POLISH AN LW POLYGONAL BARREL?
- Thread starter Drumsnguns
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If you are hoping to win a shiny barrel contest....you will have no choice but to polish it. 
Polishing your barrel will increase its friction....which is easy to see due to the fact that your velocity at the same settings will decrease with each step.
The only good thing that might come from polishing is that you might knockoff the very tippy tops of some radial tool marks....and this might lessen leading....despite increasing friction .
There is no magic trick that all barrels benefit from. Each individual barrel is a new scenario.
Mike
Polishing your barrel will increase its friction....which is easy to see due to the fact that your velocity at the same settings will decrease with each step.
The only good thing that might come from polishing is that you might knockoff the very tippy tops of some radial tool marks....and this might lessen leading....despite increasing friction .
There is no magic trick that all barrels benefit from. Each individual barrel is a new scenario.
Mike
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I like the debate about the advantages of polygonal rifling, over conventional spline rifling. Many people have excellent results with polygonal rifling, but to me its promotion is like claiming a hex nut with its corners rounded over is superior to an undamaged nut. There is an improvement over the hex nut or socket. It is called the Torx socket. Something that looks more like spline rifling. At least in principle...
Of course, there are several variants of polygonal rifling, as there are of spline rifling. They are not all all equal. Shooting results are what matter, as well as cleaning intervals.
My suggestion is to shoot the barrel you have and see if it performs well enough, and does not lead up in a detrimental way. If so, leave it alone. Making a barrel look very shiny inside, while rounding over the crown, or causing variations in the bore diameter will not make it "better".
If the barrel does not shoot very well with any good projectiles, then you can't make it worse. Check first for burrs at the transfer port and crown. If none, then a light polishing can't hurt. If a little polishing helps, try a little more. When a litle more does not change anything, stop. Good performance, or performance improvement is the goal, not "bling".
Of course, there are several variants of polygonal rifling, as there are of spline rifling. They are not all all equal. Shooting results are what matter, as well as cleaning intervals.
My suggestion is to shoot the barrel you have and see if it performs well enough, and does not lead up in a detrimental way. If so, leave it alone. Making a barrel look very shiny inside, while rounding over the crown, or causing variations in the bore diameter will not make it "better".
If the barrel does not shoot very well with any good projectiles, then you can't make it worse. Check first for burrs at the transfer port and crown. If none, then a light polishing can't hurt. If a little polishing helps, try a little more. When a litle more does not change anything, stop. Good performance, or performance improvement is the goal, not "bling".
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I don't know what your experience has been but I picked up about 20 fps in my D48 when I polished my barrel.If you are hoping to win a shiny barrel contest....you will have no choice but to polish it.
Polishing your barrel will increase its friction....which is easy to see due to the fact that your velocity at the same settings will decrease with each step.
The only good thing that might come from polishing is that you might knockoff the very tippy tops of some radial tool marks....and this might lessen leading....despite increasing friction .
There is no magic trick that all barrels benefit from. Each individual barrel is a new scenario.
Mike
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If polishing increased velocity, then the barrel was too rough to begin with. I have seen that starting with a visibly "dark" bore on an airgun.
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I noticed a drop in velocity after I polished a CZ barrel in my Krait. I haven't shot it much yet to say if this is temporary as it leads back up.
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After easier cleaning, consistently better grouping between cleanings seems to be why people polish their barrel bores. If they lose 10 or 20 FPS in the process, why does that matter?
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You can lose 100fps or more polishing a barrel.
The more air you need to push a projectile at a given speed...the more you will struggle for accuracy due to muzzle blast. The gun will also be much harder to attain decent velocity spreads at a reasonable level of efficiency with higher friction.
Mike
The more air you need to push a projectile at a given speed...the more you will struggle for accuracy due to muzzle blast. The gun will also be much harder to attain decent velocity spreads at a reasonable level of efficiency with higher friction.
Mike
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OK, Mike, 100 FPS is significant. Is that with pellets of slugs? Observed with what caliber and at what power level?
Do you know what the ideal surface finish is for shooting lead airgun pellets? Is it the same for slugs? Does it matter if the user wants to shoot at 800 FPS and below, compared to 1000 FPS (for slugs) ?
Does the ideal finish apply to .177 through to .50; or is there some scaling involved?
I have read that high-end hand lapped PB rifle barrels are often finish lapped with 400 grit abrasive imbedded in a lead lap. Those doing the lapping admonish the owners not to polish the bores, Yet mechanical cleaning of the carbon ring, and eventual throat thermal cracking, using JB compound is recommended. Along with a little full length cleaning. JB compound starts as 1600 grit and breaks down finer with use, so is it a polishing compound or not?
