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Discussion in 'Gunsmithing' started by whitetail predator, Feb 10, 2011.
what is the actual purpose and benefit of crowning a barrel
The crown is the last influence you or your rifle have on the bullet, how that bullet leaves the barrel will make or break your accuracy. You want the bullet to leave the barrel cleanly and evenly, if you have a bur or have a crown not perfect to the bore this will not happen. Not an expert but I'm finding that when I re crown them it makes a big difference! Surely someone else will give you a better answer but that's my two pennies
It aids in the bullet and gasses leaving the barrel cleanly and uniformly.
The bullet is under complete control of the barrel until it exits and any unevenness in
the gas escaping can push the bullet off course.
When a proper crown is used It is sharp and square to the bore. This is the reason it is best
not to clean from the muzzle end of the barrel if possible because you can damage the crown
and hurt the accuracy.
J E CUSTOM
This is something that's interested me for a while. I think most gunsmiths recommend an 11 degree target crown. Is 11 better that 10 or 12 degrees? Somebody must have done tons of research to determine that was the best angle for bullet and gas release. Yet the factories don't do this. Why? Must be cheaper to just put that bubble thing on there. Then when gunsmiths put a brake on the muzzle, gas goes every-which-way. So most agree that crowning is not critical and just make a 90 degree cut. Strange. Personally I think that as long as the crown is perfectly perpendicular to the bore, and free of burrs or damage, the shape of the crown really doesn't make much difference. But I've been wrong about less important stuff before.
There is nothing magic about 11 degrees. Even and square to the bore interior is most important.
I don't know if that's a true statement.
(1) You still need a decent crown even if it's not recessed, although I prefer recessed even though you're installing a brake. Simply making a 90 degree cut would leave burrs.
(2) Gas doesn't "go everywhere" until the bullet is free of the barrel. The brake should be about .020" over sized. If the bullet touches the brake, you're screwed.
Well, you're right. I didn't really mean they just saw the barrel off and leave burrs. I guess my point is as long as the barrel is cut true to the bore, I'm not convinced the shape makes a lot of difference. I'm sure it's recessed somewhat to help prevent damage.
As far as a brake goes, the bullet is free of the bore before gas escapes, and the same goes for a muzzle with no brake. With a brake, the gas pressure exiting behind the bullet has diminished as some pressure has gone out the ports. But there is still some pressure on the base of the bullet. So everything needs to be square to keep the gas from upsetting the bullet. However, when there is no brake, it is especially necessary that everything be true and square because the full gas pressure is still bearing on the base of the bullet. I'm convinced that's why some short range benchresters really prefer flat based bullets instead of boat tails. At distances of less than 300 yds, The square base is more important than a higher ballistic coefficient. I'll climb down off the soap box now.
No problem gr8whyt.
Recessed or not, I agree that square is the main thing.
Someplace over 30 years ago, I recall reading that the military did exhaustive testing which showed that 11 degrees was the optimum crown angle for best accuracy. Not by a huge margin, but still significant enough to be the recommended angle of choice.
However, I also recall that this assumed a perfectly perpendicular cut.
The theory behind the 11degree crown was based on high speed photography testing done by
It appeared that the gas escaped at close to 11o from a squared of crown and that more than
11 degrees of crown caused the gas to contact the surface of the crown and possibly having an
effect on the bullet.
So they theorized that any thing from 0 degrees to 11 degrees had no contact with the surface
of the crown preventing any effect of the shock wave or gas on the bullet.
After some firing some crowns have Smoke trails that match the rifling from contact of the
escaping gas and 0 to 11 degree crowns have little or none.
I am not sure that the High pressure rounds perform the same as the test done by the military
but a good clean exit of the gasses does make a difference.
The gas and unburnt powder can and will erode the crown over time and it needs to be re cut
when this happens.
The 11 degree crown has a thicker edge than a square crown (90 Degrees compared to 101
degrees of the 11o crown)from the bore and should last a little longer. Does it? I cant prove
it, so the choice is from 0 to 11 degrees.
