22lr Rim Thickness

22 Long Rifle ammo is finicky. Tell us all about it here.
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ppkny
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22lr Rim Thickness

Post by ppkny »

Has anyone did any rim thickness measuring? I bought a rim thickness gauge and started to sort my SK Standard and SK Match Ammo. I wondering if it's really worth the trouble. I checked 3 boxes of each and the biggest variation I got was .0015. Most are within .0000 to .0005 of a measured lot through all boxes.
What's the normal? I've never done this before and don't no what to expect.
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Re: 22lr Rim Thickness

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If I were to measure any one thing on rimfire ammo, it would be from the top of the rim to the leading edge of the driving band.

Measuring only rim thickness ignores important details like where the variation in the rim thickness is in relation to overall brass length as well ignoring bullet seating depth. Who cares how consistent the rim thickness is if your bullet depth is all over the place.

Measuring from the top of the rim to the driving band of the bullet will enable you to be consistent in bullet placement when chambered; whether it's how much of a jump you have or how much contact with the lands there will be, because then the variations of the brass length or rim thickness won't matter in the grand scheme of things. You know the key metric is where the bullet is in that barrel. Measuring in this manner will show that rim thickness varies outside of the chamber and could possibly impact ignition by being farther from the barrel and being striked by the firing pin sooner, but I doubt it would be enough to ever be measurable in performance or possibly even lock time.

Recording rim thickness or OAL or weight or most any single measurement on rimfire ammo ignores too many other aspects of the cartridge you would also have to measure to provide context to your original measurement.

If the rim thickness varies...ok...what does that variation impact? Did the location of the leading edge change? Was the the base that changed? Was it both? Something else you'd have to measure.

Weight changes...ok...was is brass density? Lead density? Power variation? Primer variation? Lube amount? Who knows if you can't measure it all separately.

OAL varies....ok...so the tip of the bullet isn't in the riflings so you'll have to see where the driving band is in relation to the bullet tip and what if the rim thickness isn't consistent so you'll measure rim thickness to see where the variation is OAL is.

Trying to control the variables in factory loaded ammunition is a rabbit hole you don't really come out of because you would have to disassemble each round to properly find the individual measurements that we can and do with hand loaded centerfire ammo.

In the game of silhouette, I personally think what accuracy improvements that can be gleaned from measuring ammo (outside of lot testing) isn't really score impacting so I don't currently measure anything because like you asked, I don't think it's worth the trouble. That and I think the ammo Lapua/SK/Eley is producing these days is great and doesn't require such tedious activities. Maybe if you're trying to get all you can from some cheap ammo you just want to burn up, but if I were to measure anything, it would be the leading edge of the rim to the leading edge of the driving band to ensure the bullet is seating consistently.

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Re: 22lr Rim Thickness

Post by ppkny »

DoDaddy,
Thank you very much for your thorough explanation of the dynamics in what it takes to make a precision bullet/cartridge assembly. You are spot on. In regards to 22LR ammo your statement:

DoDaddy wrote::Measuring from the top of the rim to the driving band of the bullet will enable you to be consistent in bullet placement when chambered; whether it's how much of a jump you have or how much contact with the lands there will be, because then the variations of the brass length or rim thickness won't matter in the grand scheme of things. You know the key metric is where the bullet is in that barrel.

This seems to be a key factor in all reloading along with all the other variables. The theory behind the rim thickness check, as I've read, is to make sure all bullets SEAT the same way in the chamber when the bolt is closed regardless on where they are in relationship to the lands. Granted where the driving band is in relation to the bullet tip is the ideal dimension check.

I do hand load for my 6.5, 260, 30-30 and 357 guns so I am familiar with most loading procedures in controlling these variables but with 22LR the manufacture has the control. I should have mentioned that I was also weighing the cartridges but is it the bullet, powder charge or the brass that is the variable?

