# Technical, about rifling.



## rogervan (Oct 23, 2009)

Hi there,

I am wondering whether the pellets fly down the barrel of a rifle on the rifling, like skating on rails. Or, do the pellets have close clearance to the sides of the bore?

Roger


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## spentwings (Apr 25, 2007)

It's called web search 101.
http://en.wikipedia.org/wiki/Rifling
http://www.pyramydair.com/airgun-accuracy-barrels.shtml
http://en.wikipedia.org/wiki/Air_rifle


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## fprefect (Oct 14, 2009)

rogervan said:


> Hi there,
> 
> I am wondering whether the pellets fly down the barrel of a rifle on the rifling, like skating on rails. Or, do the pellets have close clearance to the sides of the bore?
> 
> Roger


There is no "clearance" at all. The force of the initial blast of air will fill out the "skirt" on the rear of the pellet causing the pellet to form an airtight "seal" as it travels down the barrel thus not allowing any of the expanding volume of air to "escape" around the edges of the pellet.

The same is true of the lead or copper jacketed lead bullets fired from all certerfire rifles. The depths and heights of the lands and grooves will vary from rifle to rifle, but without the airtight seal, the rifle is little better than the smooth bore muskets that preceded the rifled barrel having an effective range of only about 75 yds.

F. Prefect


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## rogervan (Oct 23, 2009)

fprefect said:


> rogervan said:
> 
> 
> > Hi there,
> ...


I know what the skirt does. It's function gives air rifle pellets their unique shape. I was referring to the pellet in front of the skirt - does it hug the barrel or is there some degree of clearance? Does choke provide a last split-second, maybe two-inch-long alignment for the pellet if it's not hugging wall as it travels down the barrel?

About boattail bullets in center-fire rounds, they as a class do not have a skirt to hold back gas. So is it right to say that a portion of the projectile is tightly hugging the barrel as it travels down? Is there choke on the barrels made for boattail ammo? This is an important question for me. Actually I see now that any gas behind the round will be like plasma, and since it is violently expanding, being held back by the round, if any of it could find a way to leak past the bullet on it's way down the barrel, it would melt grooves in the side of the round. Ergo, the bullet itself is sized to make a gas-tight seal as it travels along the entire length of the barrel. Correct?

Thank you for your time,

Roger


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## fprefect (Oct 14, 2009)

rogervan said:


> fprefect said:
> 
> 
> > rogervan said:
> ...


Whether the bullet is totally made of lead or covered with a copper jacket that allows a bullet to be fired at higher velocities than lead, the seal between the projectile and the grooves must be complete to obtain the maximum velocity as well as accuracy.

It might seem, at least on the surface, that more force could be applied to bullet if it were designed in the manner of a pellet or a .58 cal. minie ball where it would appear all of the force is being applied to the rear of the bullet. But in reality, it makes no difference whatsoever. The completely jacketed boattail bullet that you alluded to where the contact with the groves occurs more toward the center of the bullet, as long as the seal is complete, the entire force will be applied to the projectile, pushing it down the barrel regardless of the shape or where on the bullet the seal is made between the bullet and the rifling.

In the case of the pellet, if you where to take a micrometer and measure the diameter of the front and rear, you will find that it touches the lands and grooves with only the wider rear section of the pellet. In the case of a misaligned pellet, or a pellet with a skirt that has been "caved in" to even a small degree, the impact point will in all likelihood be "off", and sometimes by a wide margin. In other words, rifling cannot straighten out a misaligned projectile and an inaccurate shot will be the result. There are probably some pellets that have the same diameter all along the entire body of the pellet, perhaps in an effort to achieve a higher degree of accuracy, but I'm not aware of any as the larger the surface area in contact with the lands and grooves in the barrel, will tend to produce additional friction, therefore reducing the muzzle velocity of a pellet or bullet.

