Thoughts on headspace difference affecting POI

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Fisher-Dude

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I just developed a load for a 300WM that was putting shot after shot into the same ragged hole.

I used 1F RP brass from factory ammo for my test loads, and didn't bump the shoulders back. They chambered fine, and no signs of pressure right up to max in my test loads.

I always use 0F RP brass for my hunting loads. My 0F brass is .016 to .018 different on my headspace gauge from the test loads.

Would this affect POI to any degree? Or would the near instantaneous case swell upon firing negate any pressure change?

Before I load up a pile of rounds with my most accurate load for hunting season, I'd like to hear your thoughts.

Of course, I'll be hitting the range again just to be sure (even though my test loads ended up dead on the bullseye). Thanks!
 
It will change but it depends on what accuracy you want and how far you intend on shooting. Load some of each and test at your max hunting distance.
 
A change of headspace alone won't change the POI.

BUT, that shot case will move forward to push the case shoulder against the chamber shoulder when it get hit by the firing pin. If the bullets are seated long and they engage the rifling as the case slides forward this will soften the blow of the firing pin and can cause missfires. I have seen this several times.

Test the ammo before the hunt.

If you put a shim on top of the shell folder you can reduce the amount of shoulder set-back.
 
One thing that needs to be mentioned, NOT all chambers are created equal.

One of the things headspace helps to control is cartridge cant in the chamber.

If your firearm has a a large or even oversize chamber, cartridge cant can cause POI changes that are increased with excessive head space.

Tight chamber conditions along with tight headspace will make a firearm more accurate as long as other conditions are tight as well.

This can be especially true with Flanged cartridges, such as the 303 British and 30-30 Winchester.

If your chamber dimensions are close to mean and your headspace is within spec, POI shouldn't be badly effected if the headspace is a few thousandths on the + side. It should shoot acceptably

There is a good article in the August Handloader magazine about loading dies and how they effect accuracy.

Factory ammo will often shoot really well in rifles with tight chambers and not so well with chambers that are larger than mean spec and get progressively worse as the chamber gets larger.

I was always impressed with the ability of WWII and pre WWII Mausers to have thousands of rifles made with close to identical chambers.

I could often take out a half dozen rifles chambered for the 7x57 or 8x57 and use the same load in each caliber and not be able to tell which rifle the cartridge was fired in.

One fellow I spoke with told me that the factories used to make up a reamer and use it for a specified number of cuts at a pre determined depth, then exchange the reamer for a new or freshly resharpened one. Usually the resharpening only consisted of a proper stoning, which was almost a trade in itself.

When the reamer was sharpened, the shoulder/case mouth was also ground back the same amount and the reamer would be marked with the number of times it was sharpened, so the lathe operator could set up his machine accordingly.

Whatever, it all sounded good to me and in all honesty, I've only had to have a few reamers reground, which sometimes proved detrimental.

Headspace does more than just control iginition issues, it also controls consistent center of bore axis with center of bullet axis, which is critical for the bullet entering the leade consistently, rather than at a haphazard, albeit very slight cant, depending on the angle of the rifle.

The extractor should not touch the rim at any time while the cartridge is chambered, etc.
 
Ganderite said:
A change of headspace alone won't change the POI.

BUT, that shot case will move forward to push the case shoulder against the chamber shoulder when it get hit by the firing pin. If the bullets are seated long and they engage the rifling as the case slides forward this will soften the blow of the firing pin and can cause missfires. I have seen this several times.

Test the ammo before the hunt.

If you put a shim on top of the shell folder you can reduce the amount of shoulder set-back.
Click to expand...

300 WM, it only can go forward until the belt meets the recess in the chamber for it, generally. Then the brass will flow forward to the shape of the chamber. Which may, or may not, cause interference between the bullet and the leade. Lot going on in that chamber. - dan
 
If there's a difference in case capacity between virgin and 1x fired brass there will be a pressure (and velocity) difference when running the same powder charge. In my limited experience I've usually have to dial the powder charge down 0.2 to 0.5gr on fired brass compared to virgin to get back in the node I found with virgin brass. 1x fired brass is often 25-50fps faster than virgin brass with the same powder charge from what I've seen. The difference seems to be less with my precision cartridges, more in my hunting rifles.
If your node isn't very wide it could be enough difference to push you out of your accuracy node.
 
Unholysavage said:
If there's a difference in case capacity between virgin and 1x fired brass there will be a pressure (and velocity) difference when running the same powder charge. In my limited experience I've usually have to dial the powder charge down 0.2 to 0.5gr on fired brass compared to virgin to get back in the node I found with virgin brass. 1x fired brass is often 25-50fps faster than virgin brass with the same powder charge from what I've seen. The difference seems to be less with my precision cartridges, more in my hunting rifles.
If your node isn't very wide it could be enough difference to push you out of your accuracy node.
Click to expand...

I would expect the 1f brass to have more capacity, not less.
 
It will, but the volume difference between virgin and 1x will be less for a tighter chamber than a sloppy one. And it depends on how much you resize the fired brass. A neck sizing or slight shoulder bump would retain more of that volume difference than a full length resize.

I've always had to back off the powder charge 0.2-0.5 grains from my virgin brass load. And more times than not there's been enough difference that I've had to redo the load workup on 1x brass. I've read people's theories on why 1x fired brass has higher velocity with the same powder charge. Some claim that the process of stretching virgin brass on first firing smooths out/reduces the pressure peak, which doesn't happen with formed brass. Seems reasonable I suppose, but I'm not an engineer. All I know is that my 1x fired brass, with only a 0.002" shoulder bump, is consistently 20-50 fps faster than my virgin brass with the same powder charge.
 
OP,
Full length size your brass to bump shoulder back a few thousandths for hunting ammo.

Virgin brass-the velocity/pressure variation is due to internal ballistics to fire form said brass.

1X fired either neck sized or shoulder bumped does not have to expand as far to fill the chamber so velocities will be higher.

To eleviate the internal ballistic issues, anneal the necks for the optimal ES,SD variables.
 
Thanks everyone for the replies.

Got back out to test the 0F - answer is: no effect on POI at all.


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In a hunting situation I doubt you notice any significant issue at all. You POI is the kill zone on the animal. You either hit it or you don’t. For target shooting, you’ll want to set your shoulders back only about 2 thou or so. Differences in headspace can affect POI a little bit, but for most of us it has more to do with brass life and work hardening.
 
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