I also should mention that this is for 22LR Mini Palma rifle not silhouette. So other than buying Eley - Benchrest, Tenex or Match at $120/$190 a brick I'm trying to get the best performance out of my SK & SK Match ammo.

I think I can modify my gauge to check what the driving band is in relation to the bullet tip in your illustration so I'll give it a try.
Again, Thanks a lot for your help.
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Re: 22lr Rim Thickness

Post by Doodaddy »

ppkny wrote: Wed Mar 30, 2022 4:23 amThe theory behind the rim thickness check, as I've read, is to make sure all bullets SEAT the same way in the chamber when the bolt is closed regardless on where they are in relationship to the lands. Granted where the driving band is in relation to the bullet tip is the ideal dimension check.
That's why I went through so much detail because the theory behind rim thickness checking is flawed.

Let's say for instance one rim is significantly thicker than the other 45,000 rounds you checked. Does that mean it will seat differently? No, it means you have to measure other aspects of that round to give any sort of value to the rim thickness. If the body length of the brass of the brass changed due to the rim thickness, then yes, it will seat differently.

But what if the body length of the brass doesn't change and the rim thickness is 30% thicker. How does that change the bullet seating in the chamber? It doesn't, because that thickness variation didn't impact anything that entered the chamber. That's why I was talking about the leading edge (probably poor terminology...the bullet side of the rim) and the base of the rim (the opposite side). If the rim thickness varies from "perfect", one of those edge's location has changed in relation to the rest of the cartridge. If it is the non bullet side of the base, who cares? If it's the bullet side of the base, that could change bullet seating, but you won't know which of the two is occurring without measuring something else. Even then, you've found the variation in the brass, but no relation of that to the bullet so you're off to measure something else.

Which is why I say measuring rim thickness on its own an effort of futility. Measuring from the top of the rim to the leading edge of the driving band will do exactly what you're wanting: consistent chambering. It accounts for the relationship of that leading edge of the rim and the first part of the bullet to touch the lands. You won't know the variations between those two points, but you also won't need to because they all worked together for your final goal.
ppkny wrote: Wed Mar 30, 2022 4:23 amI also should mention that this is for 22LR Mini Palma rifle not silhouette. So other than buying Eley - Benchrest, Tenex or Match at $120/$190 a brick I'm trying to get the best performance out of my SK & SK Match ammo.

I think I can modify my gauge to check what the driving band is in relation to the bullet tip in your illustration so I'll give it a try.
That makes plenty sense. Now to play devil's advocate. Image

How are you going to measure the ammo without deforming the lead or disturbing the lube on the bullet? Since you're shooting SK, you'll be better off with their oil film than Eley because you'll never get an accurate measurement on Eley involving the bullet unless you wipe off that thick wax effectively ruining it. If the lube or bullet shape is impacted, you introduced a new variation chasing down the elimination of another.

Food for thought.
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Re: 22lr Rim Thickness

Post by Sporty »

I did considerable testing of variable rim thicknesses and sorted the rounds by +/- 0.0005" groups. That means one group will contain 0.0385" to 0.0395" rims. The reason I did all that work was because the 5000 round case of .22 LR Eley CMP SV ammo rims varied from 0.034" to 0.045". 80% of the case was 0.039" +/-0.0005".

One of my rifles tested is an old Mossberg 46B. You can imagine that rifle wasn't built to tight tolerances. From the bench rest, that Mossberg has its tightest group with the 0.035" +/-0.0005" rims. Unfortunately, I only found about 150 0.035" rounds in that 5000 round case.

I found match grade ammo in the Lapua CenterX and higher grade and Eley Tenex had very tight variations, or none at all, in rim thickness. Lapua is relatively thicker than Eley. Consequently the point of impact at 100 yards varied between the two but I can't tell you if it was because of the different rim thickness.

In other words, match grade ammo is so consistent that sorting by rim thickness isn't necessary.
-Charlie (Lee) Meli
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