There is also an area located in the first 1/4 to1/2 inch at the beginning of a centerfire rifle barrel that will not be rifled at all. This is called free bore and lets the bullet gain velocity faster before coming into contact with the rifling and it's much greater level of friction. However, this freebore can cause a bullet to become very slightly misaligned at the point of entry into the rifling producing a very slightly "canted" bullet as it move down the bore and producing a slight degree of inaccuracy. To avoid this condition, most bench rest shooters will work up their loads with the ogive of the bullet actually touching the lands, or only as little as .005 of an inch in back of the rifling. Needless to say, this can cause a spike in pressures as the bullet must begin it's flight down the bore without the "head start" provided by seating the bullet further back in the case producing the 1/4" or so of freebore described above, and powder charges must be adjusted accordingly. But it has been found that approx. 90% of all rifles will produce their best accuracy if the bullet is seated out to a point that is very close to or actually touching the rifling.

To be honest I'm not aware if most pellet rifle barrels have any freebore, but I would tend to doubt it as pressures are much lower and the soft lead skirt of the rear of pellet can easily overcome any excess friction much easier than could a rifle bullet jacketed in copper. Hope this answers your questions or at least part of them.

F. Prefect


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## rogervan (Oct 23, 2009)

It might seem, at least on the surface, that more force could be applied to bullet if it were designed in the manner of a pellet or a .58 cal. minie ball where it would appear all of the force is being applied to the rear of the bullet. But in reality, it makes no difference whatsoever. The completely jacketed boattail bullet that you alluded to where the contact with the groves occurs more toward the center of the bullet, as long as the seal is complete, the entire force will be applied to the projectile, pushing it down the barrel regardless of the shape or where on the bullet the seal is made between the bullet and the rifling.

I'm going to a machinist who can measure to four points to the right of the decimal point, and measure several brands of pellet, for cylindrical trueness to whether they are round, and to see if some are fatter than others. Knowing which ones are fattest will enable us to test-fire various pellets with this in mind, and make a determination among all the brands, which type shoots best.

Perfect! Thank you for responding. I'm using this information to work out a project. If it works, I'll have a very interesting post for you guys.

Roger


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## fprefect (Oct 14, 2009)

rogervan said:


> It might seem, at least on the surface, that more force could be applied to bullet if it were designed in the manner of a pellet or a .58 cal. minie ball where it would appear all of the force is being applied to the rear of the bullet. But in reality, it makes no difference whatsoever. The completely jacketed boattail bullet that you alluded to where the contact with the groves occurs more toward the center of the bullet, as long as the seal is complete, the entire force will be applied to the projectile, pushing it down the barrel regardless of the shape or where on the bullet the seal is made between the bullet and the rifling.
> 
> I'm going to a machinist who can measure to four points to the right of the decimal point, and measure several brands of pellet, for cylindrical trueness to whether they are round, and to see if some are fatter than others. Knowing which ones are fattest will enable us to test-fire various pellets with this in mind, and make a determination among all the brands, which type shoots best.
> 
> ...


Not Perfect by any stretch, it's Prefect and hope most of what I said was accurate. I think what you may find when you have the rear "skirts" of the pellets measured is that some will be considerably (if measured to 4 point to the right of decimal) larger than others and I have noticed as I'm sure you have as well that some pellets are more difficult to "seat" than others. What effect this has on accuracy I have yet to determine as all that is necessay is there be a complete seal when the pellet is fired.

One of the "tightest" pellets I have used is the Gamo Rocket, (the one with the little steel BB embedded in the nose, and I have found it to be the least accurate of any pellet I have purchased to date, so having an "extra tight" fit does not seem to have a positive effect on the accuracy of that particular pellet, at least in my rifle. But just as any bullet or pellet may perform well in one gun is no guarantee it will do the same in another, although I suspect in the case of the Gamo Rocket the added complexity of manufacturing a pellet with a steel BB in the nose is not going to contribute to better accuracy although it's penetrating characterisics may be superior to many simple lead pellets which are easier to manufacture to any given consistancy. Although I'm certain it's not going to be true with all rifles, mine at least does not seem to shoot any of Gamo's pellets all that well, even though the rifle is a Gamo.

