Airgun accuracy factors: follow on questions

[fe7565]

​What is behind the term "the airgun likes a certain pellet"? Is it because certain common airgun design/manufacturing characteristics for a particular model (i.e. FX Royal 500) when married with the common characteristics of a certain pellet (i.e. JSB 18gr, 5.51mm) produces the most desirable (i.e. stable) flight for that pellet within a certain distance?

In a previous post http://www.airgunnation.com/topic/what-makes-a-pcp-consistent-and-accurate/ the concenus for accuracy (in a simplifed way) was:

(1) well-made rifle that allows for consistent shot-to-shot power behind the pellet (good barrel, regulator, etc)
(2) a pellet that is uniform in head size, weight, shape AND a pellet that is "matched" with the barrel and the propelling power where the pellet leaves the barrel at the right speed and twist rate for stable flight during the entire travel path to the target

The question is: how do you know if the pellet leaves at the proper twist rate? I believe that the pellet has to spin within a certain optimal rotation range when leaving the barrel in order to hit the target accurately. If it flies stably for 50 yards on a 100 yard path and then it starts to wobble/spiral, is it because the rotation became less then optimal for a stable flight? Are certain pellets designed to rotate/spin stably within a certain envelope (let's say 25,000-30,000 rpm for JSB 18gr)? How does JSB arrive to that number? How do rifle manufacturer's decide what twist rate the rifle should be? How do you know if you are achieving the "designed" twist rate vs "actual" twist rate you are getting based on the unique characteristics (fingerprint) of your rifle? How do you reconcile the optimal rotation of a certain pellet vs the actual rotation a certain rifle is able to produce with that pellet?

 

[mackademics]

well for powder burners, choosing the twist rate came down to what weight bullet you are shooting. not so sure about pellets. not enough twist = not stable. too much spin = projectile disintegrates. better to over twist than to under twist. and ive noticed no airgun manufacturers really advertise their twist rates. and to add more of a "twist" (see what i did there?! clever huh), throw the smooth twist barrel into the mix. and now im thoroughly confused!

jl

 

[fe7565]

What I understand about the smooth twist barrel is that it spins the pellet just like the grooved but it does it only in the last few inches and it does a better job sealing around the pellet vs the grooved one that has higher/deeper ridges. Thus the smooth twist uses air more efficiently . The pellet in the smooth twist changes its shape from round to polygon when it's forced through the twist section and starts to spin up until it's ejected from the barrel.

So wondering still, why do certain pellets work best in certain rifles? And how you find them without hit and miss testing? And how you know that you found the best possible pellet for that rifle?

Let me shoot in the dark with some thoughts that are not based on any observed or learned facts. Let's say a pellet "A" at a certain BC in a certain rifle flies most stable when spinning between 18,000-20,000 rotations during its flight to 100 yards. If that same pellet leaves the barrel at 19,000 rotations but at around 50 yards it slows down to 17,000 rpm it may abandon its stable flight and starts to spiral, etc. Or, if that same pellet leaves the barrel at 22,000 rpm and spirals right away it may resume stable flight when it drops below 20,000rpm, albeit it's already off the target-path. Or, if that same pellet leaves the barrel at 17,000 rpm and spirals right away and never resumes a stable flight-path because it was never inside that optimal 18,000-20,000 envelope.

So what makes pellet "A" from the same brand with the same BC spin at such differing rates in the same rifle? One factor maybe head size. If the head is too small the pellet may not fully engage the grooves, thus may not spin up fully to the optimal rpm required for a stable flight. What if the pellet head is too large? The pellet may require excessive force to conform to the grooves which reduces power and velocity leading to low rpm-s (below optimal) at barrel exit.

Leaving out rifle consistency behind the pellet...am I completely on the wrong path with assuming that the optimal pellet rpm produced by a certain rifle is what determines how accurately the pellet flies...i.e. the pellet which that rifle likes the best? So are rifles designed from the get go to be grooved at a certain twist rate that will result in an optimal rpm-envelope with most industry-standard pellets? 

 

[Guest]

Sir, you asking a balistics question that has yet to be difinitevly asnered in any form of rifled projectile shooting. Many many many people have dopne extensive testing with twist rates, velocity, projectile fitting to the bore ect. and they all seem to come to the same conclusion: The gun/barrel likes what it likes and if you're lucky it will like the first few varieties of projectile you shoot through it. Though as you know it's more likely to take extensive and repeated testing of multiple projectiles to find its favorite round.

There are no current algorithms or equations that can be used to determine a guns favorite round. All due to human nature/error. From the moment a person interacts with the assembly of a gun all bets are off.

Don't forget barrel harmonics(how much and when it vibrates during the time the pellet is moving through it).

Anyways your questions are always interesting and thought provoking even if they've been asked and answered, if not to perfection many times in the past.

 

[bill_dd97]

Don't forget that Diabolo pellets are primarily drag stabilized, not rotation stabilized. Real world experience has shown us that rotation improves accuracy, but no testing has determined an optimal twist rate or groove depth for improved stabilization. There are many threads on this forum that attempt to answer your questions. Here are some examples:
http://www.airgunnation.com/topic/does-a-flatter-trajectory-translate-to-a-more-accurate-round/
http://www.airgunnation.com/topic/current-barrel-twist-rate-reality-dont-get-hung-up/
http://www.airgunnation.com/topic/twist-rate-of-fx-smooth-twist-barrels/

parts of this conversation have been removed for some reason, but there is still good info here.
http://www.airgunnation.com/topic/getting-techbarrel-twist-ratios-stabilizing-a-pellet-environmental-variables/

There is much more out there.

 

[fe7565]

Thank you both! I wish thing would be more simple or I wish my brain would work a bit better But seems like there is no simple formula/solution (yet). It may never be based on what you mention about barrel harmonics as a variable that is also specific to each rifle. Albeit I know in some cases the harmonics can be tuned out and/or mitigated by weights where it does not measurably affect the accuracy(?).

Thank you for the links to the various threads..I read only of one of the four threads before and the three others had some very interesting and relevant information that will take some time to digest fully. There are many factors at play. may not understand them all but at least will be aware of them. It's clear that there is no plain answer or a clear path to how and to what extent twist rates etc affect accuracy. There are ball park figures for a stable diabolo pellet flight which are a good start or at least serve as good guides that help keeping pellet speed within certain MV envelopes, and could help at least avoid known factors affecting accuracy.

One thing is clear that pellet head size is a large factor for a stable flight so I started on a path that many of you already done. I bought a .22 and .25 pelletgage and adjustable pellet sizers for both calibers. There are mixed reviews on their usefulness, especially the adjustable pellet sizers, but curiosity to find the holy-grail is motivating me