Varying POI - causes & remedies...?

[Kragman1]

I'm finally getting some real trigger time with my .177 R9, and its giving great illustration to something that I've never really understood:  why some rifle/pellet combinations result in large changes in POI.

Pic below shows what I mean - shot at 9 yards with no changes that I can account for:  same point of aim, scope settings, shooting position, all screws tight, etc.  And you can see the results.  Baracuda 10.65's shoot to the right as well, but they're not represented on this target.

So if anyone can give me a primer on why this happens, I would appreciate it.

I've always vaguely assumed that it was related to pellet rotational velocity & shape pulling pellets slightly off course.
But these differences are pronounced.  So now I'm wondering if the effects of normal piston stoppage at the end of the stroke is causing it, in concert with the different dwell times of the pellets.  If so, slightly different timing of the firing stroke could be very beneficial. 
FYI - I did have pretty constant side-to-side movement in my hold all day.  And had similar occurrences when shooting my Diana 34 with the same pellets.

Or, of course, I could be all wet too.....

[Novagun]

I am watching this thread with anticipation of the golden pellet. I will be disappointed I know. There are so many variables that it is difficult to isolate any one at any time .
The biggest hindrance to repeatable accuracy, I am sure is the shooter. Rough shots and shots with stump in your side. Even rested at the bench is not a sure bullseye like it should be. Some days are better than others. Try your R9 at thirry metres  R9 is the same as HW95 is it not? 

[Kragman1]

Thats right Hugh - the R9 and the HW95 are widely reported to be the same gun, cosmetics aside.

But the Golden Pellet?  Don't they make those on Mystery Island in the Bermuda Triangle?  Out of unobtanium IIRC, not gold....    😉

[subscriber]

Brian,

Vibration timing with different pellet weights can account for that shift.

[Bayman]

Brian,

Vibration timing with different pellet weights can account for that shift.

Bingo!
It's unavoidable. Different pellet types will almost never have the same POI in most any gun. Some guns and pellets are a litter better than others but they all experience it to some degree.

You can even have similar POI shifts between tins of the same pellets. It all has to do with vibrations or otherwise called harmonics.

When I first got into airguns I had a similar question. My gun would shoot the heavier pellets higher (at close range) and I didn't understand how a slower pellet didn't drop more than the faster lighter pellet. Several people here explained to me that the slower pellet was in the barrel longer and came out later in the recoil. Essentially the recoil had the barrel pointing higher by time the heavy pellet left it.

Side to side variance is quite normal between even similar weight but different type pellets. My Hw30s shoot wadcutters to the right of domes.

It's best to stick with one pellet after you find one that works for you. When testing pellets for accuracy shoot at least 5 or 6 five shot groups before judging the pellet. Different pellets use different lead alloys or sometimes no lead at all and it takes a while before the barrel releads itself with the pellets your testing. Having a properly leaded or conditioned barrel is essential for accuracy.

Once you find a pellet that works well zero the gun for it and stay with it. Otherwise be prepared to shoot a bunch to recondition the barrel and re-zero the gun.
HTH

[Kragman1]

Thanks guys.

I'm pretty familiar with the effects of barrel harmonics in firearms, but I confess to being surprised that they are playing a major role in my airgun POI. 

I figured that the effect of the piston impact at oh so slightly different times during the launching of the pellet would turn out to be the culprit.  Especially since the results were roughly linear in relation to pellet weight, and in the direction of least resistance in my hold.

Now that you guys have thrown back those red herrings for me, can someone point out where those vibrations are coming from? 
I struggle to see the barrel being affected by a sinusoidal pulse simply from the release of the piston.  I could believe that pellets can affect some subtle movement of  the entire rifle as the piston is traveling.  Especially since like most, I use a light hold.  I can't see the barrel flexing as it does with firearms.

Remember - I'm a simple man.  Still a little suspicious of fire and the wheel....

