GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: Gimmly on April 21, 2022, 09:54:17 AM
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Hi guys.
Right now slugs are in fashion, it is true that their ballistic coefficient is better and they fly better than domed pellets for long distances in calibers .22 .25.
Normally slugs don't work well on standard barrels, (I think it's due to the lack of a cup on the slug that closes off the air and air leaks through the grooves) Fx has different liners for pellets and for slugs based in a in a polygonal profile, other manufacturers such as Daystate has designed a barrel, similar to a polygonal profile.
I have tried slugs in a standard rifled barrel without choke and the results they have been horrible but with domed pellet the results haev been great
it seems that the polygonal barrel is more versatile than the standard rifled barrel
Is the polygonal profile the perfect barrel for all kind of ammunition?
regards
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Standard rifled barrels DON'T shoot slugs well ??? HUH ? They DO & CAN shoot slugs VERY WELL
POLY barrels obviously are Not round and as such ANY projectile shot threw a poly profile must be forced into the shape of the barrel to pass down the barrels full length.
A pellet that has minimal width at head and a thin hollow tail skirt EASILY shape shift to pass threw a poly profile .... SLUGS which are solid body projectiles must Shape Shift there entire length that is in contact with bores surface !!!
This miens the lead must crush and displace itself for the slug to fit bore ... This takes away / absorbs a Large percentage of the total energy ( HPA ) that is being used to launch it. This has a Poly with slugs generally shooting slower IF IT WILL EVEN shoot a slug at all ? Many who have tried shooting slugs threw poly profiles need to set them up with a LEADE that allows some jump or the slug just sits there like a tight cork in a bottle.
Also harder the lead is alloyed ... worse the situation becomes.
This stated as a loose but general FYI ... there is a lot going on here and others if inclined will add content or debate whats been stated above.
Scott S
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Maybe some barrels shots well slugs (depends of the twist rate, measures etc...) but I think in the most of them the slugs don't work well.
Fx sells different liners for slugs and for pellets, Why?
Obviously the slugs don't work well in Fs smoth twist standar barrels and they make a new liners for slugs.
I have not Pcp, but a friend have a FX Impact and he has had to buy a fx liner specifically to use slugs( In her standard barrel does not work fine)
For example Jsb Knock Out pelles for .22 calibers are in .216 & .217 I think they make different measures because their slugs don't work well in all barrels.
The Jsb Jumbo Heavy work fine in all kind of barrels...
Personally i think the problem in standard barrel with slugs is the gap between the slug and the rifling, the slug does not have a skirt that closes and adjusts the slug to the barrel and prevents the air from passing through the hole in the grooves.
The air passing through the hole between the grooves and the slug I think causes turbulences and makes that the slugs does not stabilize well, I think the polygonal barrel shape is better for slugs, slugs fit better in a polygonal profile achieving better accuracy results.
Logically it is only my opinion, I hope I have explained myself well, my English is not as well than I liked.. ::)
Regards
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Standard rifled barrels DON'T shoot slugs well ??? HUH ? They DO & CAN shoot slugs VERY WELL
POLY barrels obviously are Not round and as such ANY projectile shot threw a poly profile must be forced into the shape of the barrel to pass down the barrels full length.
A pellet that has minimal width at head and a thin hollow tail skirt EASILY shape shift to pass threw a poly profile .... SLUGS which are solid body projectiles must Shape Shift there entire length that is in contact with bores surface !!!
This miens the lead must crush and displace itself for the slug to fit bore ... This takes away / absorbs a Large percentage of the total energy ( HPA ) that is being used to launch it. This has a Poly with slugs generally shooting slower IF IT WILL EVEN shoot a slug at all ? Many who have tried shooting slugs threw poly profiles need to set them up with a LEADE that allows some jump or the slug just sits there like a tight cork in a bottle.
Also harder the lead is alloyed ... worse the situation becomes.
This stated as a loose but general FYI ... there is a lot going on here and others if inclined will add content or debate whats been stated above.
Scott S
Hey Scott -
Are you Anti-Slug out to 100 yards ..... or are there just too many variables (w/Slugs) to give a Yes or No to Traditionally rifled barrels vs. Poly?
