Barrel length vs MV, Fast twist vs. BC

[grauhanen]

There's not very much about .22LR ballistics that's published in print or online. There are many books concerning serious studies and testing with regard to centerfire ballistics but there's precious little similar about rimfire.

There is a great deal of anecdotal reporting on various rimfire forums. Among the many ideas, what’s relevant with regard to the title of this thread is the consensus that there’s a direct relationship between barrel length and muzzle velocity. More specifically, the idea is that shorter barrels generally produce higher muzzle velocities than longer barrels.

In addition, there seems to be a general agreement in the anecdotal reports that faster twist .22LR barrels produce better results at long distances because they produce better ballistic coefficients of .22LR bullets at longer distances. Better BCs may mean .22LR bullets slow down less quickly and have less drop at longer distances. Higher BC values may also have positive implications for wind drift.

I recently came across the work of a ballistician who usually focusses on centerfire. As far as I'm aware his reputation as a ballistician is good. He's done some testing with .22LR and he reports results that may be surprising. If readers don't know who this ballistics expert is from his words and results, I'll be sure to identify him below.

_____________________________

Note that below the words of the ballistician are in italics.)

First question.

How does barrel length affect muzzle velocity?

For centerfire rifles with bottleneck cartridges and full charges of smokeless powder, longer barrels produce higher muzzle velocity. Due to smaller charges of faster burning powder used in 22 rimfire, the relationship between barrel length and muzzle velocity isn’t nearly as strong. In fact, it’s difficult to detect.

He notes that although the barrel length to MV relationship is weak, the trend indicates higher MV for longer barrels. (again his words italicized)

To summarize these findings, we can say there is a very weak correlation which indicates that longer barrels produce higher velocity at approximately 1.17 fps per inch. However this is the average trend for all ammo types. Each ammo type has its own unique correlation to barrel length, but most are weak. In all, 67 of the ammo types have positive correlations between MV and barrel length which means MV tends to increase with barrel length.

By statistical standards, there is not a significant relationship between barrel length and MV for the ammo types on average, nor for any of the individual ammo types. (Bold added for emphasis.)

The next question in the next post.

 

[grauhanen]

The next question concerns whether barrel length influences the consistency of MV. In other words do longer (or shorter) barrels help cause ammo to have more consistent muzzle velocities? Here the smaller the SD of muzzle velocity, the more consistent the ammo MV (i.e. the closer they are to being similar).

How does barrel length affect the SD of muzzle velocity?

Another question we can ask about barrel length is: does barrel length affect the consistency of muzzle velocity (SD)? One possible mechanism for this would be the effect of the extra barrel length traveled after the powder charge has burned, and the effects of fouling in the additional barrel length.

The data shows that on average, there is virtually zero correlation between barrel length and SD of MV.

In other words, a longer barrel doesn't make the MV more consistent, that is to say it won't reduce the SD and hence give smaller extreme spreads.

More to follow on .22LR barrel twist rates and ballistics coefficients (BC).

 

[grauhanen]

Next question.

How does twist rate affect the average BC?

(Note: This is considered as potentially important because a faster twist rate may increase average BC. Higher BC values suggest that bullets will not slow down as fast as otherwise and will experience less drop.)

While testing revealed that there were a few ammos that had a notably better BC in fast twist barrels. Testing different lots of Eley Match showed that BC improvement due to fast twist barrels was lot dependent. (One lot showed improvement, while another didn't.)

The conclusion, however, was a little more austere.

There is generally not a strong relationship between twist rate and BC for most 22 rimfire ammo types in 1:16” and faster twists. (Emphasis in bold added.)

In other words, faster twist barrels don't significantly improve most .22LR ballistic coefficients.

_______________________

The final question.

Does twist rate affect the consistency of BC?

In other words, do fast twist barrels help make BC values more consistent from round-to-round?

Although there may be a few ammos that show a relationship between twist rate and SD of BC, in general, the SD of BC is not highly influenced by twist rate for most rimfire ammo types on average.

In general, the relationship between twist rate and SD of BC is weak.

 

[propliner]

How could twist rate affect ballistic coefficient?

 

[grauhanen]

How could twist rate affect ballistic coefficient?

