Polygonal rifling v land and groove rifling

Has anybody ever run a controlled experiment to lead up a Glock barrel to a known degree and see what happened?
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Yes, most definitely. The book pax referenced discusses the issue in detail.

The author of the chapter in question began researching the problem after blowing up one of his own Glocks. He had the means at his disposal to do so and I get the feeling that he initially felt he would be able to show that there was a problem with the pistol.

After determining what had caused the problem, he then tried to develop a reliable process/rule for using lead bullets that would eliminate the possibility of a catastrophic incident. Instead, the result was that he chose to shoot only plated or jacketed bullets in Glock factory barrels from then on.

His testing (involving shooting many thousands of rounds fired with instrumentation) revealed that the leading progressed at an alarmingly rapid rate in some Glocks while other apparently identical Glocks behaved much differently when using the same ammunition.
I have looked and looked at barrel drawings and cannot find a "passageway."

Since current practice is to make bullet diameter = groove diameter and SAAMI specifies the barrel cross sectional area including the grooves, heavy leading in the grooves does indeed "narrow the passageway."
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This issue is addressed as well. The Glock bores have a smaller cross-section than a typical land and groove barrel and all else being equal will tend to have discharge pressures 5% to 7% higher as a result. That's before any leading at all.
 
I will see if I can find the book on interlibrary loan so I can read about Mark's leading tests and save $40.

Is Glock USA a SAAMI member?
If Glock 9mms do not have a minimum bore/groove cross sectional area of .0967 square inches, they do not meet SAAMI specification. The standard is 6 groove but they give an area spec for freak rifling plans.
 
I got the book and read Mark's chapter on the liabilities of lead in Glock "polywobbly" barrels. I follow his reporting and logic.
 
Poly rifling is not supposed to shoot lead, but i've never heard a good why. It is aldo said to be more accurate and have slightly better velocities
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I'll bite.

This is personal experience/test using a Glock 26 with my sized cast bullets.

First 50-100 rounds were no big deal. Accuracy remained similar to with jacketed.

Things got interesting at about round 175 accuracy went to hell. Significant leading in the barrel to the point where I had to stop. By significant I mean somewhere between normal and turning a rifle barrel into a shotgun bore.

Unsafe? I'm no expert but id imagine an increase in pressure based on simple physics. Blow up the gun before you notice something's up? If you're a very poor shooter who wouldn't notice a group going from 3" to 9" its possible.

Either way its not ideal IMO.
 
That is pretty much what Mark described.

I think it is interesting that he did not ascribe leading to the shape of the Glock barrel bore, but its size. As we have said, the Glock barrel is undersize, reportedly to get a good grip on low quality jacketed bullets.
It would be interesting to run a similar experiment on an undersize conventional barrel. The aftermarket barrelmakers are cagy about actual dimensions, foisting us off with "match grade" etc.
It wouldn't be hard to shoot some .358" bullets in a .355" 9mm (if you have a .355" 9mm, oversize barrels are common), but strange things show up sooner in .40. I don't know where to get .404" bullets to shoot in a .4005" barrel. I'm not curious enough to set up to cast my own. Or to buy a .40 gun, for that matter.
 
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I think it is interesting that he did not ascribe leading to the shape of the Glock barrel bore, but its size.
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I think that's an oversimplification.

He definitely points out that the overall cross section is less and indicates that is a contributor, but he also attributes some of the leading potential to bore shape/geometry.

Here's a quote from page 66 of the book.

The problem lies with the barrel geometry. ​

The discussion continues onto page 67.

Put another way, instead of the rifling cutting grooves into an otherwise intact bullet, the polygon rifling mashes the entire bearing surface until corners form at the edges.

Because of this swaging and "mashing" effect, the dynamic forces on a bullet going down the bore of a Glock are significantly higher. ....SEM evaluation on test bullets...confirm that grain boundary separation and tearing occurs to a much larger degree.​
The shape and size both contribute--the swaging and "mashing", as he puts it.
 
Perhaps an oversimplification, but I know what he said and I know what he did.
It would take another series of trials to separate rifling plan from barrel size.
I have seen some desperately leaded conventional barrels, and I have seen blown cases and blown barrels with conventional rifling.
 
I have seen some desperately leaded conventional barrels, and I have seen blown cases and blown barrels with conventional rifling.
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Right, there's nothing that says only polygonal rifling leads or leads badly enough to cause catastrophic failures. Certainly conventional barrels can lead too. In fact, some Beretta pistol manuals warn against the potential for catastrophic incidents resulting from shooting a jacketed round through a heavily leaded bore. Of course, Beretta uses conventional rifling in all their handguns.
Perhaps an oversimplification, but I know what he said and I know what he did. It would take another series of trials to separate rifling plan from barrel size.
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The quoted statements quite clearly indicate that the author believes, based on his testing, that the rifling style/shape/geometry itself, not only the bore size, contributes to the potential for excessive leading. That creates a double-whammy, which may explain why other makers of polygonally rifled bores don't seem to be quite as concerned about leading as Glock is.

He may be wrong, but it's one thing to state that your opinion differs from the author's or to point out that, in your opinion, his testing was insufficient to isolate the specific issue; it's quite another to claim that the author says something that he clearly does not.

That aside, there's additional evidence that polygonal rifling is a factor in leading, independent of Glock rifling and their bore size. Gale McMillan is on record as stating that polygonal rifling had more potential for leading than conventional rifling based on his experience. The context of his quote was rifle barrels, not handguns which makes it fairly clear that it was not related to experience with Glock's particular brand of polygonal rifling nor was it specifically related to bore size.
 
