Homemade rim thickness gauge

[TOP PREDATOR]

The reasoning behind sorting rimfire ammunition by rim thickness sounded very valid to me. The concept is that headspacing varies when the thickness varies, and since rimfire ammo headspaces on the rim of the cartridge, if you can take out the variances by measuring and lotting "like" thicknesses, you will improve accuraccy. Just as in reloading, differing overall lengths and seating depths produce differing results, hampering steady consistancy.

Enter the rim thickness gauge.

As always looking for a way to economically improve performance, I finally decided to purchase a rim thickness gauge. I did a search on the net to find manufactures when I cam across a froum thread on making your own rim thickness gauge. After reading it, I immediately went to my reload bench to get started, and after just a few minutes, put together my own thickness gauge.

Items needed: calipers, and one spent or new .243 case. In the original thread, the gentleman used a .223 case, which seemed a natural choice, but I found that it scrapped off the valuable lubrications on rimfire ammo. So I went with .243. I also had some 3M non skid stick on material around so I thought I'd use it also.



I took the casing and put it on the trimmer to give it a nice flat and even lip. Ironically I used the same calipers used for the rim thickness gauge to so several measurements while turning the casing to ensure it is as flat and even as I can get it. I then took a neck brush to clean the inside of the neck of any residue. I shined the brass up a bit with never dull (for looks) and wrapped the non skid around the casing for grip and to designate it as a casing I want to keep for a special purpose.

I then simply put the casing in the calipers, closed the jaws and zeroed out the calipers.



To measure rim thickness on rimfire ammo, just slip the rimfire round into the opening of the case. The mouth of the .243 case bumps up against the rim and is not too wide as to allow the rimfire case from falling in the .243 case.



Now just put the calipers back on the .243 case with the rimfire case in it and take a measurement. The reading is the rim thickness.

My first 3 measurements....





Now to weigh a few more and see if the concept is real, which means another one of my long winded ammo tests. Will be updating with results soon.

[Jerry G]

I guess you all realize that rim thickness isn't the same. You can get many different numbers for the same rim depending how you turn the thing when you measure.

[Spudgun]

I guess you all realize that rim thickness isn't the same. You can get many different numbers for the same rim depending how you turn the thing when you measure.

Really...why? shouldnt it just be meauring at the highest point, no matter what? assuming the .243 rim is flat and level.....

[Jason]

My Eley cartridges measure .223 to .224 right before the rim. Wouldn't measuring that with a 243 introduce a lot of slop that would let the cartridge inside it be less than perfectly perpendicular and sit crooked during measuring if the rim thickness isn't perfectly consistent all the way around? In some quick testing tonight using either fired or resized 243 brass, my measurements weren't all that consistent. Using a resized 223 case, it scraped some of the lube off as you say. Using a fired 223 case, it didn't take lube off and seemed to be more consistent than using 243 brass. This was using Eley EPS ammo. The 223 case was fired in a Savage 223 factory chamber. A super tight neck in a 223 match rifle might not let the brass expand as much, I assume.

Back when I was shooting rimfire benchrest and the "buck-a-box" match was going on over at RimfireCentral, people were doing all kinda of sorting by weight and rim thickness. It didn't end up in making winning scores nearly as much as practice did.

[Jerry G]

I agree with you Jason. Years ago I tried the rimthickness guage thing. Depending how you turned the case in the holder, you could get different readings with the same case. It did tighten groups a little with the cheep ammo but it didn't do a thing for black box at all.

I have a friend that rim guages all his ammo. He has it in his mind it helps so what the hell. It kept him quiet while we were driving down the road to the match.

[ShooterP14]

I just finished my new gauge. I built it from parts I scrounged from work.

[Trent]

That looks complicated.

[Jerry G]

I guess if you like to tinker that is fine. Just accepting bad groups for practice and buying good ammo for matches is much easier.

[BCloninger]

I guess if you like to tinker that is fine. Just accepting bad groups for practice and buying good ammo for matches is much easier.

