What drives lot to lot variation performance in a 22LR?

[ToddM]

I was thinking about this the other day, it seems like much more so than centerfire rifles, rim fire rifles have much larger swings in accuracy across lots of the same ammo.

What is driving this? Is it really just average velocity variation across lots of a particular ammo, or is there more things that vary lot to lot, such as rim thickness, bullet seating depth, bullet diameters, etc. When they sort 22 ammo by lots is it just average velocity based or are they sorting by these other factors as well?

It just seems like I've seen some guns that will shoot one lot of a match 22 ammo frighteningly well, and another lot of the same ammo equally horribly. Other guns seem to have pretty similar performance between lots of the same ammo. Is it really only velocity that's driving this? Not that I have not seen lots of centerfire ammo shoot better/worse in a gun, but usually the difference is very small.

 

[B y r o n]

Ammo lots are based on manufacturing runs. Different runs can have any number of different lots of bullets, powder or cases. It can also depending on which machine the ammo was assembled on.

 

[Franko]

My first instinct is to blame the forces of darkness. My second and even less educated guess is that the tiny propellant charge weights and variances in the the amount of priming compound in each cartridge are the cause of a whole lot of heartache.

 

[penguinofsleep]

From the CCI tour and a brief glimpse into Lapua, even as someone who doesn't have any specialist knowledge in ammo manufacturing, I can see steps or places where there's going to be some amount of variation in production due to the inherent nature of the process. Add into that other factors that we don't see such as variation in raw materials or whatever else.

Now, given how relatively small and tight rimfire ammo is, these same small differences are probably a much bigger difference total percentage-wise relative to the ammo than most center fire rounds. ex: 0.05grain of powder difference in a <5 grains of powder round is a much larger % than the same 0.05grain in 40 or 80 grains of powder driving a much heavier and larger projectile. I'm sure there's much more to it as well.

 

[justin amateur]

The causes of variations in rimfire cartridges?

They are mass produced ammunition where profit is the prime motivation.
Quality and uniformity is much less important than turning our cash into their cash.
Rimfire is 1800's technology being manufactured as fast as possible as cheaply as possible.
Cheap? Fast? Good? Pick one.

Differences in base materials, components, assembly and handling
results in cartridges that are measurably different from each other.
When no two cartridges are identical, you can't expect the results to be identical.

 

[RTH1800]

The design of the priming system is complex to manufacture with precision. The tiny amounts of powder and priming compounds lends itself to variations of significant % even on small changes.

Consumers who insist on low prices.

Bullet design is poor and slight damage negatively affects accuracy to a large degree.

The low recoil and low muzzle blast allow a high percentage of shooters to actually determine accuracy which is not the case with larger cartridges.
Basically we are asking a poorly designed 150 year old cartridge to do what it was never intended to do and demanding a low price point at the same time.

22 RF is a perfect storm of poor design, and performance. It’s actually remarkable what manufacturers do produce at a low price point.

 

[Booner1334]

Add to all of the above, unlike the majority of centerfire rounds which start out as hypersonic, the rimfire bullet leaves the barrel in the transonic zone, which has its own set of problems.

 

[shoobe01]

Byron is the closest to the issue. A lot is a run, and a run is a run. What's that mean? Anything. At the end of a run anything and everything can change.

Sure, they could just load up another run with the same stuff, load a new hopper of the same powder, etc and it'll be very close (humidity and temp from storage of powder, etc will change but not much) to the previous lot.

Or:

1. Shutdown for cleaning, that means lots of disassembly and re-assembly
2. Shutdown for repair, parts change, updates. Lots of disassembly and re-assembly
3. Just... shutdown. Leave stuff sitting even over the weekend and it's changed. More with more time, temp variations etc, etc.
4. Change over to make something else. Most factories do not have dedicated lines for /anything/ so even if this is the .22 line, it's not only the particular loading/bullet/etc so everything changes and that is not unlike handload: every setting changes.

Etc. When they get back to making "the same" ammo, it's all about tolerances and matching so stuff WILL change. A bit. No way around it. It can be /very close/ but never identical.

Wear happens (hence parts replacement), so within a run the first to last produced will often (depends on process, but for stuff like this of high pressures, high speed, etc) have variations. But less so than taking the factory apart and putting it back together.


It is similar in concept to any other tolerance stuff you are used to. I am one who can take my rifles pretty much completely apart and put them back together and usually be within 2-3 clicks to re-zero (I have done that with BARREL removal!). But it is never, ever, NO clicks to rezero.

 

[spife7980]

All those small changes not to mention one lot being done on machine A and another lot being done on a completely different machine B

 

[usafa77]

Elegy identified over 500 variables.

 

[biffj]

The priming compound by itself is sufficient to spit the bullet out. It is also very difficult to keep the amount of this compound and its location consistent from round to round. If you pull a number of bullets and dump the powder you'll be able to see some cases with the priming compound where it belongs and some with it on the side of the case where it does no good. This inconsistency will affect pressures and timing and can easily cause variation in velocity. It also accounts for the occasional failure to fire where you can eject the round, rotate it and have it fire fine.
On top of that there is the variation in powder charge. Think about a 338 Lapua where you're loading about 100gr of powder. One tenth of a grain is a pretty easy charge variation to achieve with this round and is .01%. With the 22LR the charge weight is about 1-3 grains so one tenth grain can be as much as 10%. This assumes the commercial loader can maintain a tenth grain variation.

Its just a tough round to keep consistent and there are a lot of other variables to consider....

Frank