Does anyone know what type of rifling Kart uses in it's aftermarket barrels? I have a Kart barrel on a 1911 and only shoot lead. It doesn't look like conventional land & groove rifling to me. Never had a problem with leading, just curious what they use.
 
Interesting thread... I have a .22lr pistol with a polygonal barrel, which is odd since .22lr is exclusively pure (or only anodized) lead... Guess the lead issues do not apply in this case due to .22lr's low pressure and velocity?
 
The worst leading I have had was my Colt Ace .22 after shooting Sam's Club special Remington Thunderbolt.

The worst leaded centerfire I have had was a .38 Special shot with some Harvey ProteXbore zinc based bullets I bought along with the revolver. I was able to use the rest of the bullets by coating them with Corbin dip lube. I think I know what was wrong with them, but I don't care enough to repeat the experience.

MY Kart .45 barrel has conventional 6 groove rifling.
 
After reading about various tests, I became a convert to the conclusion that shooting lead bullets in a Glock (or other polygonal rifling pistol) CAN result in excess leading and high pressures. But I still don't understand why the barrels lead that much. Supposedly it is due to the bullet skidding as it tries to grip the rifling and scraping off a large amount of lead (in spite of lubrication) as it tries to go down the barrel straight. But it seems to me that any bullet that left behind so much of its mass would be horribly inaccurate, and Glock fans claim that Glocks shoot lead bullets well as far as accuracy goes.

I think there is still something we don't know about the "lead bullets in a Glock" issue.

FWIW, polygonal rifling and hammer forging are separate issues; hammer forging is used by many companies for conventional rifling. First used, AFAIK, to produce M3 SMG barrels in WWII, it is now used throughout the industry though not always exclusively by a given company.

Jim
 
Should we go back and study what the people regularly shooting cast bullets in Glocks are doing RIGHT?

They can't all be hand casting bullets exquisitely sized to fit the throat and bore, with high quality lube and metallurgically optimized alloy.
In fact I know one long term Lead User who loads the cheapest bulk commercial cast bullets available.


FWIW, polygonal rifling and hammer forging are separate issues;
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Yup. Mr Metford didn't have a Krupp machine making his well rounded barrels. Which worked quite well until Cordite came along.
 
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Should we go back and study what the people regularly shooting cast bullets in Glocks are doing RIGHT?
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Based on the testing conducted and detailed in Passamaneck's writing, they may not be doing anything right. The answer might be that they're just getting lucky--or perhaps more accurately, they're just not getting unlucky enough.

Passamaneck's testing revealed that two apparently identical Glocks shooting identical ammo leaded at dramatically different rates. One gun showed twice the effects of leading after firing only a quarter as many rounds as another. In other words, one was building up lead at 8 times the rate of the other.

Without testing a large number of guns, it's not possible to know which gun was the anomaly, but it's probably reasonable to assume that the one that leaded very rapidly was not the norm. If so, that would mean that most Glock owners may not actually have a gun that leads dangerously fast even if it does tend to build up lead (and pressure) measurably faster than a typical land & groove bore.

Passamaneck also indicated that he fired well over 20,000 rounds of lead bullets through one Glock before he finally built up too much lead during one shooting session and blew the gun. There are many shooters who will never fire that many rounds in their lifetime, let alone through a single gun. Also, had he kept his shooting sessions a little shorter, who is to say that he may have never crossed the threshold and blown the gun?

The problem is that while the odds are probably against any given Glock owner having a gun that leads dangerously fast, without careful testing, they won't know that for sure. And while they may have shot a large number of rounds without a catastrophic incident in the past (as Passamaneck had), there's no way to know for sure if the next batch of bullets or a few more rounds in a shooting session, or a slightly different loading will be enough to blow the gun.

What I get out of Passamaneck's data and analysis, and from the anecdotal evidence provided, isn't that every Glock pistol is certain to lead dangerously fast or is certain to blow if shot with lead bullets. It's more that the bore shape and size creates a set of circumstances that are more conducive to lead buildup than the typical land & groove bore and also less "tolerant" of the buildup. So when you get the outlier bore that leads more than usual--something that happens with the normal variations seen in any kind of bore, the problem develops faster and has more potential for danger than would be normal with a typical land & groove bore.
 
Does anyone know what type of rifling Kart uses in it's aftermarket barrels? I have a Kart barrel on a 1911 and only shoot lead. It doesn't look like conventional land & groove rifling to me. Never had a problem with leading, just curious what they use.
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I always thought they were broach-cut but then I read someplace the process is a combination of broach and button rifling. In any case, you will go a long way before you find a better, more accurate barrel. The finish is so smooth, they stay consistent, don't lead, and last a long time. YMMV
 
IIRC, Metford rifling is not the same as polygonal rifling. Metford rifling has grooves, but they are rounded grooves, not square cut grooves; the uncut space between the grooves has the normal curve of the bore. Polygonal rifling consists of flat sides; think of the Pentagon building. The is shaped to the bore and rides the flats to follow the twist the same way a hex wrench works.

Jim
 
Glock rifling looks rounded in cross section to me.

I don't think Sir Joseph Whitworth's barrels leaded much and they were what my highschool geometry teacher would have recognized as polygonal.
 
I don't think Sir Joseph Whitworth's barrels leaded much and they were what my highschool geometry teacher would have recognized as polygonal.
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The Whitworth bolts were shaped to fit the bore and definitely not being swaged down to fit the rifling.

Also, from what I have read, the Whitworth bolts were made of harder lead than was typical for rifled lead projectiles of the time since the bolts were already a mechanical fit for the bore and didn't need to be forced down the bore against the significant friction generated by engraving the rifling into the bullet while loading it.
 
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