303 O rings?

gregc

gregc

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with the warmer weather, am going to head out to the range with the 303s. want to continue to reload but interested in fire forming the brass to fit the individual chamber of each rifle using O rings to better fit the cartridge to the rifle. so, what are the types of O rings that you could suggest i try?

thanks for your direction.
 
You could get seriously blipped taking even a spent case out in public nowadays. Ideally someone has the O-ring number that works on a .303 case handy so you can just walk in and buy the right ones (or order online for that matter).
 
I used rubber elastic bands for dental use. Available on Amazon.
https://ww w.amazon.ca/MIBICIRI-1000pcs-Orthodontic-Elastics-Elastic/dp/B09MHGX3G2/ref=mp_s_a_1_1_sspa?crid=28FDGT4TNG4E6&keywords=rubber%2Bband%2Bdental&qid=1706389904&sprefix=ruber%2Bband%2Bdental%2Caps%2C111&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRm&psc=1&th=1
 
wesson357 said:
I used rubber elastic bands for dental use. Available on Amazon.
https://ww w.amazon.ca/MIBICIRI-1000pcs-Orthodontic-Elastics-Elastic/dp/B09MHGX3G2/ref=mp_s_a_1_1_sspa?crid=28FDGT4TNG4E6&keywords=rubber%2Bband%2Bdental&qid=1706389904&sprefix=ruber%2Bband%2Bdental%2Caps%2C111&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRm&psc=1&th=1
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This ^^. Once fired and then only neck size going forward. Otherwise, case head separation may be imminent after only one or two firings, depending on the rifle's chamber, as it was in the case of a shooting buddy's SMLE.
 
woodlotowner said:
This ^^. Once fired and then only neck size going forward. Otherwise, case head separation may be imminent after only one or two firings, depending on the rifle's chamber, as it was in the case of a shooting buddy's SMLE.
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Important to note to do that "o-ring" fire-form on FIRST firing of the brass - if it was previously fired - and case wall inside has stretched to cause that incipient separation groove - use of the "o-ring" will NOT repair the issue. Not all LE 303 British had the chambers enlarged, so may not be getting stretched cases in every firing - but happens enough to make reloading / checking / inspecting on rimmed cases like 303 different than for most "rimless" cases. Noted that "worst" for the issue were likely sold off as cheapest mil-surp - any 303 British from British military in WWI.
 
I just bought them on Amazon (or Aliexpress??)

The size is 10mm. About $6.00 for 200 rings. maybe 12mm would be better.

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When doing this are you using reduced loads to form case or standard load? I came across a really good buy on PPU ammo a few years ago, cheaper than buying all the components seperate. Could I do the o ring on these to fire form?
 
hogie said:
When doing this are you using reduced loads to form case or standard load? I came across a really good buy on PPU ammo a few years ago, cheaper than buying all the components seperate. Could I do the o ring on these to fire form?
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Yes, you would use o-ring to fire form those new factory loads - is not much point to it, if the case has already been fired. 303 British headspace on the rim - so when you have headspace checked - that is what matters - the "space" for the case rim - the body size is irrelevant for head space check. During WWI many SMLE deliberately had chamber bodies made fatter and longer - they still head spaced correctly - but case body area much "larger" than SAAMI spec - various stories why - was a "scandal" in Britain about trainloads of substandard (oversized) ammo that was bought for military use - alternate version was to accommodate bent or dirty cases from WWI trench warfare mud - in either event - British Military needed the round to chamber - then to fire - but not burst - and then extract - they did not reload - did not care whether case could be reloaded or not - something that we do, now.

Under "normal" firing sequence - the firing pin strike is going to push the case as far forward as it can go - before or while making the primer fire. The case mouth and neck - near the front - is going to expand first and tightly grab the chamber walls - that "tight" grab will progress towards rear - at some point, pressure is enough to slam the case head back to the bolt face - when that happens is when the case wall might get stretched too thin - right in front of the solid case head. Idea with "o-ring" is to hold that case tight to the bolt face - let the case body expand and confirm to the chamber - without any stretching happening. That is why you only want to neck size and DO NOT push the shoulder back - essentially, you might be transferring the "end play" from the rim to the case shoulders. When done properly in a rifle that needs it, the fire-formed rim about never hits to the seat for that rim.

If your rifle has next to no "head space" / "end play" gap in its chamber - do not need to fire form. Is the slopped out and well used mil-surp that will see the most benefit. Or, fire factory loads only - fire once and discard - that was what it was originally designed to do with the original military ammo.
 
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hogie said:
When doing this are you using reduced loads to form case or standard load? I came across a really good buy on PPU ammo a few years ago, cheaper than buying all the components seperate. Could I do the o ring on these to fire form?
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PPU brass is usually correct head diameter, so not as much worry there. The big problem is with North American brands of brass, but it sure can't hurt to be careful anyway .....catnip
 
catnip said:
PPU brass is usually correct head diameter, so not as much worry there. The big problem is with North American brands of brass, but it sure can't hurt to be careful anyway .....catnip
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catnip - the o-ring thing / headspace / "end play" is about the rim "thickness", not it's diameter - and is what I have found - PPU brass pretty good for both measurements - some North American made stuff pretty skimpy - so is likely one will see the greatest issues with that - Winchester, Federal, Remington ammo - not so much with PPU or similar. All depends on the particular rifle though - it may or may not have that issue with various brands of ammo. North American stuff supposed to be made to SAAMI specs - WWI milsurps were not - hence the dilemma. Is probable that WWII were not paying much attention to SAAMI either - most militaries had their own specs to follow - may not have cared too much what SAAMI had to say about it. Military spec'd the guns and spec'd the ammo for those guns. In many cases, those spec's were drawn up decades before SAAMI existed - 6.5x55, 303 British, 30-06 and likely many others.
 
