Question,does the last friction of an inch at the end of the barrel have more

[boscoebrea]

importance than the many inches before the end of barrel....yes I think ,butt my real question is how the pellet "spins" out of barrel...what I am trying to "picture" is how the very last part of the groove and lands in the barrel project the pellet forward.....those grooves do not change so with Everything being the same the pellet what hit in the same place every time,.....why not?if everything is the same.

say the above is true,if one was to cut the barrel and the lands and grove change position wouldn't the exit of the pellet change,I think so and the point of impact would change...now if that is so and your gun does Not shot to point of aim why not cut the barrel to change the point of impact?

Now I realize I have taken my assumptions to paper,butt I think I am right,not only that I also think there is a perfect barrel length to power and pellet .....butt hey so much easier to find the pellet and power level the pellet likes than to change the barrel "twist"...;Some barrels are made for certain pellets...must take a lot of shooting to find out....maybe there is a mathematics calculation that makes things easier.

Sorry for my long winded thought process and rough grammar,I had a lot of jungle to get through to arrive at my conclusion.Conclusion? What conclusion ?

 

[Brian10956]

If your planning on shortening your barrel do it at the back end if you cut the front off you’ll mess up the barrel crown. It works like a pitchers finger tips releasing a ball if you cut the finger the ball wont have the proper rotation. Same with a barrel cut the exit end and the pellet won’t spin consistently, it will spin but not the same with every shot 

 

[boscoebrea]

 I am talking about the crown,and if done right why wouldn't the pellet spin consistently,some barrels have to be cut at the crown.Your pitching analogy introduces a human factor...the barrel does not rotate and the pellet would be effected by the "power"of the push which would change the amount of rotation .

Now I do see that the pellet or projectile coming out of the barrel would change where it comes out because of the amount of force behind it.;thus I now see why some people weigh their pellets and keep their "power" in the sweet spot of their air guns.....like the only way for precision is to be precise and consistent.

 

[JW652]

I believe that what you are describing is a way to limit the influence of barrel harmonics. There are a lot of variables here but in its most simple form we are talking about the "whipping" of a barrel during the firing cycle - up/down - right left etc. The goal is to have the pellet exit at the same direction for each shot - instead to the right one time and the left on the next shot. Chopping the barrel might help - or not - as there are so many things going on. Slight velocity changes, pellet consistency, barrel temp, barrel stiffness - and on and on and on. A stiff, bull barrel is a common attempt to ameliorate the "wippyness". Muzzle weights are commonly used to find a sweet spot by moving it forward and back. The Whiscombe HOTS system, Browning BOSS, limbsaver doughnut and other such devices can help a lot. A good gunsmith will try to blueprint a known, good design to make each shot as dead and repeatable as possible. But even then it only works with a single projectile at the same velocity.

All the above is gross simplification and others will disagree or add their personal knowledge. It is a common challenge for firearms accuracy folks, but applies to air rifles as well. It is kinda sorta like taming recoil in a springer with a loose, consistent hold so that the pellet exits at the same point in the firing cycle with each shot. Except barrel harmonics work at a more subtle level.

 

[T3PRanch]

I don't think a rotational shift of the lands and grooves will result in a poi shift if the barrel is crowned correctly. The choke is usually in the last 3/4" of a barrel which is an area where the land and groove diameter is slightly smaller than the rest of the barrel. Removing the choke will usually create a difference in how the barrel handles each type of projectile. Typically bullets shoot better from a barrel that has no choke and pellets shoot better from a choked barrel. This statement, however, may not apply to all situations.

 

[nervoustrig]

When I read the original post, I was having some difficulty following your logic but it seemed like the function of the crown was the most relevant topic, and now it seems more clear that is the case. It’s pretty intuitive that the crown plays an important role because it is the last part of the barrel to touch the pellet as it goes out into the world.

The idea is to have the pellet skirt lose contact around its full circumference at precisely the same instant in time. That helps to ensure the high velocity gas behind the skirt does not sneak out in front where it can influence the pellet’s trajectory.

So imagine instead an extreme scenario where the the crown is cut at a 45 degree angle. Just before the pellet leaves the barrel, a gap will appear behind the skirt on one side while the other side is still in contact with the barrel. Expanding gas will push out through the gap and put a bias in the direction the pellet leaves.