F. Prefect


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## rogervan (Oct 23, 2009)

fprefect said:


> rogervan said:
> 
> 
> > Not Perfect by any stretch, it's Prefect and hope most of what I said was accurate. I think what you may find when you have the rear "skirts" of the pellets measured is that some will be considerably (if measured to 4 point to the right of decimal) larger than others
> ...


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## fprefect (Oct 14, 2009)

rogervan said:


> fprefect said:
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> 
> > rogervan said:
> ...


Roger, don't take this the wrong way, but I am not sure what exactly you are attempting to achieve? A pellet with a boattail design? I have no doubt that such a pellet could be manufactured, but given the fact that no company that I am aware of offers such a pellet for sale, they are either
1) Not as accurate in an air pressure driven rifle for some reason, or 2) to achieve the accuracy of any current pellet design, they are too expensive to produce. I really doubt it's a cost issue as shooters at the highest levels of competition think nothing of spending $2500 or more for a rifle, so any additional cost for a boattail pellet design (if it proved to be more accurate than any current designs) cost would not likely be an issue.

As far as your concern with the bullet or pellet being exactly straight when traveling down the bore, you will be dealing with different issues when the propellant is compressed air and the bullet does not have to be directly connected the source of energy providing the pressure to move the projectile down the bore. With a centerfire rifle, the bullet of course must be attached to the case holding the powder, and it is of extreme importance that the bullet be aligned perfectly straight in the case. Any canting of the bullet is referred to as "runout" and of course any "canting" that will cause the bullet to engage the rifling at even the slightest of an angle will cause the bullet to leave the bore with near the same amount of imperfect alignment that cab produce the slightest amount of wobble during flight, thus effecting the accuracy of such rounds.

With an air rifle, the projectile can be inserted directly into the rear of barrel and the only concern is that it is inserted perfectly straight. Common sense would lead one to believe that even if the pellet is inserted just slightly "off line", the rifling with have the effect of straightening out the alignment of pellet as it moves down the bore. But just as with the case of a bullet seated in a centerfire case with only the slightest bit of runout, the rifling in this case as well does little to correct the misaligned bullet. Crooked in, crooked out.

Getting back to whether a boattail or flat base bullet is the most accurate, you're get plenty of good arguments on both sides. I shoot 22 and 6mm benchrest rounds at ranges of 100 and 200 yards and if I had to choose, I think I would probably be the flatbase (with those calibers and at those ranges).

However, even though I do not shoot the longer distances such as 1000 yds., those who do will almost always be firing a boattail almost without exception as the higher ballistic coefficient lets them retain their velocites longer, thus cutting down the time the bullet is in flight and exposed to the wind that is a key factor when shooting a distances of this range, thus in most cases, producing better accuracy and smaller group sizes than and flatbase designs.

I "think" we are in basic agreement that any misalignment in a projectile as it engages the rifling is going to adversely effect the the accuracy of a pellet or bullet, but I would be very much interested in your ideas HOW flatbase bullets can be manufactured with their center of mass closer to the "true" center than boattails. I not disputing your beliefs that this can be accomplished, but I suppose what I'm most interested in is just how, during the manufacturing process this can be accomplished with the flatbase, and not the boattail.

On one one point I will have to take issue. That being that no national records are held using boattail bullets. I don't have the "recordbook" in front of me but I'm certain that boattails dominate at longer ranges and it's not because flatbase bullets have not been tried, as they have in many shapes and weights. Maybe it is just due to fact that the boattails are in the air for a slightly shorter period of time given the same initial muzzle velocity as an equal weight flatbase, but that is still a function of the bullet design. You do make some very interesting points, but I will still have to go back to the "hows and whys".

F. Prefect


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## rogervan (Oct 23, 2009)

F.P. wrote:
" Common sense would lead one to believe that even if the pellet is inserted just slightly "off line", the rifling with have the effect of straightening out the alignment of pellet as it moves down the bore."

