Anyone using an 11 Degree Crown Chamfer on a PCP Barrel?

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mtnGhost

mtnGhost

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Dec 15, 2018
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Defected from the PNW, United States
I need to re-crown the factory work that was done to the choked 7.62mm LW barrel in my EDgun R5. The accuracy has been decent for the most part, but I strongly suspect that the uneven chamfer seen on the lands under a 10x loupe (and possibly some of the chatter marks that were present) are playing a part in it not being a great barrel. 

I'm very happy with the polish work that I did to the bore. I also took a tiny bit of material out of the breech with a throating reamer, and that helped improve accuracy with the slugs I shoot out with it. There were also a few tiny chips in the factory chamfer that I was able to clean up around 90% with diamond paste and a rounded brass jib. That seemed to help reduce flyers tremendously, but I was still looking at at least two uneven lands that I didn't want to try and fix with compound back then.

Now, I need to make an educated decision one what I should do for the re-crowing job. While I could spend a day chucking it in my lathe, I have good tools for hand cutting both 11 degree and 45 degree crowns, as well as 90 degree facing tool. I want to try an 11 degree, partially due to the fact that the muzzle is threaded and it would be easier to convert the current crown/chamfer to (without touching the threads). 

I think an 11 degree would be fine for a PCP barrel like this - my big question though is how that would work with the baffled suppressor. It appears that the gases concentrate straight out of the bore with an 11 deg, and they seem to disperse a little wider with something like a 45 degree. Does that make any sense to anyone? I'd love to see if anyone else has experience in this realm before I get to work!
 
If the bevel is uneven, take a facing cut just deep enough to remove the bevel, then use the brass screw method to establish a new bevel. Even for barrels that have a uniform bevel, I usually need to dress the crown to remove the slight burr that inevitably remains from the rushed production work that favors quantity over quality at worst, or “good enough” quality at best. Even a very sharp cutter in a rigid setup (chatter-free) will not produce the same burr-free finish as a very mild abrasive. Certainly helps to reduce the amount of finish work necessary, but it’s still a good idea to wrap up with something like J-B or rottenstone on a round head brass screw.



To test for a burr, I always heard to drag a cotton swab over the crown and see if it snags. However I found it’s not nearly as discriminating as simply pushing a pellet through. The pellet should slip out of the muzzle with no more force than it takes to push it through the bore…unless it’s choked of course. In that case, I just go ahead and dress it to be certain.



See here for more thoughts on dealing with a poor crown:

https://www.gatewaytoairguns.org/GTA/index.php?topic=130555
 
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I think that I should clarify, although I have picked up the knowledge, skills and abilities to do so - I have no plans to chuck it in my lathe. I can do this with my PTG and Brownells hand tools.

I don't want anyone to waste their time, so my question is 11 degree crown on an PCP - anyone have luck / issues? What are your TJ thoughts on the gasses with a baffled moderator? (Like my EDgun R5)
 
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Okay, got it. Thanks for clarifying. As you probably know, there’s no consensus that a particular geometry is preferred from an accuracy/precision perspective. However I’ve never heard or read any advice that one helps disperse the gasses more favorably into baffles. The gases expanding into the open world just past the crown is very turbulent so I doubt it makes any difference between the common crown geometries, or at least not one that could be predicted. However it does seem intuitive that a deep cone would constrain the expanding gases in such a way that there is more opportunity for them to disrupt the trajectory.


 
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Okay, got it. Thanks for clarifying. As you probably know, there’s no consensus that a particular geometry is preferred from an accuracy/precision perspective. However I’ve never heard or read any advice that one helps disperse the gasses more favorably into baffles. The gases expanding into the open world just past the crown is very turbulent so I doubt it makes any difference between the common crown geometries, or at least not one that could be predicted. However it does seem intuitive that a deep cone would constrain the expanding gases in such a way that there is more opportunity for them to disrupt the trajectory.

