Rimfire MV vs group size

[Near miss]

Food for thought for those who are interested.

I chronoed some more rounds once again while lot testing with the bench.


I compared average group sizes against muzzle velocity extreme spread to see how well they correlate and the result was 0.6, which is quite strong correlation! Seen here as purple and black columns. Unfortunally I did not get the MV for STD+ 292 recorded due to guests there.

I then compared individual group sizes against their corresponding muzzle velocity extreme spreads and the correlation lowered to 0.22.

After thinking about this I came to a conclusion that not all MV variances are caused by a single reason, there is variance within the variance.
The causes for changing MV could be things like bullet crimp, bullet width, offset, chamber conditions and of course powder amount.
And as always, correlation does not imply causation.

This leads to the fact that individual cartridge has so many different small variables that making conclusions based on one variable alone is impossible as long the as all of the variables cannot be identified and measured.

With rimfire, a string of 10 is not data, just 10 random occurances.

 

[MPrimo]

So now you need to determine what you can control and eliminate that variable ... diameter/length measurements, weight, rim thickness, etc ... and retest to get the a better (not perfect) correlation between MV and group size (assuming no shooter error). It is quite a process! Cool data!

 

[justin amateur]

MV spread relates directly to vertical spread.
That is due to gravity and the differences in time of flight.
The rest is wind, rifle movement, barrel imperfections and cartridge defects.
Correlate visible defects to results on target and a pattern emerges.
Asymmetric bullets produce unpredictable results.
Any deformities at the transition from bullet nose to drive bands
or lack of uniformity of the drive bands or bullet skirt
will show up as strays/outliers on target.
Instead of sorting by rim thickness, cull by visible defects.
Then shoot the uniformly shaped cartridges and compare
to the results obtained with the assymetric ones.
Easy test to do and easy to confirm the cause of strays.

 

[MPrimo]

Wondering what the criteria for acceptance would be ... perfect is tough to specify so I would want some quantitative data. Maybe a gauge?

 

[justin amateur]

Doesn't require anything more than Mark I Eyeball to do a visual inspection.
Simple side by side comparison of cartridge shape, what looks different?
Is the crimp clean and straight or is bullet material compressed down onto the brass?
Are the cannelures uniform and even around the drive bands, or distorted/slanted?
Are there chunks missing or any large dents? Is one shoulder of the bullet (where nose meets drive band)
higher on one side than another. Simply rotating the cartridge between your fingers is enough
to determine which portions of the bullet are out of proportion to the rest.

Is it really all that difficult to take the time and actually look at what y'er chambering?
Would you expect center fire cartridges to produce consistent trajectories when they are visibly asymmetric?
If two cartridges are visibly different in shape, do you really expect them to fly the same trajectories?

 

[MPrimo]

What ammo(s) is that ... looks like junk!

 

[Near miss]

MV spread relates directly to vertical spread.
That is due to gravity and the differences in time of flight.
The rest is wind, rifle movement, barrel imperfections and cartridge defects.
Correlate visible defects to results on target and a pattern emerges.
Asymmetric bullets produce unpredictable results.
Any deformities at the transition from bullet nose to drive bands
or lack of uniformity of the drive bands or bullet skirt
will show up as strays/outliers on target.
Instead of sorting by rim thickness, cull by visible defects.
Then shoot the uniformly shaped cartridges and compare
to the results obtained with the assymetric ones.
Easy test to do and easy to confirm the cause of strays.

I have a project going on related to 9" twist barrel research. And I actually did a test where I shot the SK PM 712 lot at 50m with modifications. This lot shoots like 0.7" 5 round groups.
I carved one side with a knife from multiple and they showed no different at the target. Even a slight improvement. If I recommend correctly 4 of those rounds went into clover leaf and one struggled maybe 0.4" away. Flierishly.

The ballistian who is the brains in our team was somewhat surprised but he actually predicted that making one fault the dominating one might improve accuracy if there are other serious minor faults playing. Other explanation could be that the bullet just does not care, it is enough stabilized at 50m to overcome mass inbalance, in the nose mass balances itself much better than in the rear. No clue why, personally.

I will conduct this test again soon with a better lot to see if inherently better accuracy worsens. What I mean is that if 0.7" groups did not become worse, maybe 0.3-0.4" will worsen to that 0.7"?) Then I know much more.

But what is evident is that any faults in the bullets rear will cause dramatic performance degradation.
This also will be tested in practice, but I am ready to eat my hat.

I think if it turns out real, probably many good cartridges are crimped too hard and thus ruined.

 

[justin amateur]

NM, I tried the same thing with SK cartridges.
Took a file and scuffed the nose, minimal differences in results.
Do the same thing to the rear of the drive bands and watch what happens.
I'm of the opinion if the nose of the cartridge is beat to snot, new out of the box,
what does that tell you about the heel? Think about how those bullets are handled
before they are seated. If the nose is bumped and banged around on the assembly line, before seating,
won't the ass end be just as beat up? I believe that's where there's a major difference
between cheap US ammo and the European made match cartridges.

MP, those are CCI SV up top, CCI Minimags and CCI AR.
All cheap bulk ammo.

 

[Near miss]

NM, I tried the same thing with SK cartridges.
Took a file and scuffed the nose, minimal differences in results.
Do the same thing to the rear of the drive bands and watch what happens.
I'm of the opinion if the nose of the cartridge is beat to snot, new out of the box,
what does that tell you about the heel? Think about how those bullets are handled
before they are seated. If the nose is bumped and banged around on the assembly line, before seating,
won't the ass end be just as beat up? I believe that's where there's a major difference
between cheap US ammo and the European made match cartridges.

MP, those are CCI SV up top, CCI Minimags and CCI AR.
All cheap bulk ammo.

Exactly my thoughts too. They do not buff the noses at the factory. What you see is what you get.
For example federal keeps their bullets in about 3-4 feet high mountains like they were moving trash. The topmost get almost unscathed but the bottom ones undoubtly have problems.

This is somewhere within the realms of 9" twist rate research too, faster twist might keep the deformed bullet in a better check, but too much of it and it brings in other series of problems.

 

[BillOH]

With rimfire, a string of 10 is not data, just 10 random occurances.

True words.