Air pulse duration, does it matter?

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Centercut

Centercut

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Another interesting topic has surfaced that I’d like to discuss. I’ve heard many times about the firing valve opening and closing, and adjusting reg pressure and hammer strike so that the valve is shut prior to the pellet exiting the barrel...

The reasoning I’ve heard is that if this isn’t done, air behind the pellet (or slug) could destabilize the pellet in the first portion of its flight. Here’s my dilemma. No matter how long or short the valve is open, no matter what reg pressure, no matter how hard the hammer strike, there will always be high pressure air behind the projectile when it exits the muzzle.

So tell me why it matters (pertaining to accuracy), and aside from efficiency, if the valve shuts before or after the projectile exits the barrel?
 
No matter how long or short the valve is open, no matter what reg pressure, no matter how hard the hammer strike, there will always be high pressure air behind the projectile when it exits the muzzle.
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"Always"? Can't agree with that at all even though I basically share the view that the higher pressure air probably wouldn't disturb the projectile. There COULD be higher pressure air behind the projectile if a sufficient volume were released from the mechanism. There also MIGHT NOT be higher pressure air behind the projectile if a smaller volume of air were released such that pressure in the barrel was dramatically falling at muzzle exit. And dwell time and/or hammer strike would seemingly determine such.

Crown quality has been noted as directly related to this issue. A "perfect" crown would let the projectile move with equal air effect around the skirt circumference whereas a less than perfect crown might not. Or could it be a mechanical/physical bur that disturbs the projectile?

I recall a video from several years back where a gentleman took a barrel that was shooting well and gradually cut the muzzle back, using a hacksaw, at various increasing angles and the results were surprising to say the least.
 
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Adjusting everything to a point that the "accelerating force" is gone at about 80%- 90% of the projectiles way down the barrel is ideal. 

@ IDEAL conditions a projectile will be under much less pressure as it exits the muzzle. Minimizing the excessive blast behind the round. 

The mass of a projectile moving faster than the driving force behind it in the last part of the barrel will give a clean exit. 

Excessive firing valve dwell time would be like loading a slow burning powder for a short barreled powder burning gun. 

Big muzzle blast and poor accuracy. That's how barrels work.

Even a simple blow gun is a short blast tool. Long hard blast won't work. Airguns are no different. 

That's my answer........




 
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I agree with both the Crown and muzzle blast statements - although with heavy ammo, I have always had better accuracy consistency with higher pressure and lower VDT. I also believe that every pellet + rifle combination has an "appropriate" minimum air pressure needed to shoot them. 

On the flip side, for me at least - the lighter pellets don't seem to yield the same benefits for any PCPs that I've owned, and I get more consistent accuracy with minimal pressure.
 
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Long_shot

“@ IDEAL conditions a projectile will be under much less pressure as it exits the muzzle. Minimizing the excessive blast behind the round. “

I’m still trying to rationalize the higher pressure lower pressure theories above. If my pellet stops accelerating at 90% of the barrel length, then the firing valve has closed and no more air is going into the valve space/transfer space/barrel. So for a 600 mm barrel, the pellet still has 60 mm to go before it exits at the muzzle. Looking at the overall volume of the system from the valve chamber to the muzzle, we are looking at a pressure drop those last 60 mm of what, 5% of system pressure prior to the valve closing? Will that reduction of 5 or 10 bar (for a reg pressure of 120 to 150 bar) really make that much of a difference accuracy wise? Is 5 or even 10 bar “much less pressure”? Or am I making some incorrect assumptions?
 
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 I really like these post CC, it makes us really question what’s going on and why. There is so much bad info out there on the internet or people get sucked into believing something but never really ask why or the background of the people sharing the info. The velocity post for wind deflection, I’m with you, pretty cut and dry as far as I’m concerned. The bullet stabilizing at distance post, again I’m with you. This post is a good one too, I can’t wait to hear what people have to say. 

I agree with BandG that an untrue Crown will ruin accuracy but that’s more a cause of the Crown than the pulse of air. 

Long_Shot’s reply makes a lot of sense too, and initially I thought he was absolutely correct and maybe he is? However, the more I think about it, the more questions arise... Regardless of the pulse duration or force, the bullet should be accelerating ahead of any effects of the blast, shouldn’t it? If the crown is cut true and the edges are perfectly tangent to the bore centerline, the blast behind the projectile should not impart any irregularities on the projectile should it? 

I don’t know the answer. I was thinking about what was written so far, wondered about these things so I thought I’d ask. I’d love to hear more about this topic!

Sorry if my statements are similar to the above. I was typing while you guys posted.



Stoti


 
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Thanks @stoti I have a long list of topics that I will post one or two per week. There are so many fallacies and old wives tales regarding pneumatics, air and fluid dynamics, subsonic applied ballistics, etc that I’d like the AGN members to think about and discuss. Some I already know the answer but others like this topic I’d like input so I can form my own opinion as can our members. ;)
 
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Let me start by saying I’ve seen lots of discussions on this topic but no compelling experiments that characterize the effects of the muzzle blast as a completely independent variable. For example I’ve seen some good analysis of how an air stripper can be used to improve average group size, and while that may be a compelling reason to use one, it doesn’t necessarily mean it is the stripping function that can be credited. It could be the change in barrel harmonics from having added something to the muzzle. Or it could be a combination.