As any surface improvement by abrasion should follow the rifling, so does not even a 300 grit finish impart "micro grooves" that impress the projectile all the way to the bottom of the "grooves", all the way down the bore. In other words, the projectile makes full contact, regardless of the surface finish, providing the bore and groove diameters do not open up down towards the muzzle?
With airguns, there is the matter of ideal projectile to land and groove diameter matching. Certainly for .22 caliber shooting slugs, the general PB admonishment to shoot a projectile 0.001 to 0.002" over groove diameter is ignored. Such tight projectiles just add friction that robs power. It is common for airgun slug shooters to shoot slugs that are 0.001" under groove diameter, where the projectile bearing diameter makes firm contact with the barrel groove diameter only at the choke; not all the way down the barrel.
It seems that with large bore airguns the ideal projectile diameter is much closer to barrel groove diameter, with some insisting on an interference fit such as with PBs. The 0.002" larger than groove diameter rule with PBs exists, to reduce hot gas leakage past the bullet; especially lead ones. Hot gas leaking past the projectile in excess quantities will melt lead off the surface and solder it to the bore, so leading the barrel in a gross manner that is hard to remove and blows groups.
With revolvers, cast lead bullets should be sized the same diameter as the chamber throats for best "accuracy". That can be more than 0.002" over groove diameter. You don't want projectiles yawing in the throat before slamming into the forcing cone at a slight angle.
Copper jacket bullets are also prone to melting and brazing to the bore, if undersize. With the additional damage to the steel barrel, via excessive erosion, should excess hot gas leak between bullet and barrel groove diameter. Such erosion is normal with centerfire calibers, but occurs to a much greater extent with "hot" calibers, and accelerates as the barrel opens up from erosion ahead of the chamber. Airguns don't operate with hot gasses, so deliberately shooting loose projectiles by PB standards does not hurt the barrel. As long as airgun projectiles don't rattle down the barrel and have sufficient land engagement they can work.
I can't argue with good shooting results, regardless of projectile diameter rules, but what makes sense to me for airguns is for most of the bearing surface to be smaller than groove diameter (but larger than land diameter), with at least one driving band at groove diameter, to reduce air leakage, without excess friction. Altaros have a selection of lathe turned lead projectiles that seem to follow that philosophy. Part of their slugs have "bore riding" sections to center and prevent yaw near the frontal ogive; with narrow sealing bands near the rear, ahead of the boat tail.
Bore riding bullet noses with one or more tight driving bands is not a new idea for airgun projectiles (nor for solid copper PB rifle bullets). Many airgun slugs look like cast lead handgun bullets, except the "grease grooves" are shallow, and simply reduce total rubbing friction down the barrel. The snag with any of these stepped diameter projectiles, optimized for airguns, is that they cannot be swaged by conventional "slug" dies. So they need to be cast or lathe turned.
If you cast your own airgun projectiles using custom molds, you have more control over projectile diameter. But if you want exact diameters, special sizing dies are required. A pass though die will either leave the "bore riding" section un-sized, or size all sections down to the same diameter.
To make matters worse, standard airgun calibers do not seem to have standard land and groove diameters, so projectile manufacturers have to make a few diameter variants and hope for the best.
I suspect that the above complexity of projectile to barrel fit will affect the ideal barrel bore finish; making it more important in some instances, and almost irrelevant with others. The ideal barrel bore finish might even be different for stainless steel VS "carbon steel" or the cheap mild steel airgun barrels are often made of.
I think we can agree that sharp reamer marks or rifling button chatter perpendicular to the projectile travel should have its "edge" lapped or polished off, so that the propensity to scrape lead from the slug or pellet is reduced to the point of irrelevance. As long as the projectile's bearing length is greater than the distance between reamer marks or tooling chatter, the projectile should not yaw or vibrate as it travels down the barrel.
Cheap airgun barrels may be getting away with a lot of ugliness inside, by relying on the choke to direct the pellet in the right direction before release. Unchoked barrels have to get the details right, but may also be less forgiving of pellet to pellet variation, or less able to shoot a large variety of pellets well. Or, that is the rumor.
If one has ruined a barrel by over polishing it, to too fine a finish, can it be restored by limited polishing with 400 grit abrasive paste? To the extent that the surface would look slightly dull, but if you measure the land and groove diameters before and after, there would be no change, because the "high spots" or "lines" would be untouched?
Do you know what the ideal surface finish is for shooting lead airgun pellets? Is it the same for slugs? Does it matter if the user wants to shoot at 800 FPS and below, compared to 1000 FPS (for slugs) ?
