Why do we chamber to zero headspace?

[dnellans]

If you’re shooting factory ammo, this question doesn’t apply.

If your dies can size you brass back (say -.002) and the brass you’ve got is reasonable length to shoot initially (see factory comment above)....

Is there any real advantage to headspacing a chamber at zero or would a gun chambered at +.005 or -.003 all shoot the same?

 

[msinc]

Well, you could set some "custom" headspace figure and as long as the brass was sized to properly match it then the gun should not shoot any different that if it was set to some other size or factory minimum, again, as long as the ammo matched whatever that is.
If you are asking whether or not there is some "optimal" headspace setting that yields best accuracy......no not that I am aware of in general terms. That is to say, "will it shoot better at +.002" than zero?? Maybe, but not every gun. I have seen/owned rifles that were minimum, but shot better if the ammo was sized down to -.002" or -.003". I have also seen a few rifles that were way under, like Army "field", -.008" and shot just as good as they did when it was corrected.
In theory neck sizing should end up zero ammo to chamber and sometimes this works great, but we all know that sometimes it isn't so great as far as accuracy goes. One of those things that has to be played with and tried, like optimum charge or seat depth.
From a gunsmith standpoint, I think the reason for minimum in the rifle is that it will only ever increase with wear, so minimum gets maximum wear time. This is negligible however, as bolt lugs and lug ways don't really wear much anyways.

 

[dnellans]

Right - my question isn’t about how we choose to size the brass to the chamber (whatever it is). Assuming we can do that however we want, why do we (or maybe some don’t care) shoot for zero on the chamber vs any other number?

I can’t think of any good reason aside from not having to change the die settings between barrels.... wondering if I’m overlooking something.

 

[skeeljc]

It really does not matter (for handloading) as long as your sizing die can reduce the fired base to shoulder a couple of thousands. Headspace gauges are just a point of reference.

 

[rhovee]

Some brass is way short of go guage. I prefer to have the chamber cut to the brass that I am going to be shooting. Less variation between fired and new. And can help with case head separation

 

[WSnyder]

If you hand load you can headspace anywhere you want for reasons you've given. Why 0? We all need a number to target, yes? We have a gauge in hand most of the time so why not base it upon that? I think you'll find many do not chamber to 0 but 0 +.001-.002". If you chamber to 0 you run the risk of an odd die that won't size correctly (move shoulder back enough). That will make the phone ring and more work for you. As Tooley likes to say "Don't make the phone ring" or something to that effect.

 

[Dusty Stevens]

Besides being repeatable you cant measure it on some actions like a panda if you go deeper than zero

 

[msinc]

.........you run the risk of an odd die that won't size correctly (move shoulder back enough). That will make the phone ring and more work for you........

The only "risk" will be if my bench grinder burns up and throws a rod so I cant grind off the bottom of the die and set cases to whatever length I want!!! Then the phone will ring to see if Lowe's has another one.....

 

[tobnpr]

Is there any real advantage to headspacing a chamber at zero or would a gun chambered at +.005 or -.003 all shoot the same?

+.005 is pushing it- almost to "no go" clearance.
This is an issue if you handload, not so much with factory ammo as it's shot once and brass discarded.
Remember your brass stretches towards the neck every time you resize it (even if you neck size, you're gonna need to FL size eventually). That "extra" brass flows from the casehead.

When you hit "no go" clearance, you're supposedly past the thickened casehead.

Eventually it will get thin enough that the extra five thou clearance could become an issue and cause a case separation. Depends on a lot of variables- but the point is, we know that go + half or one thou at most is ideal for brass life, while still being assured of the ability to chamber factory ammo (in theory). No reason to try to do it any other way for me. That said, if it turns out I've overshot a bit and end up at go+ .002, I'm not going to pull the barrel and re-work it.

 

[NorCalMikie]

The only "risk" will be if my bench grinder burns up and throws a rod so I cant grind off the bottom of the die and set cases to whatever length I want!!!