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Any Slug/bullet has a preferred twist rate and velocity for stability.
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Polygonal vs standard rifling has been debated in the pb world for some time. Both have been proven to work.
As with anything, there are so many variables to consider. The two main ones for air are ‘how much does the projectile need to reform to properly seal bore’ (obviously the more it has to reform, the more drag it can create) and ‘what is the twist rate’.
Dave
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Fx makes a different barrel for slugs because it uses a different twist rate and usually the lighter slugs work fine in standard superior barrel the slug barrel they make uses a faster twist rate for heavier slugs
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Jorge,
The performance of one conventional or poly rifled barrel with a given slug does not predict the performance of the "species".
The rifling twist rate has to be appropriate for the projectile and the velocity you shoot it at, before the rifling style matters. The projectile must be supported in the barrel such that it does not yaw as it travels down the barrel. As the slug is not a simple cylinder of lead, the support it gets is attained by contact at both the lands and the grooves. Contact at the grooves is not essential, but can easily be achieved by means of a narrow driving band.
Slugs with driving bands at barrel groove diameter don't need to be placed at the rear of the projectile; nor do they need to have a hollow base to work. Look at the Altaros boat tail slugs - images below. Some versions have a number of bands to minimize in-barrel friction, without compromising sealing or stability. Others have a smoothly tapering "cylinder" or wine barrel shape, to ensure alignment stability and air sealing with minimal friction.
There is a wide range of projectile shapes that work, limited only by the manufacturing method used. Straight through swaging dies cannot created projectiles with multiple bands or that are wine barrel shaped. Those can be cast, or lathe turned (Altaros). So, there is no single "correct" or "best" projectile shape. There are many designs with different advantages and limitations; including to support the desired aerodynamic properties and terminal performance.
There is a balance between reducing air blowby and minimizing friction. Due to the lack of starting air pressure, friction dominates when "full length" groove diameter contact is used. A long taper, or stepped diameters where only the last millimeter of length is near groove diameter are potential solutions. But these cannot be made via pass-through swaging dies. The total contact length required for in-bore stability depends on the caliber and on the position of the center of gravity for that projectile.
The amount of air that can leak through even 10 um radial gap is not as much as one might imagine because the time constant is so short. In practice, that radial gap is interrupted by land contact, and possibly a single driving band at or closer to barrel groove diameter.
If the barrel has a choke, the maximum slug diameter that will shoot accurately is determined by the choke and not the breech of the barrel. This means that slugs tend to need to be undersized to the groove diameter when shot from choked barrels.
Unchoked barrel are preferred for slug shooting, providing the narrowest diameter occurs at the muzzle. In other words, barrels that have variations in land and groove diameter down the length will shoot larger groups, if the last 25 mm length of the barrel is larger in ID than any section before that. To ensure that there is no "flare" at the muzzle some degree of choke is often applied.
Choking would not be required if the barrel ID was parallel all the way from breech to muzzle. A very slight taper in land and groove diameter is OK, providing the muzzle is always the "tighter" end. Flip such a barrel around and large groups are almost certainly going to result.
There are so many variations within spline (conventional) and polygonal rifling that simply using those terms is not enough to predict anything. If one has the actual dimensions of the land a groove diameters; and a measure of evenness from breech to muzzle, then predicting how well the barrel will shoot, with a given projectile becomes easier.
Obtaining the land and groove diameters of a given barrel, as well as how even and burr free the breech, transfer port and muzzle are, needs to be achieved via "slugging": Pushing or tapping a slightly oversized slug through the barrel. Checking the actual rifling twist rate is also easy, by driving a tight fitting patch trough the barrel by means of a free spinning cleaning rod.
https://www.altaros.cz/en/56-slugs (https://www.altaros.cz/en/56-slugs)
(https://www.altaros.cz/1055-large_default/airgun-slugs-atp-552mm-217-suitable-for-fx.jpg)
(https://www.altaros.cz/1310-large_default/turned-lead-slugs-atp-smooth-635mm-25.jpg)
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Hi.
Your explanation is very interesting and I agree with some of your arguments, but there is some question that I can't undesrtand well at the moment.
