The o-ring method of fire forming case to extend their life

[Mayhem]

OK - I will apologise for starting a new thread on this but I just couldn't figure out which of the fifteen thousand and twenty three existing discussions that lead into this to comment on.

I just took my new to me No4 Mk1 to the range and put 40 rounds of PPU ammo through it, placing an o-ring on each case. My question is should I be concerned about how tight the bolt is to lock, when chambering the round? Whilst I didn't have to stand on it, it was firmer than I would have expected.

I haven't had a chance to reload these cases and try the 'fit' yet and probably won't for a few weeks.

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[30Three]

If your headspace is good; why do you think you need to put an o ring on the case? Have you checked the head space?
PPU is good brass so will reload well in a rifle that is within proper specification i.e. it passes the Field Gauge.
My understanding of the O-ring method was that it's was for rifles which have excessive head space; that probably should not be fired!

[WarPig1976]

Your just crushing the rubber. No big deal.

[Parashooter]

When you fire a new case for the first time, use an improvised spacer ahead of the rim - anything from a precision metal washer to dental floss can work to hold the the cartridge head firmly against the bolt face and eliminate or reduce stretch even if head clearance is significant. . . Such techniques are useful only if the rifle has excess headspace. With normal headspace, initial stretch isn't enough to worry about.

[Bindi2]

The O ring is used to centre the case in a generous sized chamber for its first firing. It has nothing what so ever to do with headspace. ENFIELDS headspace on the rim not the shoulder. If used as QUOTED case head separation will occur.

[Bruce_in_Oz]

There are two issues in play here.

1. if the actual headspace, (distance between breeching face on the barrel and the front of the bolthead) is excessive, there are several serious problems with the rifle in question.

2. If the headspace is marginal AND the chamber is "overly generous", blowing the case shoulder forward MAY enable you to "headspace" on the case shoulder.

However:

A. The case of standard .303 brass is VERY thin at the neck / shoulder region. Even in "robust" cases like the 7.62 NATO it is not very substantial and thus it "flows" and distorts during forming, firing and resizing.

B. When ANY case if fired, the FIRST thing that happens is that the primer will "back out'. This is more noticeable on rimless than rimmed cases.

Actually, the primer is first "punched in" by the striker: when it ignites, it pushes the case forward and at the same time tries to back out of the primer pocket. This initial "blowing forward" of the case results in the shoulder being pushed back, thus INCREASING the nominal "headspace". Upon ignition of the propellant charge, the case expands in all directions, forcing the head BACK against the bolt face and the shoulder forward. In a PROPERLY set up combination of ammo and chamber, these moves are measured in the thousandths of an inch, but they HAPPEN.

To give you some idea, size a rimless case so that the rifle bolt will JUST close in a particular chamber. Seat a primer as normal.

Fire the action and extract the case. What you will find is that the primer is protruding sightly above the case head. What this means is the tiny amount of power in the primer has driven the case forward and thus driven the shoulder BACK. DO NOT use these "test" cases in subsequent full-power loads unless you reform the neck / shoulder junction to provide "proper" cartridge headspace.

I have a K-98 on my workbench at the moment that has such excessive headspace that ignition, even given the substantial retention of the big extractor, is marginal, to say the least. When it fires, the shoulder is blown forward AND there is slight primer protrusion. It is a "Russian" rework and essentially needs to be stripped, have the bolt seating gauged and then probably re-barreled. It appears from the (occasional) fired case that, not only is it not the original barrel, but "someone" has freshened up the chamber a bit too enthusiastically.

The much steeper taper on the .303 case vs. the 7.92 x 57 is also a factor; "straighter" cases "stick" to the chamber walls a bit better than ones tapered like the .303.

All of this primer movement is the reason for the heavy crimping found on military issue ammo. Errant primers are likely to cause interesting functional variations / stoppages in Maxim / Vickers / Browning type guns.

[WarPig1976]

Without making a mountain out of a mole hill. Again, your just crushing the rubber O-ring. Not a big deal, been there done that.

[Parashooter]

I once believed that business about the .303 shoulder being too insubstantial to resist deformation on firing. Then I decided to test the idea. Doesn't seem to be true. Here's another bit from "Headspace 101 . . . " -

To demonstrate how we can control head clearance using only the shoulder, I filed off the rim of a once-fired Remington .303 case. After adding an extractor groove to fit a Mauser-size shellholder, I neck-sized, reloaded and fired this case 19 more times.



The load was a 180-grain jacketed soft-point over a lightly-compressed charge of IMR 4350 (giving an average velocity of 2310 fps for the 19 shots and listed at just under 39,000 CUP in my IMR data booklet). The test rifle was a 1943 Lithgow S.M.L.E. Mk.III*. 20 shots was enough for a practical test, I sectioned the case to examine the web/body junction area where thinning normally occurs.


This case, fired 19 times with no rim, has not stretched or thinned at all. I'm sure it could have continued for at least another 20 of these moderate loads.

It's clear to me that the .303's shoulder, alone without help from the normal rim, is entirely adequate to maintain "headspace" when sized in a way that preserves the shoulder location. Those handloaders who experience poor case life with neck-sized handloads should look for other factors to explain premature case failures.

[Bindi2]

My go to work rifle is a No5 which has a generous chamber. Cases fired in this rifle will not chamber into my No4 T , No1Mk3HT or my No3 T even though the headspace is tighter than some of them. I have had case head separation in this No5 never in the others. The cause of the separation was poor quality under sized brass as of new first firing in a generous chamber. Cases fired in any of the Ts will chamber in the No5 without resizing. The size of the chamber and non military quality brass (the metal) and SAAMI size (case size) are the problem not Head space.

[jmoore]

I usually don't bother with the o ring trick, but it does help when needed. (Usually used with the first firing of 6,5 Japanese. Chambers on these are "generous" and the rim isn't!)