Mulling over the "common sense" aspect of your comment, my head starts to swim. Unless I find some information ready-made for me, I suspect I'll have to experimentally solve certain machine questions.

" But just as with the case of a bullet seated in a centerfire case with only the slightest bit of runout, the rifling in this case as well does little to correct the misaligned bullet. Crooked in, crooked out."

Makes sense.

"Getting back to whether a boattail or flat base bullet is the most accurate"

Apparently, match bullets are all flat-base.

", you're get plenty of good arguments on both sides. I shoot 22 and 6mm benchrest rounds at ranges of 100 and 200 yards and if I had to choose, I think I would probably be the flatbase (with those calibers and at those ranges).

However, even though I do not shoot the longer distances such as 1000 yds., those who do will almost always be firing a boattail almost without exception as the higher ballistic coefficient lets them retain their velocites longer, thus cutting down the time the bullet is in flight and exposed to the wind that is a key factor when shooting a distances of this range, thus in most cases, producing better accuracy and smaller group sizes than and flatbase designs."

Supposedly, the boattail pulls less vacuum behind it, relieving it of that particular drag effect, which turns out to be significant. I find myself wondering why they don't make a double-ended bullet with long points, spinning at 1 in 15 rifling. That to me seems like it would be the pinnacle of the progression to more and more conical relief in the backside of the bullet. I'm sure someone has tried it, but it's not published on the Internet. Perhaps weight-forward will end up working better.

"I "think" we are in basic agreement that any misalignment in a projectile as it engages the rifling is going to adversely effect the the accuracy of a pellet or bullet, but I would be very much interested in your ideas HOW flatbase bullets can be manufactured with their center of mass closer to the "true" center than boattails. I not disputing your beliefs that this can be accomplished, but I suppose what I'm most interested in is just how, during the manufacturing process this can be accomplished with the flatbase, and not the boattail."

Cool input. Thanks. But I don't understand what you mean by moving the center of gravity. One could make two-part bullets that place the center of gravity where you want it.

"On one one point I will have to take issue. That being that no national records are held using boattail bullets. I don't have the "recordbook" in front of me but I'm certain that boattails dominate at longer ranges and it's not because flatbase bullets have not been tried, as they have in many shapes and weights. Maybe it is just due to fact that the boattails are in the air for a slightly shorter period of time given the same initial muzzle velocity as an equal weight flatbase, but that is still a function of the bullet design. You do make some very interesting points, but I will still have to go back to the "hows and whys".

Yea, according to the mfg, boattails flatten the curve. Maybe the extreme boattail flattens it a lot, but somewhere, error must start to creep in and make the round no good. I'm an inventor, and I get what probably would be called manic commitments to ideas. Maybe I need pills LOL

Nice to be talking with you,

Bellyflop


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## rogervan (Oct 23, 2009)

"I "think" we are in basic agreement that any misalignment in a projectile as it engages the rifling is going to adversely effect the the accuracy of a pellet or bullet, but I would be very much interested in your ideas HOW flatbase bullets can be manufactured with their center of mass closer to the "true" center than boattails. I not disputing your beliefs that this can be accomplished, but I suppose what I'm most interested in is just how, during the manufacturing process this can be accomplished with the flatbase, and not the boattail."

Now I get it. I'm not trying to make a special flat-tail. I propose to make a true boattail that will work in an air rifle. I have a particular idea in mind, because I know, or I assume, I need to keep the butterfly tail so it can expand against the walls of the bore. I don't know yet if an air rifle has the power to push a bullet that rubs the cylinder wall all the way down and still keep speed. But anyway, once the boattail leaves the barrel, the tail has to go away so it can fly as a true conical bullet. That's the magic, and I assume they will be quite a bit more expensive than the typical X-shaped pellet. My machinist just finished a big project, and as I can afford it, I propose to myself to work on this.