This is all theory like lots of other things in ballistics, and the reason that there is so many opinions
I like the looks and it seems to work good so I use 11 degree crowns.
But I do think that any well cut crown 0 to 45 degrees will work if it is true and clean to the bore.
If you look at some of the factory crowns they are not concentric to the bore and are there to
aid in cleaning from the muzzle without tearing the patches.
Note: never clean a barrel from the muzzle end if possible.
J E CUSTOM
JE Custom - Your reply prompted a memory flash. Don't have those very often anymore...... so I cherish them when they do!
My recollection suggests that you are correct. Thats how this whole 11 degree thing got started.
But I also remember now that a group of benchrest shooters reviewed the military data and then wore out a few barrels testing the angles further to see how the military photo results affected accuracy. They found that an 11 degree crown also seemed to produce the most consistent accuracy.
I don't remember where I saw this. It may have been in the Rifle Magazine, in Precision Shooting Magazine, or in Handloader Magazine. I suppose it could have been in one of the many books I had too - "The Accurate Rifle" comes first to mind. It sure as heck wasn't the internet. Contrary to popular belief, the internet did exist back then but a multitude of BBSs and Compuserve was about as good as it got - and they were pretty poor.
I threw out all my old magazines when my self appointed management threatened to divorce me. So I tried to find something on the net to corroborate my memory. I could only find the short description in the following link:
Barrel crowning (Bart Bobbitt)
Accuracy back then wasn't what it is now - although still damn good. So maybe recent improvements have shown this to be a flawed conclusion. In any event, I think that even if the precise angle does make a difference, it isn't likely important for anything but the finest competition rifle and probably couldn't be detected on the finest long range hunting rifle.
That said, I know of no evidence to suggest that any other angle is better than the 11 degree crown, so as they say "In the absence of any compelling information to the contrary", I'll stick with what I currently know won't hurt - 11 degrees.
BTW, I found this forum because I was looking for info on how the OEMs make their crowns. I recently purchased a Browning X-Bolt White Gold. Its never been fired. But it appears to have some crown damage in it that is hard to explain. Other damage I have seen is easy to explain by poor tooling and the like. But this is very different. The metal actually appears to have been melted at the crown edge and there are little tiny beads of metal welded to the edge.
Here is a photo taken with a digital camera in Macro mode using a bore light. The loose stuff in the barrel is just dust.
Does anyone know how this happened? My best theory is a lathe centering tool that came loose and spun in the barrel. But its hard to imagine how that could have happened at the factory. I'm contemplating either re-crowning the rifle or re-barrelling it.
Great photo Susquatch!
And, great info JE/Susquatch.
My recollection also was something like Susquatch reports. i.e. in the absence of anything better, stick with what I know.
But, I do like the point JE makes about the potential wear of a sharp shoulder vs thicker shoulder of the 11 degreee crown.
I wonder how many different bullet designs were tested in the referenced works? I can imagine that if it makes any difference at all, then the tail of the bullet would also be a factor. e.g. flat base vs boat tail
I have also heard bullet manufacturers comment that making a perfect boat tail with perfect concentricity is much more difficult than making a flat base. As such, it's equally as important as the crown itself for ultimate precision.
Not that I am any expert..... but.....
My personal experience reflects your comment. Flat base bullets provide slightly smaller groups at 100, and boat tails are better at 300. They are about the same at 200. The boat-tail advantage at 300 is probably mostly ballistics related - just my WAG. More retained velocity means less influence from conditions. This also reflects what you see being used in the 1000 yrd events - all boat tails.
This of course is in a benchrest rifle that routinely puts 5 into 1/8" @ 100 in perfect conditions - not a hunting rifle that an old man like me can actually carry without a forklift. I doubt you could ever see the difference in a hunting rifle.
I know one thing, after getting set up to do my own lathe work, the factory crown is the first thing to go!!! I haven't done many yet but everyone showed a large improvement, I've been cutting an 11 degree just cause it seems to be the universally used crown for accuracy. I wish I had a Hawkeye bore scope to just see what is going on!