To paraphrase Bryan Litz, in general the idea is that if bullets don’t spin fast enough (with sufficient twist rate), the bullets' BC may be less than optimal. To achieve the optimum BC from the bullets (i.e. the lowest aerodynamic drag), the bullets must spin fast enough. In practical terms, that means bullets drop more quickly and deflect more in the wind (other factors being equal). When bullets spin faster, the BC can be optimized for best performance.*

While this applies to centerfire, the overriding question for rimfire shooters, particularly long range shooters, is whether this applies to rimfire ammo with MVs usually in the 1050 - 1125 fps range.

There are other important questions as well, but for now it can remain there.

*For more details and ease of reference see this Litz summary https://bulletin.accurateshooter.com/2017/04/new-spin-on-bc-how-barrel-twist-rates-affect-bullet-drag/

 

[gunlaker]

That is interesting stuff. I'm not a .22LR shooter, but have competed in 1000 yard black powder target matches. These rifles, with their relatively low muzzle velocites and cast bullets are to an extent like large 22LR's with more recoil.

The trend in our type of shooting has been toward faster twists. As an example, there seems to be an advantage at long range with a 1-16 twist 45 cal vs. a 1-18 twist when shooting in difficult conditions. It's believed to be related to marginal stability and induced yawing which reduces BC. The differences between twists don't seem to matter much at shorter ranges like 500m, but it seems there is a slight advantage as distances increase. Either that or we are all victims of groupthink

Chris.

 

[propliner]

It still doesn't explain why they have better BC above 1.5. Yes the data is there but what is the scientific explanation? Is it creating some type of boundary layer above 1.5?

 

[wasa]

I'm a fan of the website "Ballistics By The Inch". Their 22LR data is here http://ballisticsbytheinch.com/22.html

They detail velocities from 2" to 18" barrels and test 10 different 22LR ammo. I find this info fascinating.

 

[gunlaker]

It still doesn't explain why they have better BC above 1.5. Yes the data is there but what is the scientific explanation? Is it creating some type of boundary layer above 1.5?

I think it's simply that if stability is marginal then the bullet is more susceptible to yawing. Yawing makes for more drag which is basically lower BC.

Interestingly we never shoot bullets with an SG as low as 1.5. I'm generally in the 2.5 range.

EDIT: I should add that's only for long range. My 200 yard cast bullet rifles bullet/twist combo is closer to 1.5-1.7 and that works very well. In cold weather the bullet get unstable and show tipping on the target so they are barely stable.

Chris.

 

[grauhanen]

The as-of-yet unidentified ballistician says two things that run contrary to the general consensus formed in recent years and posted on rimfire forums.

First, he says that .22LR barrel length doesn't directly contribute to muzzle velocity. If anything, he says, the trend is to have higher MVs in longer barrels.

If this is the case, is there any reason to choose a barrel length for the purposee of maximizing MV (for whatever purpose that might serve)?

Second, he says that faster twist .22LR barrels don't make a significant difference in BC and so offer no advantage over standard 1:16" twist barrels.

If this is the case, unless fast twist are more accurate at the shorter ranges where accuracy can reliably be tested, is there any reason to use faster twist barrels at longer distances.

Is the ballistician wrong?

 

[coleman1495]

 

[Juster]

I'm a fan of the website "Ballistics By The Inch".

Yup. Theory is really interesting, but as Einstein put it "No amount of experimentation can ever prove me right; a single experiment can prove me wrong."
A lot of it is language, such as a bullet having different BCs at different velocities for the purpose of calculation. The bullet doesn't really change it's shape.

There comes a point where an extra inch takes away the same velocity by friction as it gains, so length makes no difference at that exact comparison. But obviously a 2" barrel will give about half the energy of an 18".

 

[Grantmac]

Curious to know the conditions and ammo used for these sweeping pronunciations.

 

[Grain]

That is interesting stuff. I'm not a .22LR shooter, but have competed in 1000 yard black powder target matches...
Chris.

I believe black powder folks generally like long barrels, specially when shooting 1000 yards. Know of anyone in that sport that found the barrels were 'too long' after a certain point?

 

[gunlaker]

I believe black powder folks generally like long barrels, specially when shooting 1000 yards. Know of anyone in that sport that found the barrels were 'too long' after a certain point?