So you could in theory alter the muzzle so it favors a particular direction but it would be a trial and error affair and not likely to produce the tightest groups. Physics 101. The pellet, assuming it’s a good quality one, wants to keep moving in a straight line, the line established by the bore. Attempting to slap its rear end on the way out is an attempt to change its momentum vector.

Fortunately, pellets are flare stabilized by the skirt (think badminton birdie and how it self-corrects) so they are pretty tolerant of these shenanigans but it’s not optimal. We generally care more about consistency than we do a particular point of impact because the sights allow us to bridge the gap.

And last but not least, our pellet skirts are far less consistent than any decent crown. So just imagine for a moment a tin of pellets that are wonderfully consistent but they have a little crescent missing from the rim of the skirt. If you take care to always load each one with the missing bit in the same clocked position (say, notch at 12 o’clock) then each one would arrive at the muzzle with the notch in the same position. Thus the escaping gases coming through the notch would bias the pellet the same way each time.



However in practice, our pellets aren’t perfectly consistent...and we can’t identify the subtle differences in each one so we can load it with the exact same orientation. So if we add the complication that the crown favors spitting out gas at, say, the 6 o’clock position, we now have some pellets arriving at the muzzle with these gaps at the same position, augmenting each other. Other pellets reaching the muzzle with these gaps 180 degrees from each other, perhaps cancelling out. And everything in between.



So with all that said, chopping a barrel should have a negligible effect on accuracy provided everything else stays exactly the same...but it doesn’t. It changes velocity, it changes harmonics, the dimensions of the muzzle may be ever so slightly different, the crown may be a fraction of a degree different than before, and so on.

 

[leadfoot]

boscoebrea... I read your initial post and "see what you're saying". If there is a "fault" at the muzzle, why wouldn't all the pellet impacts be affected consistently, and any pellet going through it have the same POI. I think that is the gist of it. (Also described by nervoustrig).

My guess is that if every pellet had EXACTLY the same velocity, and skirt profile, the theory would hold, but even a few fps would alter the model due to the induced instability, as described by nervoustrig. 

It may be comparable to an arrow with one of the feathers missing. Yes, it will go forward, but even if all the arrows have the same one feather missing, it's not likely they will group as well as a properly fletched quiver full. That's just the way it is.

 

[JamesD.]

Harmonics by length or weights, plus I like 1” bull barrel. A thimble or through bolt flow & indexable barrel is a way to play with results.

 

[ptthere]

boscoebrea... I read your initial post and "see what you're saying". If there is a "fault" at the muzzle, why wouldn't all the pellet impacts be affected consistently, and any pellet going through it have the same POI. I think that is the gist of it. (Also described by nervoustrig).

My guess is that if every pellet had EXACTLY the same velocity, and skirt profile, the theory would hold, but even a few fps would alter the model due to the induced instability, as described by nervoustrig. 

It may be comparable to an arrow with one of the feathers missing. Yes, it will go forward, but even if all the arrows have the same one feather missing, it's not likely they will group as well as a properly fletched quiver full. That's just the way it is

^^^ This.

I'm somewhat noobish with pellet rifles but I have cut/threaded/re-crowned barrels on firearms on many projects over the years. I have the belief that the barrel twist and crown on an air rifle has the same importance.

I cannot understate how important the crown on the end of the barrel is. If it is not machined properly, your POI will lose repeatability.

Now in a perfect world, if you have at least a bench top lathe or mill, doing a cut and re-crown can be pretty painless. However there are also some "poor man's" techniques that can yield some decent results. One of them is (as long as you can find a way to do a perfectly flat cut) to take some lapping compound, an old rubber ball from a used computer mouse, and a lot of elbow grease, and massage the ball in circles over the bore with the compound until you have a nice even crown. Doing it lightly in every direction should take a straight hour of your time.

The other consideration is that the engineers who designed your gun probably did so with a specific barrel length in mind. The question is, was it aesthetics, function, or both? Sometimes a longer barrel can ensure higher velocity and accuracy. Shortening it can potentially sacrifice one or both depending on what your setup is.

Just my $0.02.

PT

 

[shoot44]

.