I known that the pellet stavility velocity is very important in accuracy and depends of the twist rate and the pellet velocity.
Here in Europe we have power restriccion for our airguns, In Spain is 24 J (about 18 ft/lb) I usually shot in FT at <12 ft/lb.
This is the question:
We shoot a Jsb exact 8,44 gr to 785 fps (11,5ft/lb) same configuration but with diferent airguns, we use for example a Feinwerkbau 800 a Steyr LG110 and a AA FTP 900, all of this airguns are top ft airguns and all of them have a incredible acuraccy .
Their barrels have different twist rates and the precision to 50 meters is practically the same.
Really the twist rate affects in the same pellet with same conditions?
Regards.
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Jorge,
Air rifle barrels have a traditional twist of 1:16". This is actually much "faster", with more spin than required for pellets. Nothing bad seems to happen, despite this until you shoot .177 over 20 foot.pound. Or .22 at over 35 FPE. Actually, velocity is more important, but obviously power and velocity are related:
The original FX smooth twist barrels have a reputation for shooting JSB pellets very accurately, at high power (900 FPS), out to 100 meters. These barrels work because the effective spin rate is very low. Just high enough to even out errors in the pellet shape or weight distribution. (A similar principle applies to drilling a deep hole: Spin the work piece and the hole goes down the middle. Spin the drill only, and the hole veers off to the side.)
Why such a fast twist rate in common airgun barrels? Fast twist helps in low power 10 meter applications. Or at least, it does not hurt. The typical twist for airgun barrels is a carry over from .22 rimfire barrels. Now Lothar Walther has rounded off their twist to 1:100 calibers, or 1:17.7 inches for .177. That makes some sense, except they use the same twist for their .22 barrels. So, a shortcut that is really too fast a twist.
https://www.lothar-walther.de/barrel-blank/airgun/?p=2 (https://www.lothar-walther.de/barrel-blank/airgun/?p=2)
Pellets are actually very forgiving of barrel twist rate, as they are self stabilizing due to their shape and heavy nose. This is true to about 50 yards. From perhaps 75 yards and further, too high a spin speed causes pellets to fly in a spiral, if they were shot at over 950 FPS - see videos below. This appears to be a case of the pellets having too much spin left at long range, while the forward velocity has decayed significantly. Then, the aerodynamic forces that want the pellet to follow the tangent of the trajectory, fight the very strong gyro forces - which want to maintain the original vector the pellet was pointing in when it left the muzzle. This interaction then forces the pellet to veer into a spiral.
At 12 FPE, you don't need to worry about pellet instability (unless you shoot very light tin alloy pellets at over 950 FPS muzzle velocity). Differences in twist rates between air rifles, shooting pellets at around 800 FPS simply don't matter very much.
Slugs are more likely to spiral if they are not spinning fast enough. Such spiraling usually decays more and more, to the point where the slugs tumble. That said, it is possible to over-spin a slug; but that is not as bad as spinning it too slowly.
Over powered .177 air rifle shows pellets spiraling at long range, due to pellets spinning too fast. Actual pellet spin is a function of barrel twist rate; AND muzzle velocity:
https://www.youtube.com/watch?v=_Z3YNLBs8Lo (https://www.youtube.com/watch?v=_Z3YNLBs8Lo)
https://www.youtube.com/watch?v=QFou_4VqLBY (https://www.youtube.com/watch?v=QFou_4VqLBY)
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Also see this thread about projectile stability, written by an expert: https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.0)
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Thank you for the explanation, very interesting but I think if the twist rate is not important in lower power barrels for12 lt/lb for the accuracy.
They can be others parameters more important than the twist rate? for example the grooves quantity , 6-8-10-12
symmetrical or no symmetrical grooves and lands
and perhaps for me the most important parameter the measures of the grooves and lands.
I think this kind of questions are very difficult to response in compressed air, In fire guns there is more information about this, but I don't know that this information is applicable to compressed air.
sorry for so many questions I have a curiosity mind. ;D
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perhaps for me the most important parameter the measures of the grooves and lands.
The bold part is the most important. In relation to the projectiles used.
Just as important is that these land and groove dimension be consistent down the barrel. And that pellets be consistent from one to the other.