This is art! If you don't understand it, I can't explain it!

[ShooterP14]

I guess if you like to tinker that is fine. Just accepting bad groups for practice and buying good ammo for matches is much easier.

This is art! If you don't understand it, I can't explain it!

It seams simple to me... Zero the dial on the holder, place a round in the holder, slide it under the dial, read the thickness. What could be simpler?

I can't afford $1600 a case for ammo, so I buy $250/case ammo and invest my time to make it perform close to the fancy ammo.

What fun is there (for an OCD type like me) if you can't take it to an esoteric level?

By the way, the only thing I had to buy was the dial indicator from Harbor Freight. I think it was around $5 on sale. Also , the only part of the assembly process I didn't do was milling the top of the holder so it would have less than .001 run-out. I got one of my buddies in the machine shop at work to do that for me.

[Trent]

The process is simple and understood by the photos. It's the device that looks complicated.

And I don't mean that like there is anything wrong with it.

[ShooterP14]

The process is simple and understood by the photos. It's the device that looks complicated.

And I don't mean that like there is anything wrong with it.

I take no offence. In fact if I had more time and money (I built this in just over a day) I would have used a digital indicator with a USB interface and used my laptop to record all the data. Done a distribution analysis. Categorized all the ammo into groups. Found what the optimum rim thickness was and separated those for "away" matches. (Did I mention the OCD aspect?) then finally, come match day, STILL only shoot a 24/40.

Oh well, the quest is more important than the destination.

[lone ringer]

IMHO, if it was that simple a lot more of us would be doing it. Have you measured different points around the rim? I bet you get more than one reading. I have done it myself in the past with a fancy gadget and have more confidence when separating cheap ammo by weight. Although I do not think there are any $250 per case that can shoot groups anywhere close to what expensive ammo can get even if separated by rim thickness and or by weight. I would spend my money in a good custom barrel with the hopes that it would handle cheap ammo better than a factory barrel. Would you show us a before and after pictures of your groups?

[Trent]

I take no offence. In fact if I had more time and money (I built this in just over a day) I would have used a digital indicator with a USB interface and used my laptop to record all the data. Done a distribution analysis. Categorized all the ammo into groups. Found what the optimum rim thickness was and separated those for "away" matches. (Did I mention the OCD aspect?) then finally, come match day, STILL only shoot a 24/40.

Oh well, the quest is more important than the destination.

That made me laugh! I've been talking with some buddies over in the Mossberg section of RFC and they were all saying how "it is the journey not the destination" that is important. I told them that the little Asian lady at the massage parlor in town has a different theory that "it is all about the happy ending"!

[ShooterP14]

Have you measured different points around the rim?

I measured at 5 points (12,3,6,9 o'clock and center) and got less than .001 variation on 20 different rounds (Eley Sport).

.... have more confidence when separating cheap ammo by weight.

I found more of a variation by weight due to the amount of lube on the bullets. Which, IMHO, seems that it would have little effect on accuracy.

...Although I do not think there are any $250 per case that can shoot groups anywhere close to what expensive ammo can get even if separated by rim thickness and or by weight.

I shoot Eley Sport (last paid $260/case) for practice and Wolf Match Extra for "away" match, not the REALLY cheep stuff. And since I only buy a brick or so at a time, there isn't enough "supply" to sort it properly. So, my only option there is to not mix lots.

Sorting by rim thickness will give you more consistent head spacing and a more uniform (shot to shot) primer ignition. Which WILL help to improve accuracy.

... I would spend my money in a good custom barrel with the hopes that it would handle cheap ammo better than a factory barrel.

Not everyone can afford a custom barrel and measuring rim thickness, weighing bullets, praying over them, sacrificing small animals are the only options some of us have. Remember I said I only have about $5.00 invested in this setup.

...Would you show us a before and after pictures of your groups?

Perhaps, if I can find a calm enough day here to do a fair test. Also please note 99% of this post was for entertainment purposes. I would bet that less than 1% of my missed shots were due to the ammo.