[subscriber]

The jolt at piston launch, and when stopping against high pressure air is enough to excite the barrel; up and down and sideways.  I have seen high speed video showing that.  If only I could remember where to look for it.

EDIT:   This may help until I find something better.





Also see muzzle motion at these time stamps:





5.5 FPE Slavia recoil; compared to PCP:
Notice springer muzzle bouncing up and down

[Bayman]

I too had a difficult time believing that recoil and barrel harmonics can in to play with Low power air guns. Hold sensitivity is certainly recoil related and pretty self evident. Tighten or loosen your grip and the POI changes. Hold the gun the same way with different pellets and the POI will move due to where in the shot cycle the gun is when it exits the barrel. As a powder burner shooter

I had a hard time accepting that barrel harmonics come into play until I put a muzzle weight on a gun and the POI changed with no other changes. Just removing a globe sight from a low powered Hw30 will change the POI. There's no other explanation other than barrel harmonics. As difficult as it is to comprehend, it doesn't mean it doesn't exist. I know this is not an explanation as much as an observation but sometimes like gravity, knowing it exists is more important than why.

Oh another example, the groups on my R1 started to open up in random patterns and I couldn't figure out why. Well until the muzzle weight rolled a little bit while cocking it. The muzzle weight had loosened up a touch. I tightened the grub screw a little and the groups immediately tightened up and returned to the original POI. I've since epoxied that bad boy on there.

[Ribbonstone]

Always a little suspicious about springers…..are so dependent on  constant springer-vibration-recoil-bonce and the human holding it (we are a biological springer damper) that I hate to call a difference in POA/POI for purely mechanical reasons.

Would call your photo example are pretty close in impact for two different pellets….not hard to find two pellets that would group 3 or 4X farther apart...would be a “freak” if they managed to land one on top of each other.

Is it that often that you change pellets in the middle of a shooting session?

Not going to say it is limited to springer….PCP’s/Co2’s/SSP’s  fling different pellets to different places...center fire and rim fires do too...even shot guns will shift their center pattern with different loads.

[Kragman1]

Thanks again guys.

Ribbonstone, I shot that target as a test after remarking on the very different POI for different pellets.  Keep in mind that my distance is only around 8-9 yards and the FTTs & 10.5 domes are already 1/2 inch apart. 
I do often shoot small amounts of different pellets, as I am prone to getting in brief shooting sessions at home and I like to try different pellets.  When I am shooting at the range (extended sessions) etc I tend to shoot a single pellet, whichever has been giving me the best accuracy. 

Ron, your observation about the front sight and muzzle weight are quite telling.  Thank you!

[Lt. Dan]

I have found that once I find a pellet that works well at 25 - 45 yards I'll zero the gun for it and stay with it. After shooting a few tins with consistent accuracy I'll then buy a couple of sleeves with the same lot number of that pellet.

[mpbby]

When I think I am really being able to trust the scope is holding zero and in the shooter's repeatability..,  these POI shifts needed some answer.

I believe the muzzle of a springer does a path.  BTW, I believe that the 'artillery hold' and a kind of soft rest are to minimise the whole path.

Anyway, I assume that, bigger or smaller, always there is a muzzle path, and would be interesting to know its shape.

I thought about a laser device attached to the barrel and a slow motion recording of the laser dot on a paper.  But , I don't have the camera, nor was able to convince anyone here..

Bottom line at the moment is that I ended up with a simple explanation (to my needs).

Different weight.. different time to leave the barrel.. different point of the path.. different POI.  Each pellet has its own 'path point' when leaving the barrel.

(although still curious about the shape ..) for now, this answer is being enough to zero the scope for the more consistent pellet, and be focused on the shooter tasks. 

