polygonal? land and groove? WHY?

[patriot93a3]

I need some wisdom from someone. Why does HK manufacture standard land and groove type rifling in the USP 9 and 40? My USP 45 has polygonal rifling, what's the deal?

[Big Bore]

Only the very early USP 9 and 40 had the standard rifling. Since late 94 all have the polygonal bores now. The .45 did not come out intil late 94 or early 95 IIRC.

[kassenz]

It is said the lands/grooves have a higher chance of tearing jacket lining. The Polygonal is cleaner, easier to clean, and offer higher velocities with less damage to the bullet jacket. Many people believe it is a more accurate rifling. It is used in the $10,000+ PSG1 sniper rifle.

It is believed to be a higher Quality technique, but harder to manufacture, and more expensive.

[jeray]

I have heard via several different sources including a gunsmith that conventional (lands and grooves) rifling is slightly more accurate than polygonal. HK and Glock use poly rifling to prolong service life and so than there is less carbon build up over time between cleanings, and to lessening the chance of a jam when being fired non-stop for a highly number of rounds (torture-tests). Both manufacturers specify that you are to use only jacketed rounds due to this type of rifling. Conventional rifling does not have this constraint. Both are good. Neither seems to be so benefitial that the industry chooses one whole-heartedly over the other. I think the PSG-1's accuracy comes mostly from the 25.6 inch length of the barrel, re-inforced receiver and total quality of build as opposed to the rifling.

[Scooter]

Polygonal barrels are more effecient as it provides a better seal allowing you to reduce the charge for the same velocity. That is one of the reasons why the Tubb2000 has switched to polygonal barrels.

I also think there's another reason why polygonal barrels are prefered. It's manufacturing costs. Polygonal barrels are all hammer forged. While conventional rifling can be hammer forged, broached, or buttoned. The dies, broach, or button can be easily examine to determine if it is still within specs. For example you lose the sharp corners on the rifling. But with a polygonal rifling there are no sharp corners, also harder to determine if it is within specs. So I bet you can a lot more barrels off of one die with the polygonal rifling.

[Unbreakable]

As said above polygonal bores tend to deliver more of your punch as they don't allow gas to escape through the grooves as the bullet rides the lands. I have noticed however that my polygonal bore guns (HK, Glock, Desert Eagle) tend to lose a little accuracy from 50 rounds to 100 and 100 out to 300... never shoot more than 300 rounds through my guns in a range session. Not a considerable change but the groups seem to widen as the bore gets dirtier and dirtier. At the very first I thought it might be the gun heating up and changing it's handling but even on a long day of slow shooting with little to no heat build up the polygonal bores seem to become marginally less accuarate the dirtier (ie the mores rounds fired) they get. I also considered that my hand might be getting fatigued and causing a loss in shooting ability but my standard bore pistols (Kimber, S&W, Colt, & Springfield) don't seem to have this problem. And many of them beat the hand up worse than a USP... So no fatigue concern. But then again most of my USPs after 300 rounds are more accurate than the other aobve mentioned guns freshly cleaned LOL. Never the less they still drop every round in a man sized target's chest at 30 yards no matter how many rounds you fire. I just hate seeing my groups get gradually wider and wider.

[Big Bore]