Stoti, to your question about whether the pellet is accelerating ahead of the effects of the blast…just before the pellet emerges from the muzzle, the air molecules immediately behind it are being held back so to speak. However their unconstrained velocity is generally much higher than, say, 900fps (a typical muzzle velocity). As soon as the pellet clears the muzzle and is out of their way, the air molecules are free to accelerate out and overtake the pellet. The pellet is a fat guy ambling through the doorway to Walmart at 12am on black Friday, and the air molecules are all the poor saps who have been waiting to get in and rush to the back to buy the cheap big screen TV.



This effect has been captured with high speed photography. Of course this high pressure air is suddenly expanding into a much larger volume so it begins to lose velocity really quickly, thus it doesn’t last long. It’s the residual pressure at the muzzle that determines how long it will last. If the valve is still open when the pellet leaves, the residual pressure at the muzzle will be high. If instead the valve closes quickly, the residual pressure at the muzzle will be less. 



I’m of the mind that this turbulence effect isn’t zero but that it’s small enough to ignore in most cases. Otherwise it would be an accepted fact by now that you must have an air stripper installed if you are serious about accuracy, and shrouds with little discrete baffles inside would be the pariah of the industry.
 
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Thanks, NervousT. That’s exactly what I wanted to know. I would think those expanding gasses, already having transferred much of their energy, once free of the bore, would expand and slow so fast that there would be little effect on the projectile. I’ve seen lots of super slo mo footage of the projectile leaving the bore and the gasses swirling behind and off to the side of the projectile. That’s why I mentioned the concentricity of the bore in my previous statement. If concentric, the gasses shouldn’t effect the projectile negatively, even if they do catch it for a tiny fraction of a second. Right? Or am I missing something?

Stoti
 
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In regards to stoti and my post above, I agree that a good crown is probably a necessity. But the video (or article with pictures, I can't recall specifically which it was) about cutting the crown showed major surprise on the part of the guy doing it. Horribly angled crowns cut with a hacksaw did shoot worse that before the cut when it shot very well but not nearly to the point that the shooter was expecting. If I can find that again I'll link to it. Bottom line is I don't know whether mechanical burs or air deflection are the main issue with a crown and that piece I noted really surprised me because I expected bullets to spray everywhere. They did not.
 
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Stoti, yes I tend to agree with you. If the skirt releases evenly from the muzzle and the gases escape uniformly behind it, I don’t see why it would have a deleterious effect. And again, the gases are slowing substantially once they leave the cramped quarters of the bore and suddenly have the whole wide world to expand into. The pellet has a lot of momentum at the instant it leaves the muzzle, whereas there is but a tiny mass of gas will manage to surge up alongside the pellet. Lesser still is the portion of it that will exert an imbalanced force to nudge the pellet off its course.
 
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Great information. I think what I really want to find out, is:

1. low pressure long duration (say until the pellet has travelled 90% of the barrel before the valve shuts) or,

2. high pressure short duration (say the pellet has travelled only 30 or 40% of the barrel before the valve shuts). Or, even:

3. compare a third case where the valve doesn’t shut until after the pellet leaves the barrel.

In the first case the pressure only decreases slightly before exit. In the second case the pressure will have decreased significantly before exit. And in the third case the pressure will not have decreased at all prior to exit.

Which is best for accuracy? Does it even matter? Which is best for efficiency? 
 
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Great information. I think what I really want to find out, is:

1. low pressure long duration (say until the pellet has travelled 90% of the barrel before the valve shuts) or,

2. high pressure short duration (say the pellet has travelled only 30 or 40% of the barrel before the valve shuts). Or, even:

3. compare a third case where the valve doesn’t shut until after the pellet leaves the barrel.

In the first case the pressure only decreases slightly before exit. In the second case the pressure will have decreased significantly before exit. And in the third case the pressure will not have decreased at all prior to exit.

Which is best for accuracy? Does it even matter? Which is best for efficiency?
Click to expand...

Interesting questions but how would you ever know for certain WHICH variable you were dealing with. Seems each of the above conditions would change velocity (given a constant projectile) which would change spin rate which might change stability.
 
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If you have a “good” tune at high reg pressure or low the amount it air following behind the pellet could be the same. Not excessive. Both pellets accelerate the full length of the barrel and leave at 900fps. The rate of acceleration is different. The high regulator pressure will accelerate more quickly initially and it’s average fps will be higher. It will spend less time in the barrel. The low regulator pressure will accelerate more over the length of the barrel and have a lower average fps. Within the barrel. 
 
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I was assuming no crown issues. A perfect gun with a longer or shorter dwell time. 

nervoustrig - You did a great write up. 

Photos of an "ideal" round being shot may be what your looking at though.

----------------------------------------------

I'm trying to say the gasses should be in a diminishing pressure as the projectile leaves the muzzle.

Not in a state of ever increasing pressure past the end of the muzzle.

Test 101. 

Take your best airgun that shoots ragged holes. 

Rig the valve to dump the whole AT for one shot.

That way you know it was 100% driving force clear to and beyond the end of the muzzle. 

Probably should lower the reg. pressure to achieve the same velocity as it shot before. Apples to apples comparison. 

Do 5 of those for a group. I doubt you'll be impressed. 


 
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