Does the ideal finish apply to .177 through to .50; or is there some scaling involved?
I have read that high-end hand lapped PB rifle barrels are often finish lapped with 400 grit abrasive imbedded in a lead lap. Those doing the lapping admonish the owners not to polish the bores, Yet mechanical cleaning of the carbon ring, and eventual throat thermal cracking, using JB compound is recommended. Along with a little full length cleaning. JB compound starts as 1600 grit and breaks down finer with use, so is it a polishing compound or not?
As any surface improvement by abrasion should follow the rifling, so does not even a 300 grit finish impart "micro grooves" that impress the projectile all the way to the bottom of the "grooves", all the way down the bore. In other words, the projectile makes full contact, regardless of the surface finish, providing the bore and groove diameters do not open up down towards the muzzle?
With airguns, there is the matter of ideal projectile to land and groove diameter matching. Certainly for .22 caliber shooting slugs, the general PB admonishment to shoot a projectile 0.001 to 0.002" over groove diameter is ignored. Such tight projectiles just add friction that robs power. It is common for airgun slug shooters to shoot slugs that are 0.001" under groove diameter, where the projectile bearing diameter makes firm contact with the barrel groove diameter only at the choke; not all the way down the barrel.
It seems that with large bore airguns the ideal projectile diameter is much closer to barrel groove diameter, with some insisting on an interference fit such as with PBs. The 0.002" larger than groove diameter rule with PBs exists, to reduce hot gas leakage past the bullet; especially lead ones. Hot gas leaking past the projectile in excess quantities will melt lead off the surface and solder it to the bore, so leading the barrel in a gross manner that is hard to remove and blows groups.
With revolvers, cast lead bullets should be sized the same diameter as the chamber throats for best "accuracy". That can be more than 0.002" over groove diameter. You don't want projectiles yawing in the throat before slamming into the forcing cone at a slight angle.
Copper jacket bullets are also prone to melting and brazing to the bore, if undersize. With the additional damage to the steel barrel, via excessive erosion, should excess hot gas leak between bullet and barrel groove diameter. Such erosion is normal with centerfire calibers, but occurs to a much greater extent with "hot" calibers, and accelerates as the barrel opens up from erosion ahead of the chamber. Airguns don't operate with hot gasses, so deliberately shooting loose projectiles by PB standards does not hurt the barrel. As long as airgun projectiles don't rattle down the barrel and have sufficient land engagement they can work.
I can't argue with good shooting results, regardless of projectile diameter rules, but what makes sense to me for airguns is for most of the bearing surface to be smaller than groove diameter (but larger than land diameter), with at least one driving band at groove diameter, to reduce air leakage, without excess friction. Altaros have a selection of lathe turned lead projectiles that seem to follow that philosophy. Part of their slugs have "bore riding" sections to center and prevent yaw near the frontal ogive; with narrow sealing bands near the rear, ahead of the boat tail.
Bore riding bullet noses with one or more tight driving bands is not a new idea for airgun projectiles (nor for solid copper PB rifle bullets). Many airgun slugs look like cast lead handgun bullets, except the "grease grooves" are shallow, and simply reduce total rubbing friction down the barrel. The snag with any of these stepped diameter projectiles, optimized for airguns, is that they cannot be swaged by conventional "slug" dies. So they need to be cast or lathe turned.
If you cast your own airgun projectiles using custom molds, you have more control over projectile diameter. But if you want exact diameters, special sizing dies are required. A pass though die will either leave the "bore riding" section un-sized, or size all sections down to the same diameter.
To make matters worse, standard airgun calibers do not seem to have standard land and groove diameters, so projectile manufacturers have to make a few diameter variants and hope for the best.
I suspect that the above complexity of projectile to barrel fit will affect the ideal barrel bore finish; making it more important in some instances, and almost irrelevant with others. The ideal barrel bore finish might even be different for stainless steel VS "carbon steel" or the cheap mild steel airgun barrels are often made of.
I think we can agree that sharp reamer marks or rifling button chatter perpendicular to the projectile travel should have its "edge" lapped or polished off, so that the propensity to scrape lead from the slug or pellet is reduced to the point of irrelevance. As long as the projectile's bearing length is greater than the distance between reamer marks or tooling chatter, the projectile should not yaw or vibrate as it travels down the barrel.
Cheap airgun barrels may be getting away with a lot of ugliness inside, by relying on the choke to direct the pellet in the right direction before release. Unchoked barrels have to get the details right, but may also be less forgiving of pellet to pellet variation, or less able to shoot a large variety of pellets well. Or, that is the rumor.
