Tag: CZ

.22 LR Rim Thickness vs. Accuracy

By Joe Berk

Here’s the latest accuracy rabbit hole I’ve gone down: Rim thickness as an accuracy factor for .22 Long Rifle ammo.

Kevin’s .22 Long Rifle ammo sorted by rim thickness.

My friend Kevin told me about a hypothesis that holds you can get even cheap .22 ammo to shoot well if you sort by rim thickness.  I researched this online, and like most things online, opinions are all over the map (Abraham Lincoln once famously said not everything you read on the internet is true).  Kevin told me about this and suggested it might make for an interesting blog.  I felt it was worth investigating, and I’d never considered .22 Long Rifle rim thickness as a variable until Kevin mentioned it.

Federal Value Pack .22 Long Rifle ammo.
Velocity data on the Federal Value Pack ammo. A bit optimistic, as it turns out.

Kevin had a specially fixtured dial indicator for measuring rim thickness (see the photo on top of this blog) and he lent it to me for this blog.  I sorted a bunch of Federal Value Pack 36 grain .22 ammo.  It’s cheap ammo.  I’ll get to the results in a second.

First, take a look at the SAAMI specs for sporting .22 Long Rifle ammo:

…and for match grade .22 LR ammo:

Yeah, I couldn’t see any difference between the two, either.  You’d think match grade specs would be tighter, but they’re not.  But check out that tolerance on rim thickness for both grades of .22 Long Rifle ammo…it varies from 0.036 to 0.043 inches.  That seems like a huge tolerance to me.

If you read the SAAMI .22 Long Rifle chamber specs, the dimensioning there is the same for the chamber’s rim cavity on a sporting versus a match chamber.  The chamber rim cavity can range from 0.043 to 0.051 inches.  It would seem that a max thickness rim in a minimum length chamber would have line-to-line contact between the bolt face and the back of the cartridge.  Any cartridge case that is less than 0.043 inches thick, or any chamber with the rim cavity longer than 0.043 inches, would allow clearance between the bolt face and the rear of the cartridge.  Thicker rimmed cartridges would have minimal room to move around in the chamber, and should be more accurate.

The rim thickness versus accuracy hypothesis sounds like it could make sense.  Differing rim thicknesses will influence headspace (.22 Long Rifle ammo headspaces on the rim), and that could influence accuracy.  Also, as Kevin pointed out, rim thickness will affect firing pin strike on the cartridge case, so if the thicknesses are identical, the firing pin strike should be the same (and that should result in improved accuracy).  It all sounds good, anyway.

I used Kevin’s dial indicator and measured rim thickness until I got tired of doing so (it took 62 cartridges for me to get there).  I found four different rim thicknesses:  0.038, 0.039, 0.040, and 0.041 inches.  I could see that the rim thicknesses were normally distributed with the cartridges sorted on my desk, but I thought it would be cool to prepare a bar chart and make that finding a bit more clear.

A rim thickness histogram. The rim thicknesses are normally distributed.

All the cartridge rim thicknesses I measured were within the SAAMI specification, but the SAAMI specification is liberal.

I headed to the West End Gun Club the next day with my most accurate .22 rifle, a beautiful CZ 452 Varmint model with a 14X Mueller scope.

My CZ 452. I saw this photo on a forum and wrote to the owner asking if he’d consider selling the rifle. The answer was no. But a year later he contacted me to ask if I still wanted to buy it. With wood like that, my answer was “Hell, yeah…”

The plan was to shoot five-shot groups at 50 yards off the bench using the rim-thickness sorted Federal El Cheapo ammo.  Here’s how that turned out:

50-yard groups with ammo sorted by rim thickness.

All my shots felt right when the firing pin dropped, so I’m pretty sure that I wasn’t throwing the shots around due to poor technique.  This rifle normally shoots much tighter groups at 50 yards with standard velocity ammo.   The intent here was to see if I could get it to shoot better with cheap ammunition sorted by rim thickness.

Here’s how it all shook out:

Results show that for Federal Value Pack ammo, sorting by rim thickness does not improve accuracy. Note the average velocity figures; they are all well below Federal’s claimed 1260 feet per second value. All group size data in inches; all velocity data in feet per second.

My take on the above data is:

      • All the rim thicknesses I measured were well within the liberal SAAMI specification tolerance band.
      • Not surprisingly, rim thickness is normally distributed (something I’ve found to be true for most manufacturing processes).
      • Rim thickness, at least with my inexpensive Federal Value Pack ammo, has no effect on accuracy.  Well, almost no effect; that last group with 0.041-inch rims jumped in size.  But it was only one group.  I looked for more 0.041-inch-thick rounds to test this again and I couldn’t find any.
      • Rim thickness, at least with my inexpensive Federal Value Pack ammo, has no effect on average velocity.
      • Rim thickness, at least with my inexpensive Federal Value Pack ammo, probably has no effect on velocity variation.  Although the table above shows differences, it’s likely those differences would disappear if the sample sizes were larger.