I was speaking specifically of the head diam. of the case itself, often several thou. larger than N.A. ammunition.The rim thickness is another issue, and, like you mentioned, the PPU and similar cases seem to be better in that area as well. The larger diam.case helps in preventing the dreaded "banana" that some rifles would spit out.The "o" ring works well , and I have used it many times,but was trying to point out the natural advantages of case brands that were not available, or at least not common,in the past. ...catnip
 
i haven't bothered on a rifle, but do use an o-ring on .410 shells in my 3" chambered Lee Enfield Indian No.1. The rim on the shotshell is slimmer than say a 444 Marlin brass. I was getting light strikes and misfires, works good with the o-ring.
 
If you have a "Staple's" store close by, they will have a selection of multi colored rubber bands used in offices.

You don't need to take a case with you, take a Sharpie felt pen and select the rubber band that looks like it has to stretch a bit to slip onto the ogive of the tube

About a toonie for several hundred.
 
When it comes to pressure and case movement, I believe the following

1) Firing pin/primer pressure pushes case as far forward as is allowed by chamber
2) pressure builds slowly as the bullet is pushed OUT of the case, into the rifling
3) bullet obturates the chamber expansion overall, causing the pressure to build behind the bullet and inside the case
this is done quickly enough so if pressure is high enough, the neck/shoulder/body expands into the chamber
4) once the neck/shoulder/body (all expanding at the same time) is tight against the chamber walls, the base, being free to move, as it has NOT expanded due to thickness/hardness, moves back against the bolt face, stretching the brass in the area of weakest brass

any method of holding the brass back against the bolt face is going to work, this all assumes the action is not stressed to allow the case to stretch more than the headspace will allow

I'm not sure how the mechanism of oiling the case works. to me, it is counterintuitive, as pressure is on all sides, how can it push the case "back" and not "out"?

There have been a few experiments about headspace issues with rimless and firing pin strikes vs primer force for shortening the case's headspace.
with the rimmed cartridges, the headspace can only be altered so far, and that is a lot less than a rimless

now onto Number 5)
flexing of the action so even a properly tight chamber on a 303B in a No1Mk3 allows case stretching... even an O-ring will not help that.
only reducing pressures will

I have bought some 1F 303B brass that is not safe.
because of that, I think no one should buy/sell 303B 1F brass from a No1Mk3 SMLE. There is no way to know how it was fireformed.

but that's just me
 
"oiling the case" - I think that comes from the British proofing procedure - they are not actually measuring circumferential pressure like SAAMI does - I think they are measuring an axial pressure - we would probably call that "bolt thrust" - maybe it works out to be the same. I think their protocol requires their test cases to be dipped in a specific oil before firing - is alleged to make it more difficult for the brass case to grab or "adhere" to the chamber wall - thereby maximizing the thrust on the rifle's bolt lugs. I had read there is a pressure limit over which "oil" will not actually work to do that, but it is their process and they have been doing so for many decades.

Is "normal", I think, for "regular" firing (not British proof testing) to want the chamber and case to be smooth and dry - but the point of their proof testing is to demonstrate the system is adequate to handle wet cases, among other things.

I would question your point #4 - from sectioning some cases, the case neck and front area is usually thinner, then the case wall gets thicker near the case head. If pressure within the case is equal, then, I think, the case neck and front part of the case grabs tightly to the chamber wall, first. As the pressure increases, that "grabbing" continues towards the rear - is the point, as I understand it, of that "o-ring" thing - in "normal" firing, the front part of the case is swelled tight to chamber - can not move - so the solid case head is then driven back against the bolt face - meaning some portion of the case wall - usually just in front of the solid case head has to stretch. Something like an "o-ring" holds the case head to the bolt face - will be next to no case head movement when the firing pin hits the primer - meaning the brass case can expand to conform to the chamber, without any wall stretching occurring.
 
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I tried this last fall with IVI rounds from the 90s in my No 4 MKII and the bolt would not close with the thinnish o-ring recommended. Not sure if I have a good chamber or if I should try fishing line next.
 
I believe the Mark II were made after the Second World War. The "o-ring" thing seems to have most effect on rifles with larger head space - maybe not to FIELD length, but closer to that, than GO length. Is possible that your Mark II may have really tight head space - nothing to do with the chamber on a 303 British - is about the fit (clearance) for the case rim between the bolt face and the rear end of the barrel. Is really small dimensions - a sheet of note paper is typically 0.004" thick - difference between GO and FIELD is often like 0.010" or so - so there is less than three thickness of paper between "not enough" and "too much".
 
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