If one has ruined a barrel by over polishing it, to too fine a finish, can it be restored by limited polishing with 400 grit abrasive paste? To the extent that the surface would look slightly dull, but if you measure the land and groove diameters before and after, there would be no change, because the "high spots" or "lines" would be untouched?
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I'll be honest....I didn't read the whole post yet.
I can tell you that Muller Rimfire barrels are hand lapped to 180 grit silicon carbide or 120 grit aluminum oxide. Paul is among the best in the world and the successes of muller barrels in the Rimfire BR world strongly reinforce that statement.
If you try to lap a barrel that someone has polished....you will not be able to move the lead lap after it is cast. Sometimes you can cast a very short lap (like 1") and get it to move...but it will still be very prone to locking up on the push stroke. I normally cast 4-4.5" laps. It's easier to pull tight patches of 120 or 180
through the barrel to rough up the surface enough. May take a lot of that, though.
I can tell you that Muller Rimfire barrels are hand lapped to 180 grit silicon carbide or 120 grit aluminum oxide. Paul is among the best in the world and the successes of muller barrels in the Rimfire BR world strongly reinforce that statement.
If you try to lap a barrel that someone has polished....you will not be able to move the lead lap after it is cast. Sometimes you can cast a very short lap (like 1") and get it to move...but it will still be very prone to locking up on the push stroke. I normally cast 4-4.5" laps. It's easier to pull tight patches of 120 or 180
through the barrel to rough up the surface enough. May take a lot of that, though.
Interesting that people state you will lose velocity due to friction from a polished barrel, I have polished quite a few barrels with no adverse effects, in fact quite the opposite.
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There is no arguing with facts and verified results. What is missing from the above is context:
The linear grain size of 180 grit crystals averages 0.003". That is taller than your typical .22 rimfire land to groove height of 0.002". Why is that not too rough in the instance of Muller Barrels? I am speculating, but I bet that their barrels are much harder than your typical mild steel airgun barrel. Of course, most of the grit height is imbedded into the lead lap, so it does not cut grooves into the barrel bore surface 0.003" deep.
For context, if you want to precision grind a fine finish into the taper in a lathe spindle, you do not use an 800 grit wheel; you use an 80 grit wheel.
Despite that seeming way too rough, you will battle to cut hardened steel with too fine a grit because too many grains will make contact with the work at one time. Too much contact area would require so much combined force to cut, that the tooling will spring excessively, making it difficult to reach the required specs on the finished part. It will also take "forever" to cut away the distortion in roundness that often occurs with quench hardening.
By using a coarse grinding wheel, proportionately fewer granules or grits make contact with the work at the same time. This enables cutting pressure to be reached at individual grains, without so much total force on the tooling as to spring it significantly.
Now, cutting mild steel with an 80 grit grinding wheel is likely to result in a rough looking surface, because it cuts so easily. For that, finishing wheels are required that have a much finer grit. But then precision tooling interfaces (such as a taper) are never made from mild steel.
It takes more energy to impart a fine finish to a hard steel surface, but far finer finishes are achievable that are also precise in shape, than for soft metals. You can buff soft metals to a mirror finish easily, but try to buff something to a precise shape.
So, while Paul Muller uses coarse grits to lap their rimfire barrels, with great success, I am going to propose that such coarse lapping is a "corner case" for airgun applications; rather than representative. of best practices. Airgun shooters regularly use JB compound, with improvements reported for consistent grouping, while making barrels easier to clean, when they eventually need it. I think this does not prove JB compound is best. It simply means that the people using it have a common experience. I would add that how any abrasive is used matters more than exactly which brand or grit.
A respected manufacturer who posts on GTA, fire laps all his new big bore air rifles with 360 grit valve lapping compound, much to the chagrin of those who like a mirror finish in their barrel bores. I like fire lapping for its effectiveness and simplicity. I also know that it should not be overdone, from experience.
Hard lead lapping is best, but not for those who have not learned how to do it properly. Also, it should be done on barrel blanks, so both ends can be cut off and thrown away. Hard lead Lapping a finished barrel is risky, so it should only be done if the barrel shoots poorly, because of reasons that cannot be fixed by less aggressive means. If you lap a finished barrel that shoots really well, you likely are not going to improve it. The opposite is more likely.
