Headspacing off the rim/belt versus headspacing off the shoulder.

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Dave L.

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I've been wondering if Remington's claims about their new .300 Ultra Mag of doom hold any water or are just a sales pitch. They've been claiming that the cartridge 'headspaces off the shoulder instead of the rim, allowing for greater accuracy.' I read this in the Remington catalogue that I picked up at Al Flaherty's.

Is this true or are they simply pulling my leg? I always figured it was the tolerances of the ammunition and how snugly it fit in, not where it was held.

- Dave.
 
Its mostly advertising doggerel, and depends on the precision of the chamber, I would say. In a well cut chamber, a shoulder headspaced cartridge will likely have a tighter gas seal, resulting in more consistent pressures. The case fireforms as well during firing, something a rimspaced cartridge may not do as efficiently.
But a well made rimmed chamber will seal well too. My guess is that the rim of the chamber is more susceptible to wear and damage, and more dependent on ammo rim shape and condition, so there would be inherently more variability in pressures on a rim spaced chamber.
 
I have heard (yes hearsay) that BR shooters that use belted cartridges headspace off the shoulder rather than the belt for gretaer accuracy. There was an article 15 odd years ago in one of the gun rags on this very subject.

Don't ask which, don't remember.
 
I agree that "'headspacing off the shoulder instead of the rim, allows for greater potential accuracy". Still, the 300 Win Mag made a pretty good name for itself at 1000 yds.

That having been said, with the recoil put out by the RUM family, they do not lend to the most accurate of shooting. Very poor sellers - I guess they need all the help they can get.
 
If a cartridge headspaces off the rim or belt, the rest of the chamber can be rather generous, and the case will fireform. Belted magnum chambers have a reputation for being more generous than necessary, and adjusting a belted magnum sizing die so that the round will headspace on the shoulder is a common practice. This is felt to give better accuracy and longer case life.
As far as that goes, variations in belt thickness aren't uncommon.
Given an equal level of precision in chambering, neither rimless, nor rimmed, nor belted has an advantage for accuracy.
 
Interesting, looks like everything I've read about guns to this point - marketing craze mixed in with pseudoscience with no indicators as to where the sales pitch ends and where facts begin. :(

Sounds like Tiriaq and Skip both have got pretty solid points. I would've guessed that too. Interesting note about wear and tear on the rim, I hadn't taken that into consideration either.

Either way I'm not a great fan of the Remington Ultra Magnum series' cartridges...

- Dave.
 
It seems that introducing a new line of perfectly good cartridges is way of selling perfectly good guns to people who already own perfectly good guns shooting other perfectly good cartridges.
 
I think the primary advantage to headspacing on the shoulder is brass life for the reloader.
Headspacing on the rim, the shoulder is blown forward further with every shot.
Back in my magnum days, I set my dies to headspace my 338WM on the shoulder for this reason. I agree that it should also be more accurate, but, my own shooting didn't show a noticable difference.
Headspacing on the rim/belt, has the advantage of a looser fitting cartridge with less chance of it having stiff bolt closure.
 
I have read that in reloading belted cartridge casings you are best off to resize minimally, more to just fit the chamber rather than full length resizing to lessen the amount that the shoulder of the casing is moved back for better accuracy.
 
Most reloaders partially-size (or neck-only size) belted cases, thereby headspacing them off the shoulder, not the belt.

But, on a belted case, the headspace is measures from teh belt to the bolt face, and thus the smae headspace gauge is used for the 378, 458, 7mm, and any other cartridge based on the same case. Distance between the shoulder of the case and the chamber is not part of the spec as it is with rimless cases, and it is the tolerance here that affects accuracy.
 
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