In the meantime, do you want a way to keep your dog off the furniture? Sprays, etc. don't work. I have special thin light wire mats that you lay side-by-side on the sofa, chair, sofa arm, wherever, that your dog will not lie down on. They are simple, not dangerous, cheap, and come up in a short pile that you slip behind a sofa when company comes or when you're home to oversee your dog's behavior. I hope to see money from this in the near future, because no one is remodeling their kitchens anymore.

All in fun, with great interest,

Roger


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## fprefect (Oct 14, 2009)

Nice talking with you as well. You have some interesting and probably valid ideas. Got no use for the dog mats though as mine has shown no interest in the furniture, but being half Schipperke, I probably have the most often vacuumed floors in the neighborhood. Why a dog would shed this time of year I have no idea.

F. Prefect


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## rogervan (Oct 23, 2009)

"There is no "clearance" at all. The force of the initial blast of air will fill out the "skirt" on the rear of the pellet causing the pellet to form an airtight "seal" as it travels down the barrel thus not allowing any of the expanding volume of air to "escape" around the edges of the pellet.

The same is true of the lead or copper jacketed lead bullets fired from all certerfire rifles. The depths and heights of the lands and grooves will vary from rifle to rifle, but without the airtight seal, the rifle is little better than the smooth bore muskets that preceded the rifled barrel having an effective range of only about 75 yds."

"The same is true". I kind of believe that copper jacketed bullets do not mushroom at the back end to make the gas-tight seal. Instead, the body of the columnar bullet in it's back and middle sections hugs the bore tightly, preventing gas from escaping. It takes a lot of power to push them through the barrel. I'll get some quotes to back this claim.

roger


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## spentwings (Apr 25, 2007)

:iroll: If you had any working knowledge of firearms/airguns you would know that's not what he meant.
That's why I posted/suggested the links...learn through reading then ask questions if there's a need for clarification.


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## rogervan (Oct 23, 2009)

spentwings said:


> It's called web search 101.
> http://en.wikipedia.org/wiki/Rifling
> http://www.pyramydair.com/airgun-accuracy-barrels.shtml
> http://en.wikipedia.org/wiki/Air_rifle


None of those citations answer the question. Maybe we need Advanced Search With Staying On Topic.

Roger


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## rogervan (Oct 23, 2009)

fprefect said:


> rogervan said:
> 
> 
> > Hi there,
> ...


Well the skirt does have a measurable clearance of 0.0000. I was actually asking about the pellet, you gave me info about the skirt. No big deal. I might have already mentioned that I cut the skirt off a 16 grain pellet, and pushed it gently down the barrel. It doesn't quite clear the rifling; I could feel it as a ticking sensation as the pellet moved down the barrel. The pellet gets nowhere near the walls of the bore. That might be because there is not enough power in the air blast to push that kind of configuration down the barrel. Subsonic .22 rimfires shoot as fast as our air rifles, but there is enough push at first to slide the lead down the barrel, sliding along the sides of the tube with zero horizontal clearance.

hmmm...I wonder if I counted the "ticks" as I push the skirtless pellet down the barrel, can I figure the rate of twist.

The only way I can explain why the "ticks" happen, is that the rifling is very low rotation, so that the skirtles pellet moves with no rifling contact every inch or so. I'll do that again and do some reckoning, and let you guys know. I suppose I can wait till Monday and call Gamo, as well.

Now I can see that the Diablo shape is a direct descendant of Mine's Mini-ball.

Roger


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## spentwings (Apr 25, 2007)

rogervan said:


> spentwings said:
> 
> 
> > It's called web search 101.
> ...


:eyeroll: Mein gott!
Another quick 101 search gave me _*the diabolo pellet (the most common airgin pellet shape) is generally referred to as an hourglass shape. The forward end or "head" and the base or "skirt" of the pellet are of larger diameter than the midsection. The skirt is essentially hollow. The reason for this is that airguns operate at a much lower pressure than firearms. The base of the pellet must be soft and expandable for it to enlarge and fit the bore tightly and engage the rifling. However, only the head and skirt are in contact with the barrel so friction is reduced, allowing maximum speed down the barrel. *_
To your relief I'm sure...I'm done with you. :jammin:


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