Yeah they are pretty long . Most people I know use 32" or 34" barrels, although occasionally there will be a few 30" ones. In my opinion the extra length in our sport doesn't change the velocity much, it just gives a little more sight radius, and more barrel weight to keep the rifle steady and reduce felt recoil. My latest long range rifle weighs about 17lbs without the scope. It only has 30" barrel, but it's a super heavy profile.

I have heard a few people complain about extra fouling build up in 34" barrels but I haven't found that to be a big problem.

Don't a lot of .22LR shooters back bore their barrels so they only have maybe 16" or so of rifling?

I don't know much about competitive rimfire shooting, but I do own a very accurate CPA Stevens 44-1/2 reproduction with a 28" Douglas XX barrel on it. It shots maybe a little better than the Anshutz MPR I had for a while. The barrel probably isn't optimal length in some respects, but the rifle balances very well for offhand shooting.

Chris.

 

[grauhanen]

Curious to know the conditions and ammo used for these sweeping pronunciations.

If you're asking about the basis for the conclusions drawn about barrel length vs. MV and barrel twist vs. BC, the ballistician in question is Bryan Litz.

It seems Litz's Applied Ballistics Lab tested a number of different .22LR ammos (over 80 of them) across five different barrels.

Of course, the Litz imprimatur isn't necessarily a guarantee that his testing and conclusions are unassailable.

 

[Grain]

Yeah they are pretty long . ...

I have heard a few people complain about extra fouling build up in 34" barrels but I haven't found that to be a big problem.

Don't a lot of .22LR shooters back bore their barrels so they only have maybe 16" or so of rifling? ...

Chris.

Long barrel's fouling, and/or creating greater drag, similar comments to rimfire, very interesting.

There is information out there that show's a 22lr has reached max velocity at 16", so some believe that's the sweet length for accuracy. As in all things rimfire, everything is debated very rigorously.

 

[Grantmac]

If you're asking about the basis for the conclusions drawn about barrel length vs. MV and barrel twist vs. BC, the ballistician in question is Bryan Litz.

It seems Litz's Applied Ballistics Lab tested a number of different .22LR ammos (over 80 of them) across five different barrels.

Of course, the Litz imprimatur isn't necessarily a guarantee that his testing and conclusions are unassailable.

Testing in a temperature controlled indoor range won't say much about real world performance. Plus faster match ammo has only recently become available.
So the question stands. Does the testing represent reality.

 

[grauhanen]

Testing in a temperature controlled indoor range won't say much about real world performance. Plus faster match ammo has only recently become available.
So the question stands. Does the testing represent reality.

Litz didn't say anything about the conditions under which the tests were performed. If the testing was done in a temperature controlled indoor range, it may have the advantage of potentially establishing a reasonable base line against which to compare other tests. Testing outdoors can never establish "real world" standards as such comparisons can't be apples to apples. In other words, uncontrolled conditions are virtually impossible to replicate.

If Litz's results can be repeated in testing, then they have validity. There are, however, a number of concerns that can be raised about his methodology that make his conclusions questionable and at the very least premature.

As for faster ammo only becoming recently available, that's simply incorrect. For decades, Eley, Lapua, and RWS have made (and continue) to make .22LR match ammo varieties with faster MVs than standard match ammo.

What's now different and new are ammo varieties with the words "Long Range" on the packaging. Those words have an appeal to many shooters in PRS and ELR type. Historically, however, except for the many new enthusiasts for faster ammo in PRS and ELR, serious competitors in the longer distance .22LR prone events (100 and 200 yards) have prefered the slower match ammos. Biathlon shooters like faster ammos because they perform more like standard match ammo in the colder temperatures (all .22LR MVs fall with the temperature).

 

[trky chsr]

I worked with a fellow who had worked in a gunshop for over 30 years and had the opportunity to play with a lot of rifles. With the theory of longer barrels providing more velocity in a 22 they decided to test it out with a chronograph. They found a longer barrel did produce higher velocity, to a point. After (I believe) 24" velocity actually dropped. They didn't test for accuracy at different velocities, they were more interested in proving/disproving coffee shop theories. TC