HS is often quite over stated in its absolute importance. SAAMI gives a pretty wide berth with these values because they can. If you shooting in the "fire and forget" community where you only run commercially loaded ammuntion, then you are risking very, very little. Almost nothing. In fact, too tight of HS will cause BIG accuracy problems. Proven this a bunch of times.
.0005" and on up to .006" won't make a gun shoot better or worse. It will however potentially cost you some money and aggravation if you fail to understand what can happen if you don't address it.
If you are "white sheeting" a particular job, then you have an advantage. You can set your brass up very easily a couple ways. the easiest is to just fire form it with the bullet seated intentionally long so that the case head is against the bolt face when the bullet is jammed into the lands of the barrel's chamber. If you have a plunger ejector, removing it works to your advantage in this instance. This way the case is truly pushed against the bolt.
The other way is a false shoulder. Neck it up, then squish it back down, but leave a small ring of the larger neck. This will "pinch" the case between the bolt face and the neck/shoulder junction of the chamber. More work, but your able to seat bullets at the desired COAL this way. You'll have it right when you feel a light pinch on the bolt as you roll it into battery.
What this does:
Brass moves around when its fired. We know this. When the striker hits the primer, it wants to push the case forward in the chamber. One of 4 things stops that.
1. Case body taper
2. Case shoulder contacting chamber shoulder
3. Bullet engaging the lands of the throat
4. Case belt contacts the machined ring in the chamber
When any of these take place the case stops moving forward. Ignition immediately follows and the fire gets lit. As pressure comes up the case body swells like a balloon until the chamber walls and bolt face stops it. Then the bullet is coughed down the hole.
The issue that'll give you fits arises when the case is scooted forward inside the chamber. The surface area of the case body latches onto the chamber wall. Quite dramatically. The friction it creates locks it into place because a pressure vessel exerts itself at a right angle to whatever its contacting. If the gap between bolt face and case body exists, the case will stretch itself as that portion becomes the path of least resistance. Brass with only maybe .04" of wall thickness won't stand up to the tens of thousands of pounds of pressure created inside a modern high performance cartridge case. This condition always makes the web of the case thinner in its cross section.
Now resize your brass and don't pay attention. Take it back to SAAMI length vs what your chamber is. Now you fire again and the web gets even thinner. Do this more than 3 or 4 times and the cases pop apart. Case head separation...
You can do this regardless of where HS is set if you don't pay attention to what you are doing at the reloading bench. Stick with the two suggested practices lined out in the beginning of this and it'll go away. Ensure you setup your sizer die correctly as well otherwise you put yourself right back in the same spot.
So there's that part of this. Now, the fun shit: The complication can really start when you have a barrel that tanks and your sitting on a big ol pile of hand made ammo already loaded up. 2x when another shop fitted the first barrel but now you are taking your work someplace else. Getting the HS the same is tougher than most seem to realize. Reamers can be made very differently and still live well inside published SAAMI guidelines. If my "6mm Wiz Bang" has a radius callout of .02" at the neck/shoulder but your brass was fired in a chamber cut with a .04" we will be chasing HS issues. Now cascade this to the rest of the features/transitions:
*body/shoulder
*shoulder/neck intersections
*body OD at the web
*throat
*FB, etc...
SAAMI is only a moderator of sorts. They are not the police. Tooling companies can do whatever they want and as we know, some are far better at this than others. Thinking you can do it "by feel" is also plagued with issues. Least it was when I told myself I could do it. (I can't and gave up trying to force it a long time ago)
The better path out of this is to save up a few more dollars and have a few barrels fitted simultaneously. You are at far better odds of being able to truly swap and go. I do this for a lot of "gamer gun" guys in the spring. We'd hang 6-8 sticks a season for Jake Vibbert alone.
Hope this helps.
C.