Sorting by rim thickness is an interesting hypothesis, but my limited testing to assess the hypothesis shows it’s not worth the effort (at least with Federal Value Pack ammo).  I have Remington and Aquila target ammo, and I’m going to measure rim thickness on those two brands in the next day or so to see if the rim thickness variability is lower.   I’ll fire that sorted ammo through the CZ to see if the rim thickness hypothesis holds up with better ammo.   It’s another good excuse to get out to the range (not that I ever need an excuse).

Stay tuned, my friends.

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The Rimfire Series

By Joe Berk

I liked good buddy Jose’s blog about his Marlin Glenfield .22 rifle, and we thought it would be a good idea to include a Rimfire Series category here on ExNotes.  When the idea first surfaced, I thought we might have done a blog or two on .22 rimfire firearms.  When I searched through our blogs, I found that we’ve already posted six .22 blogs.  For your quick reference, here they are:

A Tale of Two .22s (a CZ Model 452 and a Remington Model 504)

A .22 Colt Trooper Mk III

¡Siluetas Metálicas!

First Person Shooter

A 200th Year Ruger .22

A Tale of Two Springfields

We’ll be including a category for these on our Tales of the Gun page, too.

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Watch for upcoming rimfire stories in this series, including a blog or two on the GSG .22 1911, the Ruger Single Six, the Smith and Wesson Model 41, a Mannlicher CZ Model 455, a Trainer CZ Model 452 , a Winchester Model, a Remington Custom Shop Model 504, a Ruger Mannlicher 10/22, a 200th year Ruger 10/22, a target grade Ruger Mark III, a vintage Winchester Model 62, and more.  Yep, we like our rimfires.  Big time.  Stay tuned, Amigos!

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The CZ 2075 9mm Rami

As concealed carry powerhouses go, it doesn’t get much better than the CZ 25 Rami.  One of my good buddies owns one and I had an opportunity to play with it.  I was impressed.  On the plus side, it is an all-metal gun with a flat black finish, a hammer (greatly preferred by yours truly over a striker-fired pistol), great sights, and a marvelous trigger.    My bud had a trigger job on his, and it was awesome (light and crisp, with zero creep, just like it should be).

The three-dot sights on the CZ are crisp and non-gimmicky.  There’s no glow-in-the-dark nonsense and they stand out.  In the photo below, the sight picture is just the opposite of what it’s supposed to be (you want the front sight to be in sharp focus, and the rear sight to be a bit fuzzy, but I couldn’t get my iPhone camera to cooperate when I shot these photos).

I liked the feel of the CZ.  I didn’t get to fire it, but I think I might have an opportunity at some point.  The one you see here is chambered in 9mm.  The Rami was also available in .40 S&W, which I think might be a bit much for a gun this small.

The Rami was discontinued in 202o with the introduction of newer CZ models.  I haven’t seen the new handguns, but I can’t see how they can be any better than the CZ 2075.  This pistol just feels right.

More Tales of the Gun!

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The Rimfire Series: A Tale of Two .22s

Rimfire rifles are cool.   The ammo is inexpensive (when you can find it), there’s no recoil to speak of, they are accurate, and they usually cost less than centerfire rifles.  Usually.  Unless you go for fancy wood and high end rifles.  Both the rimfires you see above fit that description.

The one on the left is a CZ452 Varmint model and it is a stunning rifle.  I bought it used and came to it in a unusual way.  I’d never owned a CZ before I bought this one.   I heard they were accurate and I’m a sucker for a pretty piece of walnut.  I saw this one on an Internet rimfire forum, and I knew the chances of finding one like it in a store were slim.  So I wrote to the owner through the board’s messaging system and asked if he’d be interested in selling it.  “No way,” came the quick response.  I forgot about it and then one day about a year later came the email.  The guy needed cash and I needed that rifle.  It was a match made in heaven, and I bought it as you see it here, complete with the Mueller scope.  It’s as accurate as I hoped it would be (it’s the most accurate .22 rifle I own).   Patience pays big sometimes.

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The one on the right I came to own in a different manner.  About 15 years ago Susie and I were in Rapid City, South Dakota.   Rapid City is a cool little town and it has a very cool gun store.  First Stop Gun is a dream come true:  A real gun shop, with an eye for high end guns, blue steel, and good wood. I didn’t buy anything on that visit, but having learned about the gun store I watched for their listings on Gunbroker.com.  One day, they posted an ad for a Remington Custom Shop Model 504.

I pounced on the 504 and I’m glad I did. Remington (as we knew it) is no more, the Custom Shop (as we knew it) is no more, and the Model 504 is no more.  This one checked all the boxes for me…great wood, a Custom Shop rifle, and a rimfire.  Yeah, you might say I paid too much for it, but the value is only going one way (and that’s up).   I’d say I didn’t pay too much; maybe I just bought it too soon.  And no, it’s not for sale.  It shoots well and the Model 504 has the feel of a full size centerfire (check out the recoil pad on this rifle).  It doesn’t feel dinky like many .22s do.  It’s just a fun gun to shoot and it’s a fun gun to look at.  I do both a lot.

Do you like photos of high end handguns and rifles, and fancy walnut?  Then here is where you want to be:  Tales of the Gun!

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