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I can kinda visualize that. I wish there were some high-speed videos with different crowns/ chamfers firing to show the gas escape patterns. The best I could find is a guy on youtube with his 11 deg crowned barrel in a fishbowl with food coloring flowing out of the bore 😂 


If the crown is machined properly, makes no difference.
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In all actuality, you're probably right. If it winds up shooting like crap when I know the 11 degree crown is perfect, then I'll put a 45 degree chamfer on it and try again 🤷‍♂️


I re crowned a couple of my barrels. Used 11 degree on them. Seemed to work fine. The barrels were definitely buggered up before the re crowning.
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Nice! On a lathe or with hand tools?


 
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My thoughts is if you using a lathe just zero barrel id , then true end , then I would do an 11 degree crown and polish it using lathe and cloth later and it will be fine ,



OK now first the crown must be square to the bore or if it is uneven you get an uneven discharge and gas leakage causing spiriling or such , if it iss not square it causes wobbles , and if it has a flaw or ding can cause all kinds of issues , so you want the angle cut square to bore and it is fine ,



Now as for 0 degree , multi angle , 11 degree or 45 degree , does it matter ? yes and no , ,, yes you can tighten groups , I found slat slugs more like a o or 45 crown , where as boat tails more accurate with 11 degree , Now I have some crowns 0 degree which is ok as well but problem with a 0 degree is It is easier to ding as it is flush with bottom so If you want a 0 degree undercut it first so it has a taper then cut your zero ,



But my answer to you is do an 11 degree and I can send you a brownells crown cutter and a 30 caliber pilot with a o degree ,11 degree 45 and 90 degree ,

your lathe will also do it but you can crown with my tool in 5 minutes work out removing barrel , ps if you wanted to remove barrel and chuck it you can use cutter on lathe but with t handle it it cuts fast

LOU



1573692257_14170397885dcca361cb7174.58905360_p_080586909_1.jpg



 
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As to choosing the right degree, being that competition rimfire is only relatively slightly higher in FPS, maybe research what they are doing to crown their barrels for accuracy.

And keep in mind, along with the initial crowning of the barrel, what about where it is exiting the crown but then dumps into the moderator or shroud? I have a theory that a crown and possibly additional air/gas relief slits cut at a bevel where the moderator is attached to the barrel would help the turbulent air flow get away from the projectile as it first expands inside the moderator or shrouded barrel area as well.
 
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ha you have same kit , I did crown my 25 caliber edgun and it helped , My 30 caliber I did not as it show well and If something is good I leave it alone , I have crowned probably 50 barrels now and with lathe either building a new barrel or shortening , and getting other guns to shoot better ,

My thought is this , would I recrown to 11 if I had no problem NO , but I do use 11 degree on my pcp guns and all my hi Powered rifles , My 22 R5m I did recrown to 11 degree and My boat ail slug groups tighted up by 35% , the gun shot well before , but not as well with a boatail it shot better with MP slugs but I then re crowned it and damn . it is a lazer now with the boat tails I use , However my flat based slugs shot almost 5% worse , Maybe I test too much lol IDK ,

But I am only interested in how My slugs shoot , as with a pellet I never saw a crown make a difference , as long as it is true and no edges are sharp ,which is why I dislike using compound with a drill or a cone with valve compound , I wouldnt do this

Put some machinst blueing . make sure you brownell pilot is perfect and put the cutter in and turn it with no pressure u can see if it is straight before touching it ,

The barrels are usually very clean in edguns and while I did face my .22 long I spent alot of time getting it to spin .0002 when I went to square it , It was perfect under .0005 , not .005 like 1/2 of 1 thousandth , I did a hatsan last week it was off .008
 
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I've got the same barreling tool with multiple pilots. Plus the facing tool. 

I never lathe a face or crown due to the fact I can not index the center of the bore which on airguns from not match the OD. 

I have crowned both pcp and spring guns with great results. Plus as mentioned if you do not think the performance IS there the 45 can always be worked. 

I have faced on the lathe and after that machine dyed the face followed by the barrel tool. Both times using a fresh pilot that fit the bore as it should and yes the face was off vs the barrel tool. Does this really matter? Many say no. I still use and keep fresh pilots for my barrel tools. Used properly they will last a long time. Very little chatter marks if any. It's easy to remove any chatter marks if you know what your doing. JMO💀
 
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