I don't want this to be taken as a flame and is not directed toward anyone, only I felt compelled to reply because IMHO there is just too much incorrect information being put out about polygonal VS conventional bores. I do not understand how people think that conventional land and groove rifling lets gas escape around the bullet. Throat erosion and gas cutting that occurs right in front of the chamber happens because of a combination of several things, not just the very tiny amount of gas that might be going around the bullet before it completely obturates, but from the burning of the powder and the tremendous heat and peak pressures that occurs in those first few inches the barrel. If gas were shooting past bullets all the way down the barrel, barrels would burn out in no time flat, accuracy would non-existent, and extreme spreads would be sky high. None of these events are seen in conventionally rifled bores any more so than they are seen in polygonal bores.
If you slug a conventional rifled bore you will find the groove diameter is the same as the bullet's diameter. And don't think for a minute that a jacketed bullet will not obturate to fill the bore if slightly undersized, say shooting a .308 bullet in a .309 bore. They will completely obturate and seal the bore the same as a soft lead slug will. We are talking about pressures here that will blow a barrel apart if obstructed; you don't think that it won't obturate something as soft as a bullet, hogwash. In a .45 ACP you shoot a jacketed bullet of .451 to .452 diameter. The groove diameter of the .45 ACP conventional rifled barrel is .451 but can vary as much as .001 inch in a crummy barrel. The land diameter is several thousands less than that. The bullet does NOT ride on top of the lands, the rifling cuts into the bullet. No gas escapes around the bullet any more in a conventionally rifled barrel than does a polygonal bore. Also, if polygonal bores are so much more accurate which I often hear stated, how come you do not see them showing up in all the BR guns and LR guns? You don't. People who shoot LRBR will pay any amount for a barrel if it will give them an edge, and polygonal bores do not. They are accurate, sure, but they are not more accurate than conventionally rifled bores. Sure, you can find an individual polygonal bored barrel that is more accurate than an individual conventionally rifled barrel, but that is purely anecdotal, meaning nothing to the overall picture. If polygonal boring were more accurate you would see Lothar-Walther, K&P, Lilja, McMillan/Wiseman, Shilen, Wilson...all the top barrel makers going to polygonal bores. They aren't. You don't get better barrels than what these makers produce, so while there MIGHT be advantages to polygonal bores on a combat rifle/pistol there are none when it comes to accuracy. That is just pure sales propaganda, period. Sure, one can find all the anecdotal "evidence" one wants to make a point such as my HK out shoots my Colt, but that proves nothing. Look at what the top barrel makers in the WORLD produce, and an "ain't" polygonal bores. And furthermore, if polygonal bores are just as accurate as conventionally rifled bores but offer less throat erosion, then do you think for one minute that if these top barrel makers could sell a barrel that is just as accurate but last longer without having to be set back and rechambered they wouldn’t? Of course they would because if it were true, a barrel maker that came out with a barrel that was just as accurate AND held its accuracy longer would put the others out of business in a heartbeat. In the world of LRBR shooting, what works is used, they will do or try anything to give them any edge at all, even if it means killing a chicken at midnight under a new moon. They hold few loyalties.

The other claim often made is the polygonal bores offer less resistance so they shoot the bullet faster. Again, hogwash. Take a bullet in a conventional rifled barrel. Shoot a particular load over a chrony and get a velocity. Now, take those same bullets and coat them with molly and shoot them over the same load in the same gun, what happens? Velocity goes down because of LESS resistance to the bore, and because less resistance means less pressure develops in the barrel. To get the pressure back up to where it was before, you add more powder. More powder means more gas expanding, and by the time you add enough powder to get the pressures to where they were before you have a larger volume of expanding gas and the end result is that the bullet is going faster with a molly coated bullet when loaded to the same pressure. That is why you cannot take a load worked up with molly coated bullets and substitute a non-coated bullet. The non-coated bullet will raise pressures sky high. So, if one sees higher velocities with a given load out of a polygonal bore versus a conventional bore, then it must be because HIGHER pressures are being developed in the polygonal bore, not less, so the polygonal bore must be creating MORE resistance to the bullet, NOT LESS.
I have shot loads though many .45s in polygonal and non polygonal bores, and the velocity difference between barrels of the same length are well within the standard deviation of individual barrels. One HK might shoot a hair faster than the conventional bored barrel, another HK shoots a little slower, same as with the normal variation found between any set of barrels.
I know HK and Glock make all kinds of claims about the superiority of polygonal versus conventional bores, but nothing I have seen proves any, and some of the claims made by supporters are against the laws of physics. You cannot decrease resistance which lowers pressures and increase the velocity of a bullet, it ain't gonna happen.
The only claim often made that could hold any water is that they last longer. IF the polygonal bore offers less resistance, then less pressures develop in the barrel, then of course that barrel is going to last longer, same as with shooting molly coated bullets in match barrels with conventional rifling.

