Thinking out loud about pellet stability

[BeemanR7]

Everybody comprehends how important is the crown of a gun barrel. It must be true and concentric with the center of the bore so that when the projectile leaves the barrel, it is not sent askew, but true. So, much time and effort and expense are expended to make sure that the crown has no flaws.

But if true stability is the goal, doesn't the same mentality follow that the end of the projectile (the portion that leaves the barrel last) is held to the same tolerance and precision as is the crown of the barrel? Common sense says, "YES".

Now we know that the skirts of our pellets are lousy by comparison with the well manicured crowns of our barrels, and that making a pellet with as much precision and concentricity is never going to happen. So, what's the solution to pellet stability without pellet skirt perfection?

The common sense answer to me seems to be to tune the gun so that the valve is closed and the pressure behind the pellet as it leaves the barrel is as close to zero as possible, thus putting the irregular skirt of the pellet in a neutral pressure environment as the pellet leaves the barrel.

Any discussion?

 

[intenseaty22]

This has been discussed recently in different sections, sort of. The pellet skirt imperfections anyway. I dod a quick experiment that I posted on there. My theory is that the pellet is literally reshaped to almost perfect as it is pushed through the lands and groves and exits the barrel in relatively good shape. In my experiment, it did affect accuracy, but only marginally at 30 yards for the kind of damage I did to them. You figure that most pellets don’t have that severe of damage. 
http://www.airgunnation.com/topic/not-happy-junk/

 

[BeemanR7]

intenseaty22,
I looked up the thread and your post (experiment) and saw the pics of your 'bitten pellets'. I comprehend that the explosive pressure behind the pellet tends to expand the skirt to conform to the bore (it's lands and grooves). But that's not what I'm talking about. I'm talking about the flat portion of the skirt.....the trailing edge of the skirt.....that portion of the skirt that is ninety degrees opposite the bore of the barrel. I'm suggesting that to achieve perfect stability, that flat surface must be perfectly square to the bore as is the crown of the barrel or stability cannot be achieved unless the pressure of the barrel behind the pellet is reduced to near zero as the pellet leaves the crown. To achieve like perfection on the end of the skirt, one would have to load each pellet into a lathe and burnish the end of the pellet to match the precision of the crown. Either that, or load the pellet into a die drilled just deep enough for the pellet, and then burnish the end of the pellet on a piece of glass to make it perfectly flat and perfectly 90 degrees to the longitudinal axis of the pellet with the same care and precision with which we treat the crown of the barrel.

In reference to your experiment, look at your photos again. Ignore for a moment the 'bites' (bent skirts) and look instead at the flat surface of the pellet skirts at 12, 1, and 2 o'clock positions in the mag. See those tiny notches or nicks in the flat perpendicular surfaces? THAT'S what I'm talking about. Each one of those nicks results in a jet of gas that escapes asymmetrically as the pellet leaves the bore, sending the pellet askew, even if the skirt is blown and expanded to perfect conformity with the barrel walls. Those notches or nicks need to be burnished to a mirror finish, just like the crown of the barrel, but at precisely 90 degrees perpendicular to the crown, else perfect stability cannot be achieved.

Now, I know that nobody but possibly an Olympic champion would invest the time and effort to burnish the trailing edge of their pellets to precisely match the crown of their barrels. I'm suggesting that the only other way to null the effects of an imperfect trailing edge would be to time the valve opening and closing so precisely as to eliminate any asymmetrical jets of gas upsetting the pellet as it leaves the crown. That means that the pellet leave the barrel with no pressure behind it.......to accelerate the pellet early and time the flow of pressure to end before the pellet leaves the crown so that there is neither positive nor negative pressure in the barrel as the pellet leaves.

Does this make sense?

I'm wondering if there's more than one reason why FX makes their pellet probes the way they do. FX's pellet probes bear right on the flat trailing edges of the pellet skirts. This would have the effect of burnishing the trailing edge of the pellet to a perfect 90 degree surface to the crown of the barrel. Maybe not all the accuracy of the FX guns can be attributed only to the Smooth Twist or Smooth Twist X barrel treatment. Maybe some of that accuracy and precision really results from the way FX loads the pellets into the bore to begin with.

 

[intenseaty22]

I see what you are referring to now, but it’s almost too deep for me to delve into. My brain is way too rusty for that. 
But...I’m trying to picture what you say about the “no pressure behind it as it leaves the barrel” and I feel as though that would be against the laws of physics in a HPA gun. The gas (air) is compressed in the tank and it’s expansion IS what’s propelling the pellet. The only scenario I see that could work like that would be a sort of catapult type force upon the pellet. Or even a Rail Gun? Like I said, this goes beyond me. 
I just want to hit a nickel at 30 - 50 yards. Or roughly the space between the eye and ear of a squirrel. And actually, 30 suffices me for now.

 

[Crusher]

Testing the skirt is something I do. I turn the pellet skirt down in the tin lid and rub it around in a circle. If the skirt isn't perfectly flat, it will be after a few circles. Give it a try. you will see what I mean.

Crusher

 

[BeemanR7]

"Crusher"Testing the skirt is something I do. I turn the pellet skirt down in the tin lid and rub it around in a circle. If the skirt isn't perfectly flat, it will be after a few circles. Give it a try. you will see what I mean.
Crusher

I've done it too, Crusher. It really works. The end of the pellet is as perfectly flat as an eyeball can perceive. I just have no way to know if it's still perfectly 90 degrees from the axis of the pellet. And if it's not, then it will skew a bit as it leaves the crown. The point of all this is that it is not practical to perfectly true the end of each pellet. And the only other way to insure that the pellet doesn't skew when it leaves the crown is to time the valve so that the pellet is fully accelerated before it reaches the crown and there's neutral pressure as it leaves.

I noticed something right away when I got my FX Wildcat. It was unbelievably quiet compared to my other guns. It almost sounded like it was broke. So, beings that the gun was new to me, I got out my chronograph to check velocity. 870FPS as normal shot after shot. But there was no noise. How can this be unless every squirt of air is fully expended before the pellet leaves the barrel? And the efficiency of the Wildcat is extraordinary compared to my other guns too. I think FX has accomplished theoretical perfection with their valve timing and duration and squirting just enough air behind the pellet to fully accelerate it prior to leaving the barrel and there's little or no excess air to blow an uneven skirt one way or the other. In other words, when the pellet leaves the barrel, it may even experience a slight negative pressure (suck) on the skirt just as it leaves. How else could FX achieve such accuracy with pellets having skirts that we all know are not nearly as true as is the crowns of our barrels?

Something else I noticed while watching one of Ted's videos where he was talking about how he tunes his Impact and his Crown. He said he could HEAR the harmony! He adjusts the valve spring and the regulator pressure and the valve limiter until he achieves a certain SOUND. He's learned to equate that sound as proof of the harmony that he's seeking. Could it be that the sound that he hears is when he has perfectly squirted just enough air to accelerate the pellet to the desired speed for that pellet and no more? Is that pellet under the neutral environment that I've described so that there's neither pressure nor vacuum in the bore as the pellet leaves the crown?

Both Matt and Ted recommend setting the reg and the hammer spring to the desired speed, and then turning the valve limiter until the speed just starts to drop. What does that say to you all???

It says to me that they have manually duplicated what FX perfected with the Wildcat. A neutral or near neutral condition just as the pellet leaves the barrel. The acceleration is over before the pellet leaves. There's little or no pressure or vacuum to skew the pellet as it leaves. This would result in a dramatically more quiet gun. A short, quick squirt of air that is fully expended and has done it's job completely before the pellet leaves the barrel. There's nothing left. Not even noise. You can HEAR it!