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The Over-Rated Crown

 
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  #1  
Old 01-21-2013, 08:57 PM
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The Over-Rated Crown

Quote:
You probably think having a perfect crown is important to accuracy. I think so too although this article will make you wonder.
Background: I was told that one of the major chronograph manufacturers checked the velocities of the rifles of three officers so they could determine their come-ups for various ranges. The rifles were described as being similar, as was the ammo. What was interesting was that the bullet's Ballistic Coefficient (BC) when fired from Rifle 1 was different than the bullet's BC from Rifle 2 and from Rifle 3. In fact, each rifle could be distinguished by the BC it produced. (As an aside - Was the rifle with the highest BC also the most accurate?) So the question is - What caused the difference? It wasn't the ammo, since the same ammo was used in all three rifles, and it wasn't the shooter. It must have been the rifle, but what part of the rifle? My first thought was that maybe the crowns were a little different, perhaps by cleaning rod wear or a less than perfect crown to begin with. It made sense to me that a perfect crown would allow the bullet to start its flight with very little yaw, resulting in the highest BC possible for that particular bullet. On the other hand, a crown that is off center or has a ding in it would allow gas to exit non-uniformly and thus kick the base of the bullet a little to one side as it left the muzzle, resulting in a wobbling bullet with greater drag and a lower BC.
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This is a thread for discussion of the article, The Over-Rated Crown, By Alan Marshall. Here you can ask questions or make comments about the article.
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  #2  
Old 01-23-2013, 07:34 PM
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Re: The Over-Rated Crown

Alan,
A facinating article to say the least!
Many of us have had experiences about barrel crown and the like. We all see and do what we believe to be correct? at the time any way! This is an amazing 'trial' to get some factual answers. I would have never believed what your were going to write.
Thanks for sharing
L-46
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Old 01-23-2013, 09:01 PM
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Re: The Over-Rated Crown

I have a confession. When I slugged the barrel of my 1 MOA .444 Marlin I used a wooden dowel to drive the slug notnot. When it broke, the dowel was stuck good and I decided to insert a screw in the broken section so I could pull out the dowel. In drilling the hole for the screw I damaged the barrel across a grove about 0.25" below the crown. I kicked myself around the house for a week until I went to the range and found I still had a 1 MOA rifle.

I feel better now. Thank you.
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Old 01-23-2013, 09:01 PM
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Join Date: Aug 2010
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Re: The Over-Rated Crown

I have a confession. When I slugged the barrel of my 1 MOA .444 Marlin I used a wooden dowel to drive the slug not. When it broke, the dowel was stuck good and I decided to insert a screw in the broken section so I could pull out the dowel. In drilling the hole for the screw I damaged the barrel across a grove about 0.25" below the crown. I kicked myself around the house for a week until I went to the range and found I still had a 1 MOA rifle.

I feel better now. Thank you.
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  #5  
Old 01-24-2013, 09:06 PM
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Re: The Over-Rated Crown

Great article! Thanks!
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Old 01-27-2013, 07:28 PM
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Re: The Over-Rated Crown

Wow is the only word that comes to mind here. I do not doubt your evidence. My only thing is the amount of barrels I have recrowned that shot better after. It would be interesting to see what the test results would be like in a more run of the mill gun. It seems the ones that have been helped the most in my limited experience are those that are of the sporter non custom category. I'm with you in I now have more questions than answers. This is certainly interesting. I cannot believe the results from the carbide routing job! Just wow. Excellent article.
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Old 01-28-2013, 03:41 AM
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Re: The Over-Rated Crown

Alan,
I have seen, as others have, varing results with recrowning. I makes me wonder whether calibre, velocity, bullet jackets, BC, and other factors have an effect on the results.
My story is a little different, maybe to the other end of the scale.
I was given a .204 some months back, it was new, from a reputable manufacturer, out of the box, and did not shoot a 10 inch group!
Long story short, my friend, the dealer asked me to take the rifle and see if could be fixed, I carried out a few basic tests to see if there was a simple solution.
Upon visual inspection, all i could see was a small indent on the crown. I was sceptical that this was the trouble. Hey 10 inches at 100yds!!
I took a tight patch and felt the bore for any variations, none where found.
The rifle had fired an unknown number of shots, during the run in, by the owner, and it was returned to the importer for a warrantee claim, so i gave it the sweets and JB paste to be certain the bore was clean. it came up pretty good.
I took the bore scope and had a good look inside, nothing was out of place for a factory rifle.
I went to the range and fired 5 shots, I was also given the test target, the customers target was a good reference as the new target was similar, 10 inches! wow!
now on further inspection i concluded that the group had in fact blown out, in favour of the damage to the crown ie indexed at 10 o'clock if you like in line with the damage.
Not certain what to do next, and knowing my friend had just bought a cheap action, i proceeded to see if i could change the 10 inches down some what, to something less than an inch, on the target.
Those of you who shoot 204's will know they are very capable, with less than half inch groups being obtainable out of the box.
Not having the correct tools to recrown the barrel and not really troubled by the result i took the burr off the damaged part of the crown with a screw driver to see if the resultant target showed any improvement. To my surprise the consequent group was around 8 inches, not good but different.
I knew if i was successful that a competant smith could take 1/8 of an inch off and reshape the crown so i was underterred. i took a drill and removed some more material trying to get below the damage part or the bore.
I did this in stages as to see if i was getting any where. Another 5 shot group showed an improvement down to 6 inches, than 4 inches. At 4 inches some where landing in the bull, while others scattered out in a radius, I had achieved my goal. I stopped there as to not cause further damage to the barrel and it was time to let a machinist complete the work in a professional maner.
The barrel was soon recrowned and now shoots as it should. I know we are not comparing apples here, but i was very taken by your findings.
At any stage you are able to conduct more tests, it would be great to add some other calibres? To see whats really in a crown!
Good Hunting!
L-46
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