IMO if there is any advantage of one method over the other it has to do with manufacturing costs. If anyone can make a barrel that is just as good as another barrel but make it cheaper, then that is what they are going to do. HK and Glock went the polygonal route and the barrels work. Why change. Same with all the others. They chose the other route, their barrels work, why change. I think what we see here are two methods to achieve the same end.
We’ve all heard raves about new barrels that come out from time to time, gain twist, tapered bores, RH vs. LH twist, more or less number of lands and grooves…and as yet has any one method proven to be light years ahead of the other? No, they have not. Marlin invested tons of money in their Micro-Groove barrels and they work. But yet they don’t work as well with all bullets so they have gone back to conventional rifled barrels for those who regularly shoot lead. And while with jacketed bullets the Micro-Groove works very well, it does not work any better, not when you get right down to the brass tacks of it. And don’t forget the gain twist and tapered bored barrels which are rare birds indeed. They work sure, but do not offer the hoped for results often claimed, but once invested, the makers continue to make them that way for as long as they can because as with any venture, there is a client base that will swear by this or that without any real evidence to support their prejudice (sort of like me with the .45 ACP). If any of them offered any real gain, then those barrels would dominate the shooting industry and the firing line, they do not.
In fact, the only barrels that do dominate the world of firearms IS the conventionally rifled barrel. Many other barrels are good, some very good, but other than the occasional anecdotal story, there is no solid, irrefutable evidence that I have seen that suggests any one method is any better than the other.
OK, now take all the above with a grain of salt. It is worth exactly what you are paying for it.

[Unbreakable]

Just to sign in on my above statement. I am no expert. I have read time and time again here and other places that the same loades from a Poly barrel chrono in at higher FPS than conventional bore. Is this true... I have no idea. I have accepted it as a possibility but like I said I am no expert when it comes to the science end of the firearms world. Big Bore seems to have more of an in depth view of this and as always I'd trust his view. I have no idea how fast or hard my bullets are coming out of each gun other than what the box claims. As for accuarcy... it's very true to say that My HKs are more accurate than my Colts is no substantial proof. I may just get along with the USP better. The only USP I have ever had trouble shooting accurately was the 45 Compact. I went through two of them and didn't like either one. But that can reasonably come down to shooter preference and ability. Losts of guys here rave about the Compact 45. I will, however, stand by my observation that my Poly bores seem to lose a hint of accuracy the dirtier they get. I have noticed no such deviations in my Conventional bored guns. To say I stand by it doesn't mean this holds true for every single poly bore out there. It may just be mine. It may once again be shooter error. But it is something that I have come to notice and can almost foresee when going out to a day of shooting. Today I took my 40 cal USP out to dial in my new adjustable sights. Took about 40 rounds to do to my liking. Then we spent the day shooting at everything from Stand Up steel targets to soda cans to cigarette packs. After about 280 rounds I was missing the cigarette packs more often than I was hitting them at about 50 paces and I just new it was because my gun was gooped up pretty good by then. And this is with a 3"X4" target... not the broad side of a barn. So like I said earlier we're still talking usable accuarcy. Poly vs. Conventional... I like Poly... My view is most definately lop sided as I will prefer what my HKs have as I shoot the best with them. I believe they do last longer... and other than that I choose my guns by name brands that I have experience with a trust. I shoot just as good with my Kimber BP Ten II as I do with my buddies $2,500.00 Wilson. That to me either means I am a good shot with my Kimber or a bad shot with a Wilson LOL either way I don't see where the other $2,000.00 comes in. And once again that means nothing. All that means is that my mind is pretty much made up that I shoot as good as I'm gonna within the $500.00 to $1,000.00 gun range and have no need for the super pistols. I trust Poly... I like poly, and that's what I suggest you go with. No science behind it or evidence to support it as I do not own the expensive testing equipment to proove one over the other. But that's my preference and I guess I'll have to live with it. Make your own choice.

[patriot93a3]

Thanks everyone for your input. I know I can always rely on intelligent information and insight from the members here at HKPRO. There is always someone willing to take a few minutes and add a reply. Thanks again

[tjloeb1]

Here is a link to a great source that describes rifling in great detail and shows the different types and how they are manufactured:

http://www.firearmsid.com/A_bulletIDrifling.htm

Tom