Fx impact and slug tuning

[Airgun-hobbyist]

thing i have noticed on the fx smx barrel system and feel free to correct me but that is that the pellet or slug can curve for example if you shoots at 30y 50y and 100y sometimes it seems to me like the 50y POI can be of to the right and back in at 100y have any of you noticed that to?



if so then it is very importen to test at diffentent distenceys as well.

If it is doing this you may have a cant issue going on with how the gun's set up. The scope and gun are not in a good alignment. Check this thread out, and see if this is also what you are experiencing.

https://www.airgunnation.com/topic/impact-shooting-to-the-left-the-further-the-distance-help/

As other's said, if the projectile is too small then you may be getting a bigger group, but also looping, spiraling impact pattern at different yardages as well. This can be easily tested with different sized ammo and changing the speed.

 

[mughram]

i feel you because i have same results until i change slug liner to pellet liner



using slugs doesn't mean you have to use slug liner ...



i have good results in slugs ( NSA 27.5g in 0.218 ) 1 to 1.5 inch groups in 100y in 10 shots using pellet liner

also in H&N 25gr 0.217 @ 930 fps grouping in 200y in 10 shots less than 3 inches ! i have proofs 

regarding JSB slugs i used it with my pellet liner and i have less than 1 inch in 100y 5 shots @ 1005 fps

try pellet liner 



please note ( STX need to be clean very often ) use some bore cleaning compound you will be surprised to see what comes out of it )



the reason i am telling you this ( my STX barrel used to group less 1 inch @ 100y ) until one day i find it open up for more than 2.5 inches after cleaning it with " Compound " accuracy back to normal )

 

[spyde82]

i feel you because i have same results until i change slug liner to pellet liner



using slugs doesn't mean you have to use slug liner ...



i have good results in slugs ( NSA 27.5g in 0.218 ) 1 to 1.5 inch groups in 100y in 10 shots using pellet liner

also in H&N 25gr 0.217 @ 930 fps grouping in 200y in 10 shots less than 3 inches ! i have proofs 

regarding JSB slugs i used it with my pellet liner and i have less than 1 inch in 100y 5 shots @ 1005 fps

try pellet liner 



please note ( STX need to be clean very often ) use some bore cleaning compound you will be surprised to see what comes out of it )



the reason i am telling you this ( my STX barrel used to group less 1 inch @ 100y ) until one day i find it open up for more than 2.5 inches after cleaning it with " Compound " accuracy back to normal )

Holt sh...t that worked put the pellet liner in and alined the liner, bum 1” groups at 100y

 

[Jlands]

1............For a simple illustration, let's say the barrel just oscillates up and down. As it oscillates up and down, it will swing up to its highest point (moving away from the POA on the high side, causing wider groups if the projectile exits here). It will then swing back down, passing through the mid-point of the oscillation (where it is back on the POA, causing tighter groups if the projectile exits here), continuing down to the lowest point of the oscillation (again moving away from the POA on the low side, causing wider groups if the projectile exits here)...........

2........harmonic tuning doesn't require that the projectile exit the barrel while oscillating past its original POA, but rather that it just exit the barrel at as close to the same point in the given oscillation every time. This will give the tightest groups, even if POA and POI do not match.

First, thank you for the time and effort to write a detailed explanation. I hand load rifle ammunition and i have a feeling you do too..... Or could. I quoted two points in your text for a reason. Even simplified, you gave 95% of the basis of harmonic tuning. Might as well gave the other 5%. I can tell "you" know what i am going to say, but this is for others.

For the first quote, the oscillation travels in what can be best visualized as a cork screw. So instead of going to 12 o'clock, to center, to 6 o'clock, to center and so on. It more or less goes to 12 o'clock, to center, to 4 o'clock, to center, to 8 o'clock, to center, to 2 o'clock, and so on. This is because of the twist rate of the barrel or more so, the mass of the projectile rotating inside the barrel. This is why the oscillation follows the twist direction. The picture below shows and exaggerated look at what the front of the barrel is doing. 

