Thinking About Rim Thickness

W

WhelenMan

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Here's another question along the same lines of the other current post on ammo sorting that I was thinking about yesterday. When one is sorting rimfire ammunition just how much runout is there around the part of the rim towards the bolt that may give different readings dependant upon where the indexing point might be. Also how much runout is there on the front of the rim where it contacts the barrel? Anyone ever measured this and determined if there's a thickest portion of the rim where it will make full contact with the barrel and tightest headspace with the bolt and indexed accordingly. I know it would be a pain to try to sort this way but just answer as a "what if?" question.

The sorting tool I'm using is homemade and makes contact with the center of the rimfire face but I notice that in some rounds there is enough runout to be able to get different readings according to the rotation of the round in the tool. Would a difference in the forward rim thickness result in an ever so slight misalignment of the cartridge with a sloppy cartridge to chamber fit? Just thinking. I know, I know, the wife says it could get me into trouble.
 
I have found the same thing. Kinda felt that the bullet may be started into the lands and be fairly strait before the rim might touch anything. Also with a headspace of .43 or so and if the rim is thinner, it might not matter.
 
Straight from the box

I found that the best use of my time, was to work for ammo, and buy the better quality stuff?
 
Yeah right.I checked Eley match and guess what,they are not consistent in weight (up to .8grs difference max min),rim thickness (10/100mm difference max min) nor concentricity (apauling:half have more than 3/100mm runout)
They are considered to be top quality...
compared to Lapua center-X the difference is huge:they are very consistent
But my sorted Eley outshoots the Lapua which I didn't sort before.
 
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Hey Guys

Having spent way too many hours weighting, and measuring, ammo let me add to this discussion.

Eley match ammo normally has a rim thickness of .038 to .039. Most rifles are head spaced at .042 to .044. When most people insert a round, they push it forward into the chamber until they meet resistance. Then they close the bolt, and the bolt face pushes the round forward until it goes in as far as the head space allows.

Think about it! The round is still three or more thousandths from making contact with the barrel face.:eek:

Now when you pull the trigger, here comes the firing pin, it hits the case rim drives it up against the barrel face (.003 or more) and boom goes the round. Now one would think this movement would not be a good thing for accuracy!.

If you agree, I have a little test for you. Just try this. Make yourself a little tool that you can use to push the round all the way into the chamber so that the rim is up against the barrel face. This way when you pull the trigger the firing pin can not move the round, all of it's energy will be used to drive the pin into the rim of the case, rather than energy lost by movement of the case. Now you can try to push the round all the way in with your fingers but it's hard to do, and you will not get them all the way in. Use a tool. You want the inside of the case head all the way up against the barrel face.

Shoot a few groups like this and compare. Please post your results!! I think many will be surprised!
 
I will give it a try tomorrow.All in all it sounds logic...so why not.
 
I am not a bolt rifle shooter but in most singleshot traditional rifles, the rim cut out is a little thinner than the rim and there is a crush fit of the rim into its recess as the block is closed. I am surprised that that is not the case with a good bolt action.

Brent
 
I would think that the dual extractors in my rifle would actually push the round against the boltface before they opened and engaged. It may not be enough pressure though. I have noticed that any extra lubricant build up will prevent a round from seating fully without extra pressure. You can see that even when checking cartridges for rim thickness. Maybe we need to clean our breech faces more often. It would be interesting to smear a little extra lube around the foreward portion of the rims of some cartridges and see what the accuracy was as compared to some cartridges that were cleaned along with the chamber face for each round. It's probably been done somewhere along the line but I may have to do that just to see.

Regarding depth of the rim cutout I would think that the headspace and crush fit can't be done in a rimfire because of possible accidental discharges what with the wide range of firearms the ammo must fit. Supporting that rim can be on the barrel or the bolt depending on the rifle and may be full circumferance, partial circumferance, or none depending on the rifle.
 
Tony, does the gun you're doing this with have a chamber that fully engraves the drive band on the Eley?
 
BrentD said:
I am not a bolt rifle shooter but in most singleshot traditional rifles, the rim cut out is a little thinner than the rim and there is a crush fit of the rim into its recess as the block is closed. I am surprised that that is not the case with a good bolt action.

Brent
Click to expand...

It would seem most BR smiths chamber to about a .043". Bill Calfee, some time ago, ran and published a series of tests showing [to his satisfaction anyway] that in a custom BR application, a bit of headspace excess is preferable to a crush. I'd suspect it's a combination of headspace/chamber issue and there are lots of chamber variations.
 
Tim

The chamber I tested engraves through the first band but stops before the second band. The results are so dramatic I believe the results will be the same if one band or both bands are engraved. If the chamber were so deep there is no engraving, may change the results, but I've not tried that.
 
Well Tony,I shot 2 times like that.I made a little tool from wood to push the rounds completely in.The extractors can't push them in this far.It takes some force to fully chamber the Eley rounds.
Conclusion:I must honestly say I can see some improvement in consistency.Maybe it's only mental but it works for me.Yesterday I shot a 247 and I screwed up on the 2 last shots myself shooting too relaxed thinking it's easy now... .But I must add our benches at the range aren't very stirdy and any movement is directed in wobble.The holy grail is about to be found:D
 
Pieter

From your last experiment you are probably thinking if I headspace this thing so the bolt pushes the case head right up against the face of the barrel/chamber with no clearance, this thing will shoot like a house on fire. Right!


Sorry to tell you this, but it will not! And if you get just a little crush on the case head, it will shoot terrible. For whatever reason there has to be a little clearance, and it don't seem to matter if it is .001 or .005. but It's got to be there. Headspace is not critical. Anywhere between .039 and .045 seems to work. If you shoot Eley. If you shoot Lapua you need a little more usually .42 or greater.

When I first did this experiment I thought I had found something, but no just another dead end road.

:mad::confused:
 
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