An exception to this, is if a PCPs barrel retention screws were over tightened, and have dinged the barrel all the way to bore. Then focused lapping to get rid of the tight spots might be indicated. The other exception is if the bore has several tight spots along its length from manufacturing, then lapping those out is likely to make the barrel shoot better. You can't harm a barrel that already shoots poorly. You just have to know when to stop. If you "improved" a barrel into oblivion, you were going to replace it anyway
. If you can shoot .22 pellets from your .177, you probably lapped too much I think "best grit size" for lapping barrel bores should be offered as a conditional statement. One that includes the steel the barrel is made of, its hardness before rifling and after rifling (hardness is method dependent). Also, if the coarse grit lap is followed by a finer one. I would like to ballpark discussion, then focus on specific details, in context. Else we might be arguing absolutes, using either pocket gophers, or pocket battleships as our reference.
As Thomas Air PCPs have a very good earned reputation, I would me more interested in how your barrels are made and prepared? I expect you to remain tight lipped, because that info is proprietary. If you are lapping them with 180 or 140 grit abrasive, I bet they are not made from mild steel.
Everything should be discussed in context. Arguing absolutes without context is silly, lazy or potentially deceptive.
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Again...didn't read the whole thing...but I treat airgun barrels the same as he treats Rimfire barrels. Rimfire barrels are 416 stainless steel. I don't change my method whether I'm lapping a 416 airgun barrel or a softer variant.
I'm really not looking to debate my methods or results and not suggesting that you are challenging them...I just thought some might find it interesting since it's not at all aligned with commonly accepted airgun practices.
Ultimately....the forums are an interesting study on how people think and the things that influence their beliefs.
Mike
I'm really not looking to debate my methods or results and not suggesting that you are challenging them...I just thought some might find it interesting since it's not at all aligned with commonly accepted airgun practices.
Ultimately....the forums are an interesting study on how people think and the things that influence their beliefs.
Mike
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As for big bore slug shooting, I have “polished” every barrel, and never noticed a velocity reduction. Though normally I only do 10 or 20 strokes each of progressively finer compound, finishing with 600 grit. My various slugs are all bore riders with rear driving bands that are sized .001” under groove diameter. And lubed with lithibee.
I had a so-so .22 LW Polygon barrel with some substantial tight and loose spots, so I did a very heavy polishing on the tight spots and after, pellet accuracy suffered noticeably, but slugs got better.
I usually perceive an improvement, at least for slugs.
Maybe it does not matter all that much, but I like to see a barrel with a low average roughness inside (even “shiny”), with obvious but not overly aggressive rifling. The slow acceleration of an airgun projectile does not need heavy rifling engagement for the projectile to track with the rifling (my opinion).
I had a so-so .22 LW Polygon barrel with some substantial tight and loose spots, so I did a very heavy polishing on the tight spots and after, pellet accuracy suffered noticeably, but slugs got better.
I usually perceive an improvement, at least for slugs.
Maybe it does not matter all that much, but I like to see a barrel with a low average roughness inside (even “shiny”), with obvious but not overly aggressive rifling. The slow acceleration of an airgun projectile does not need heavy rifling engagement for the projectile to track with the rifling (my opinion).
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Thanks, Mike
In summary; if a method works well, it should not be dismissed as "wrong".
In summary; if a method works well, it should not be dismissed as "wrong".
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That was a lot to catch up on!
In my case, I noticed an improvement in accuracy in my 11 yd test location. Which is saying something IMO if you can notice a change at that distance. I have not tested further yet. The velocity difference wasn't huge. But I never noticed a change with FX liners. This is the first CZ barrel I've polished.
I polish with a bore swab and firelap, first with JB bore paste and then with Autosol. Maybe wrong order? I was never able to find reliable grits of either.
I also have a Mullerworks 22lr from DI Precision, which I've bore scoped. Now you've got me interested to scope the CZ and see what it looks like.
In my case, I noticed an improvement in accuracy in my 11 yd test location. Which is saying something IMO if you can notice a change at that distance. I have not tested further yet. The velocity difference wasn't huge. But I never noticed a change with FX liners. This is the first CZ barrel I've polished.
I polish with a bore swab and firelap, first with JB bore paste and then with Autosol. Maybe wrong order? I was never able to find reliable grits of either.
I also have a Mullerworks 22lr from DI Precision, which I've bore scoped. Now you've got me interested to scope the CZ and see what it looks like.
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Here's the polished Krait CZ, .25 cal. Velocity drop was maybe ~20 fps, though I don't remember the original chrono conditions. And I wonder if I'll get a bit back after it leads up.
Here's an unpolished Uragan 2 CZ, .30 cal. I will be polishing this one as well soon.
22lr coming shortly.
Here's an unpolished Uragan 2 CZ, .30 cal. I will be polishing this one as well soon.
22lr coming shortly.
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