 

[nervoustrig]

In that case I would say no it does not matter...the clocking or indexing of the lands and grooves does not matter. Certainly not if the rifling is uniform. However let's just say the rifling button used to form the rifling was asymmetric. For example the grooves on one side are just a smidge deeper than on the other. When the pellet leaves the barrel, it is rotating. Therefore whatever bias this hypothetical rifling might impart on the projectile, it's constantly rotating about the pellet's forward axis of travel which has a net cancelling effect over a distance. The same mechanism by which rifling helps mitigate minor geometric or gravimetric imbalances in the projectile itself. Granted it's not 100% effective at cancelling these effects but clearly it's very effective as evidenced by how well a rifled barrel performs compared to a smooth bore. 

 

[nervoustrig]

Now in a perfect world, if you have at least a bench top lathe or mill, doing a cut and re-crown can be pretty painless. However there are also some "poor man's" techniques that can yield some decent results. One of them is (as long as you can find a way to do a perfectly flat cut) to take some lapping compound, an old rubber ball from a used computer mouse, and a lot of elbow grease, and massage the ball in circles over the bore with the compound until you have a nice even crown. Doing it lightly in every direction should take a straight hour of your time.

Yes, with a bit of care it's possible to do a good crowning job with caveman tools. What I run into most often is a crown that was done poorly so burrs remain. Typically the chamfer is uniform so all it needs is to be dressed with the brass screw technique using a hand drill. One such example, before and after:



But on occasion I get one that has a terrible uneven chamfer or damaged rifling at the muzzle (from using a piloted crowning tool) that has to first be chopped off. Before I got a lathe, I would wrap painter's tape around the barrel to establish a reasonably square cut line for a hacksaw. Then I'd fixture it in the drill press to dress the end square using an emery wheel in a Dremel. Light touch of the rim of the emery wheel to the muzzle until it's uniformly abrading material around the full circumference. Then the brass screw method to form a clean chamfer with no burrs. An example of that, before and after:

 

[boscoebrea]

Wow,your thoughts and answers put a smile on my face. Putting my thoughts to paper is really hard for me and does lead to some confusion as to what I really meant.....the good part is everyone helped me and made sense and I think each answer opens the thought process.

Shoot 44 got what I meant,nervoustrigger brought in the pellet skirt..which expands and would help negate the problems I had pictured....I guess I brought in too much "old school powder burning" thinking....the soft lead and pellet skirt makes much of my thought process not so useful.

All that makes me curious about using slugs,like it would seem slugs would need more velocity to

be anywhere near as accurate as "pellets,butt that is a different subject that needs a different heading.

Thank you guys for expanding my thoughts and education.

 

[ptthere]

Now in a perfect world, if you have at least a bench top lathe or mill, doing a cut and re-crown can be pretty painless. However there are also some "poor man's" techniques that can yield some decent results. One of them is (as long as you can find a way to do a perfectly flat cut) to take some lapping compound, an old rubber ball from a used computer mouse, and a lot of elbow grease, and massage the ball in circles over the bore with the compound until you have a nice even crown. Doing it lightly in every direction should take a straight hour of your time.

Yes, with a bit of care it's possible to do a good crowning job with caveman tools. What I run into most often is a crown that was done poorly so burrs remain. Typically the chamfer is uniform so all it needs is to be dressed with the brass screw technique using a hand drill. One such example, before and after:



But on occasion I get one that has a terrible uneven chamfer or damaged rifling at the muzzle (from using a piloted crowning tool) that has to first be chopped off. Before I got a lathe, I would wrap painter's tape around the barrel to establish a reasonably square cut line for a hacksaw. Then I'd fixture it in the drill press to dress the end square using an emery wheel in a Dremel. Light touch of the rim of the emery wheel to the muzzle until it's uniformly abrading material around the full circumference. Then the brass screw method to form a clean chamfer with no burrs. An example of that, before and after:

Another great example of DIY technique. I tried that once on a rifle and had what seemed like "satisfactory" results, but I handed that rifle off shortly after that so I didn't have the opportunity to evaluate any long-term accuracy with different rounds. It's also been awhile since I have done any serious gunsmithing.

This thread has been a very interesting read.

PT