[Kragman1]

Marcos, I have always been aware of my "wobble track" once I have established a decent shooting position.  It is almost always influenced by my heartbeat, which is usually a major factor.  I'm old and fat these days, so I cant expect to shoot between my heartbeats anymore.  When I shot those 3 groups I was varying mostly horizontally, mostly due to imperfect rifle support.  Pretty good, but not great. 
So the theory that firing the rifle is in some way causing repeatable POI shifts is extremely believable to me.
Now - what portion is due to recoil/rifle shifting, how much to dwell time/point of release differences, how much to barrel harmonics, and how much to issues not yet discussed?

At the moment, I'd say the lions share is due to the dwell time/point of release factor, as the POIs favor my wobble path (which was pretty consistent).  That can't be the only thing though since the differences are exaggerated.  Especially the 10.5s & 10.65's - my sights were never anywhere near their POI.

This is a very interesting topic to me, and I simply don't know enough to support an opinion.  But I'm trying to unravel it, so I hope more voices are heard!

[Back_Roads]

I have seen pellets corkscrew if the gun does not like them, or a dirty barrel or the like they may be grouping at closer ranges but open up a few more yards out, also will have a different POI with the changing distance from the target.

[Novagun]

So far the information produced here is much as I expected. It still leaves me immersed in the mysteries of air gunning. I believe that to be true for .22 LR. In perfect conditions they struggle for a 100 score. The topic of barrel harmonics is inextrciably linked to shooting performance.  I will start a new thread which I hope will run parallel to this one. All the time I am aware that mist of us have observations but no scientific means of establishing what actually goes on with each shot.

[splitbeing]

Consider changing the weight on the barrel to test the harmonics theory. You could add or remove a globe sight or even use a zip tie to attach something snuggly if no muzzle breaks are available.

My CO2 pistols even have different POIs with different pellets.  My suspicion is spiraling at different velocities with those shorter rifled barrels.

[Kragman1]

All the time I am aware that mist of us have observations but no scientific means of establishing what actually goes on with each shot.

That fits me to a "T"...!   😏

[subscriber]

This instrumented muzzle motion post is pertinent to the discussion: 
https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg156150612#msg156150612

If you find it interesting, click on page one to take a look at the rest of that thread.

This barrel vibration chart appears at the above link:

[Mike -]

MUCH of this harmonic thing ALSO has to do with the material thickness.

While I'm not familiar with an R9, I know that the harmonics can have an effect in barrels like the thin, FX barrels.  The cross section of an FX barrel is very thin.  This is the gun that pushed all of these harmonics tales.  The thicker barrels that have a wall thickness of .150" or more are very...much less susceptible to vibration or, harmonics as the kool kids like to call it.

The effects go down significantly...with thicker walled barrels.  Someone mentioned that your gun is similar to the HW100.  I have an HW100 Bullpup.  That is a pretty thick walled barrel, and I seriously doubt that that any 8gr. pellet and the air pushing on it will sent the barrel into sever enough harmonics to alter the point of impact.

Mike

[gloob]

The graph that Subscriber posted is very important. But it leaves out an important variable.

The amount of time between peak recoil and pellet exit is not fixed. It's dependent on several factors such as the power of the spring, mass of the piston, the length of the barrel, and the weight of the pellet. Most of those factors are fixed. But the weight of the pellet is user's choice.

If you shot a heavier pellet out of the same rifle and repeated this graph, you'd see two things. First,  the time between peak recoil and pellet exiting would be longer. And this could push the exit time into the part of the graph where the vibrations are larger.

The second thing you notice with relatively heavier pellets is this timing isn't as consistent. The pressure in a break barrel drops relatively fast after it peaks. A relatively heavy enough pellet will reach it's slower max speed earlier in the barrel, with maybe 6+" (even 12" in extreme cases) left to go before it exits the muzzle. It will be subject to a lot of drag/deceleration in that last few inches. The amount of this deceleration is variable, depending on how tight that pellet is in the bore. So pellet-to-pellet variation starts making a huge difference. You get more fliers.

When you shoot the pellet weight that reaches max speed within only a few inches of the muzzle (for this particular rifle), the pellet-to-pellet variation in head diameter/drag doesn't matter as much.