Cartridge Headspace Dimension?

[Ballistix]

Hi all.

I have a set of Hornady headspace gage bushings for my digital caliper. I want to find out what new headspace dimension my rifle fire forms the cases to so that I can compare with saami sized unfired cases. The end game is neck sizing and shoulder bumping but I want to start full length sizing until I build up some experience and can source the Forster die I want.

What choice of case/s from below should I use as the headspace dimension to use as saami spec to compare against and aim to get closest to?

Below are the cases I can choose from:
A) A new unfired and unloaded case
B) A pulled and decapped unfired case
C) A live round (not keen on that!)
D) A modified case that you buy with the Hornady oal gauge
E) A once fired from my rifle case

Thanks in advance.

[Laurie]

You cannot use these gauges as a SAAMI standard gauge. They are designed to be used purely for comparison against fired cases and are not machined accurately enough, nor do they use hard enough material for the purpose you intend. If they were made to exact SAAMI and machinist's spec, they would cost a GREAT deal more!

The purpose of these and other such devices is to compare a sized or bumped case against a selection from your chamber - that's what matters anyway, not what SAAMI or CIP lay down. Both chamber and case industry dimensions aren't set in stone, they can't be, as they are subject to allowed tolerances for reasonable manufacturing vagaries, and those dimensions plus their worst case tolerances, shortest chamber in conjunction with longest case, must still be such that the latter will chamber in the former.

So, the object of the exercise is to take a selection of fireformed unsized cases from your rifle's chamber which will normally only vary by 0.001 and to knock the shorter value back by maybe 0.004" for a straight-pull, 0.001-0.002" for a precision turnbolt rifle. You will usually find that new brass is a bit shorter still, sometimes rather a lot shorter. In the case of good quality new .308 Win brass, a LOT shorter than after firing in a 7.62mm chamber. if you want to size some cases so that they will chamber easily in any rifle in the calibre, then measure new brass and set the die up in the press to bump the shoulder back to that value. You are then returning to what the factory gives you, although there may still be chambering issues for those fired in a rather wide chamber ahead of the case-web (or had very heavy previous loadings) if it's attempted to reuse them in a 'minimum SAAMI' chamber.

Incidentally, deprime your fired sample cases before measuring. Fired primers may give false readings through being slightly proud of the case-head. There are no safety issues in measuring the case on a live round, although you wouldn't be able to do it if the bullet length outside of the case is such that it is longer than the gauge body.

[Alpha1]

I struggle with shoulder bumping I still can not get my head round shoulder bumping.?

[Ballistix]

Thank you very much indeed Laurie. Just the education that was needed.

To conclude.

I will deprime my once fired cases, measure the headspace on a few of those and then ensure that my subsequent resized cases are sized back to the optimal headspace dimension which would be .002 inches less than the average 'once fired' measurement.

sign92

[Laurie]

Ballistix - yes, you've got that right. (When you've got the die set to give the readings you want on newly sized cases, it's also a good idea to do a double-check at least on your first go at this by chambering three or four case so sized to ensure they do chamber without resistance.)

Alpa1 - it's very simple in practice. Full-length and body dies squeeze the case body down to a slightly smaller O/D. As the case is pushed into the die body, any reduction in O/D makes it slightly longer overall and in particular, increases the distance between the case-head and the shoulder section. In theory, neck or part-sizing using a FL die shouldn't work as the case shouldn't chamber for this reason, but in practice it often does. How we guarantee the sized case chambering is that the final itsy-bitsy bit of its movement into the die at the end of the press handle operation sees the shoulder section of the die chamber contact the case-shoulder and push it back. With most cartridges, setting the die as per the maker's instructions, ie die body hitting the top of the shellholder with a bit deeper screw-in to account for press frame flex puts shoulder back to pretty well where it is on a new case - ie within SAAMI or CIP tolerances.

That may or may not be optimal for the rifle's chamber. That is while it usually guarantees easy chambering, there may be a larger gap than ideal, ie slightly more 'headspace' than is ideal for precision and case-life even if the clearances are within the SAAMI or CIP tolerances. (Firing a case sized with a .308 Win FLS die in a 7.62X51mm Nato chamber may create slightly exessive headspace in fact outside of allowed tolerances.) In any event, when the trigger is pulled, the firing pin pushes the entire cartridge forward as far as the chamber allows it and any headspace gap transfers to the rear end now lying between the bolt-face and the cartridge head face. When the primer detonates, it backs out of its pocket up to the bolt-face, then as the main charge ignites and pressure builds up, the case-body sides ahead of the solid web are forced out hard against the chamber walls and stick there, ie 'obturate'. The back end of the case is forced hard back against the bolt through the rear of the case-body stretching, this action also reinserting the now fired primer cup back into its pocket.

If this 'bumping' + stretching + 'bumping' on the next resize is more than marginal, the case OAL will grow rapidly on each firing / resizing cycle needing frequent trimming, and the case walls ahead of the thick web will thin a little on each cycle leading to early incipient separation. Excessive headspace plus low pressure loads also often see primers slightly backed out on the case-head and/or excessively flattened giving a false impression of high pressure symptoms.

The answer is to set the die up initially not screwed far enough in so the sized case has inadequate headspace and won't chamber, gradually screwing the die into the press until it chambers as a crush fit, then tiny amounts more until cases reliably chamber without any resistance to turning the bolt handle down. To do this propperly, the expander stem should be removed from the die, and the bolt's firing mechanism should be removed to allow easy bolt closure and feel for any chambering resistance.

The more expensive but easier and more reliable way is to use good callipers, a comparator body and appropriate case 'headspace gauge' (an incorrect term, but all we've got) and use them to set the amount of bump with fired cases from that chamber used as the gauge. This can be done to a single thou' accuracy, but you find that it takes a tiny change to the die setting in the press to achieve that, and must be rechecked on every sizing session.

With a conventional turnbolt, rifle operation gives the user a lot of feedback, ie the final little bit of bolt-handle operation, but this is entirely missing on straight-pulls / semi-autos. It is ESSENTIAL to measure fired cases and set the die properly for such rifles, the norm being to 'bump' shoulders to give 0.003-0.005" clearance in the chamber. This avoids excessive headspace using the default setting, or equally bad, potentially worse, inadequate clearance but just enough to allow bolt closure with inadequate lock-up which is not obvious to the shooter thanks to the power and speed of spring-assisted bolt group movement. In some rifle designs, this will stop the firing pin falling fully and just cause a misfire, in others ignition may take place leading to damage or even firearm destruction. The US M14 / M1A is notorious for the latter and suffered many nasty incidents in the days when it was the primary US High-Power Service Rifle tool, occasionally too with out of spec factory ammo.

Finally, some cartridges, perhaps because of a wildcat or semi-wildcat background may see dies machined such that they bump shoulders huge amounts causing grossly excessive headspace clearances and case stretching if the die is adjusted in the press using the default method. I have seen that with 6mm BR using both Redding and Forster dies.