View attachment 1586023234_21024722195e88cb422f9754.64466082.jpg



So, the second quote and why i quoted you. Again, I know you know this, this is for others. The general consensus is that the best possible accuracy is obtained when the barrel crosses the center point "because" of this spiraling rotation. With everything else equal, bullet weight and uniformity, velocity and environmental factors, hole in hole accuracy is achieved at tuning for center point.



I do 100% agree on the use of the VSA. The valve spring adjuster is vastly important to efficiency and to a good degree, the accuracy. This because of cleaning up and reducing wash air from behind pellet. this can turn a 1" group to an 7/8" group or a 1/2" group to a 3/8"........... or better.

Thanks

 

[Jlands]

Also. This is why actually, "shorter" barrels are "seen" as more accurate because of "less" barrel whip which equates to easier tuning. In reality, barrel length "in general" has no effect on accuracy. Length does affect speed which could make a projectile more accurate. But that is a component of the speed the projectile is best at. Same as twist rate. 

 

[Doug-T]

No substitute for spending the time and effort to thoroughly test multiple slugs over multiple shooting sessions to figure our which ones work best. I thought I could short change the process but still ended up trying 20+ types while shooting 500+ slugs over 2 months. Sample packs are great. Have a firm plan. I used 3 shots at 30 yards to cull the herd then tested the ones that showed promise at 100 yards. 10-12fps changes in velocity between each group.

 

[Chuck26287]

First, thank you for the time and effort to write a detailed explanation. I hand load rifle ammunition and i have a feeling you do too..... Or could. I quoted two points in your text for a reason. Even simplified, you gave 95% of the basis of harmonic tuning. Might as well gave the other 5%. I can tell "you" know what i am going to say, but this is for others.


For the first quote, the oscillation travels in what can be best visualized as a cork screw. So instead of going to 12 o'clock, to center, to 6 o'clock, to center and so on. It more or less goes to 12 o'clock, to center, to 4 o'clock, to center, to 8 o'clock, to center, to 2 o'clock, and so on. This is because of the twist rate of the barrel or more so, the mass of the projectile rotating inside the barrel. This is why the oscillation follows the twist direction. The picture below shows and exaggerated look at what the front of the barrel is doing. 

View attachment 1586023234_21024722195e88cb422f9754.64466082.jpg



So, the second quote and why i quoted you. Again, I know you know this, this is for others. The general consensus is that the best possible accuracy is obtained when the barrel crosses the center point "because" of this spiraling rotation. With everything else equal, bullet weight and uniformity, velocity and environmental factors, hole in hole accuracy is achieved at tuning for center point.



I do 100% agree on the use of the VSA. The valve spring adjuster is vastly important to efficiency and to a good degree, the accuracy. This because of cleaning up and reducing wash air from behind pellet. this can turn a 1" group to an 7/8" group or a 1/2" group to a 3/8"........... or better.

Thanks

Thanks, JLands. Yeah, I have reloaded for firearms, but primarily pistol. Only an miniscule amount for rifle. I had just started with my first .223 when I moved over and got into airguns, so I haven't even developed that first custom handload for my powder burner. I'm just having too much fun with airguns. It's amazing how the basics of harmonics is the same, but the tools for tuning are all rifle adjustments vs cartridge adjustments. I'm a very quantitative person, and I loved handloading my cartridges and the testing of a new load almost as much as shooting them once developed. I loved watching the cause and effect in action, and the reward of getting the best I could get. That's actually why I went with the Impact... the versatility of the tuning tools available is like being able to handload, and never having to stop and got back to the reloading bench to load the next round of test cartridges. Just air up again, and keep tweaking.

I also appreciate the final 5% of the illustration of harmonics. I didn't realize the oscillation followed twist direction. Makes perfect sense, just hadn't thought about it to that degree yet. In my explanation, I actually got a little more in depth that I had meant to. I initially just wanted to question disregarding the use of the VSA, and show why it did indeed matter, and was a valuable tool in the tuning process.