case gauge so the leg of the 'L' contacts the nose of the bullet,
And tighten the hose clamp around gauge body & clamp, holding the 'L' bracket to the side of the gauge.
When a loaded round is dropped in, the round in in the 'Go' zone, and the bullet is nosed up to the 'L' bracket, I have a VERY FAST visual reference to know the bullet didn't get seated too deep or too long.