Better yet? Chuck a shell holder up side down in a drill press and run it down on an oiled sharpening stone to get the measurements you want for specs. Mark it and set it aside.
Works like a champ and looks factory so you mark it so it doesn't get mixed up.

 

[Rset]

+.005 is pushing it- almost to "no go" clearance.
This is an issue if you handload, not so much with factory ammo as it's shot once and brass discarded.
Remember your brass stretches towards the neck every time you resize it (even if you neck size, you're gonna need to FL size eventually). That "extra" brass flows from the casehead.

When you hit "no go" clearance, you're supposedly past the thickened casehead.

Eventually it will get thin enough that the extra five thou clearance could become an issue and cause a case separation. Depends on a lot of variables- but the point is, we know that go + half or one thou at most is ideal for brass life, while still being assured of the ability to chamber factory ammo (in theory). No reason to try to do it any other way for me. That said, if it turns out I've overshot a bit and end up at go+ .002, I'm not going to pull the barrel and re-work it.

How does this make sense when a 6 dasher is a 6BR with the shoulder blown forward .100 and reshaped to 40 degrees?

 

[DaveTooley]

We all need a Maginot Line for defense. 0 to 0+2 is defensible.

 

[carlsbad]

My 2c. i don't want the casing touching the chamber at the front. I want the round centered in the chamber by the bullet in the (tight) freebore. So you need some clearance in the shoulder. For accuracy, that clearance isn't important. .002 to .020 would work. But too much shoulder clearance causes brass failure. The clearance is minimized to extend brass life, thus we often settle on .002. zero would not be ok with me, although via springback we sometimes get it when we don't want it. If I chamber a round and feel resistance indicating the shoulder is touching, I'll pull it out and use it for a fouler. --Jerry

 

[Henryrifle]

Perhaps this opinion is limited by to my experience shooting .223 and .308 in F-T/R. Due to the load I choose to use for the 90 VLDs, I get 4 to a maximum of 5 firings out of my brass before the primer pockets are ruined, and, as such, I want to be able to shoot virgin brass with little to no change to my powder charge. Setting the headspace at zero allows me to do that. I do the same thing on my .308 F-T/R rifle in order to shoot virgin brass in spite of having brass that has been reloaded 7 or more times.

Henryrifle

 

[Clark]

Looking at Sammi specs and comparing it to factory rifles and factory ammo, I would expect the chambers to be as short as possible and the ammo to be as long as possible, for the tightest fit. The factory rifles are typically 0.002" of extra headspace, but still in tolerance. The ammo is a few thousands smaller than largest possible, but still in tolerance. This means 0.005" is typical slop. The exceptions are belted magnums and 303Brit. The Belted magnum rifles are only 0.002"sloppy, but the belt can be 0.010" loose. The SAAMI cartridge drawing for the 303 Brit shows a rim thickness from .054" to .064". The most typical rim I measure is 0.063". The brass is not the problem. As Ackley said, the actions are stretchy. When I calculate how much stretch I get:
movement = .0041" @ 65k psi chamber pressure
movement = .0038" @ 60k psi chamber pressure
movement = .0029" @ 45k psi chamber pressure
movement = .0019" at 30k psi chamber pressure

That is not the problem. The lack of replacement bolt heads mean that most No 4 rifles have oversized headspace.

What I prefer to do with belted magnums is to buy a lifetime supply of brass, characterize the brass, rebarrel and headspace to fit the brass... too bad for the next guy when I am dead.

 

[Greg Y]

Nearly every rifle case is tapered. If you increase the headspace the diameter of the chamber grows (belted magnum aside). If the chamber is .004" deeper than minimum, the area of the case web is also likely oversized.

 

[DaveTooley]

Oversize is a bit of a reach. In theory it does get larger but falls comfortably inside standard tolerances for the industry. Lets say a case has .010" taper per inch. You would have to go .100" deeper to increase the size just .001". Using those numbers an increase in the depth of .004" would increase the diameter .00004" That's 40 millionths. We can't measure that without very specialized metrology equipment.