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 Post subject: Lapping a MilSurp Crown
PostPosted: Fri Mar 02, 2012 6:07 pm 
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Mil-Surp Owner
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I have been doing this for years and years with great results in the accuracy department. I learned it from my Dad in the 60's so I surely did not come up with this by any means and I am sure many here do it also. With that out of the way....

Our MilSurps come with barrel crowns that often are fairly worn, scratched etc. This will/does affect accuracy more than many folks think. You can always buy the tools needed for a recrowning project. But more times than not all the crown needs is a easy to do lapping job.
We would be just polishing out the scratches, burrs etc. from the crown. These scratches/burrs will/do push the gases/bullet from one side to the other versus an equal/smooth release from the barrel. I have many instances of going from a 3" @100 yard gun to a 1" @100 yard gun with just doing nothing but this.

I just finished a Chinese T53 that went from a "so-so" accuracy gun to a nice accurate gun and thought I would photograph and post up the process. I suck at pictures so please forgive the quality but hopefully some of you interested in this will get the idea.

Be prepared to spend about $5-6 dollars.

You need some valve lapping compound. I just get it at NAPA. Any auto store should have it though.
Image

Couple of 1/4" brass screws. Home Depot has them. The heads on the quarter inch brass are perfect for 7MM/8MM/7.62/30 cal etc.
Image

Put your brass screw in a drill or cordless screwdriver. Your not going to be removing metal so you don't need much power here. Your just polishing. Get you a "glob" of lapping compound on there.
Image

Begin lapping your crown by lightly seating the screw in the barrel crown. You are going to use a slow speed on your drill. Go in a circular motion with your drill. This insures a uniform job. Just keep it firm but you don't push "hard" by any means.
Image
I then light sand the crown face with 600 grit sandpaper.
Image
And finish off the job with spinning some scotchbrite around on the end of the brass screw.
Image

You end up with a smooth, bright, uniform crown.
Image

Last thing I always do is drag a clean Q-Tip from the barrel outward. Making sure not a single snag is there.
Image

There I just spent less than 30 minutes and about $5 bucks. All the scratches etc. are gone and I have a perfectly uniform polished barrel crown. The difference in accuracy is truly remarkable.

You can reblue it very easily and your done.

I hope this may help somebody as much as it has helped me in improving the accuracy and enjoying my old MilSurps more.


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PostPosted: Fri Mar 02, 2012 6:15 pm 
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I just made this a Sticky. Well done and informative. Good job TheSarge.

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PostPosted: Fri Mar 02, 2012 6:16 pm 
interesting...question: you show the drill at an angle to the bore...is that correct? Or would you want it straight inline with the bore? Im guessing you warble it around (my technical term) at the angle you show?


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PostPosted: Fri Mar 02, 2012 6:19 pm 
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very nice! i did something similar by hand with an old hungarian M44

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PostPosted: Fri Mar 02, 2012 6:29 pm 
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Great post - very informative - thanks for sharing. :clap:


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PostPosted: Fri Mar 02, 2012 6:43 pm 
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VinnAY....You rotate the drill in a clockwise motion. This insures a uniform polishing of the crown.
Thank you and I am glad some may find this helpful. It still to this day amazes me the accuracy enhancement from this simple operation.


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PostPosted: Fri Mar 02, 2012 6:59 pm 
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As you stated in your post the crown is very important. The reason is that the very high gas pressures pushing the bullet down the bore, want to leave by the easiest means. If there is a scratch or gouge at the crown the gasses will vent there first. This jet of gas will push the bottom of the bullet away from the jet. " This will cause a "blowing" the base of the bullet to one side.This imparts a very bad wobble of the bullet in flight.
As you should know, you cannot do anything to the the base of the bullet. A dent,deep scratch,malformation or gas jet will cause the bullet to be unstable in flight. You will have very poor or no accuracy.
A good crown will show a star pattern as the gases pass the rifling and leave a powder fouling on the muzzle.

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PostPosted: Sat Mar 03, 2012 9:53 am 
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Excellent explanation and tutorial. Very well presented.

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PostPosted: Tue May 22, 2012 1:27 pm 
I actually tried this myself last night and hope I did it well, I'll have time to test fire this weekend! :thumb:


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PostPosted: Tue May 22, 2012 2:13 pm 
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I posted this same "Gun Kinks" technique on other sites and was ridiculed for the effort. I'd just purchased a like new Buck Mark rifle that had a spread like a shotgun because of a flattened land at the crown.

I actually did this in the shop, then did a little more grinding at the range. The top targets were where I originally started. Things really tightened up with a few cranks on my hand drill.

I figured if it didn't work, I could always get it re-crowned.

Here are the before and after pics.


Attachments:
File comment: Didn't cost me anything except about 10 minutes of crank time.
DSCN3925a.jpg
DSCN3925a.jpg [ 867.18 KiB | Viewed 15746 times ]

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PostPosted: Tue May 22, 2012 3:15 pm 
That's one helluva an improvement...WOW :o


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PostPosted: Tue May 22, 2012 3:22 pm 
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I thought there was a write-up using a carriage bolt head in the same fashion

This would be a better idea.
I guess because the brass is softer than the steel, there is no way the screw can do the work, or cause damage.


Then there was using a Lee case mouth cutter, with a gauge with shims around it, to CUT a flat crown

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PostPosted: Tue May 22, 2012 3:28 pm 
I was really looking at my work that it was even and not lopsided, that's my only nagging thought, that I got it even. I looked for a carriage bolt (Brass) but they were all huge, way too big for this work.


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PostPosted: Tue May 22, 2012 3:36 pm 
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Great Post!

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PostPosted: Sat Sep 01, 2012 4:51 pm 
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I just found this thread, thanks for the tip.

DogZebra wrote:
I posted this same "Gun Kinks" technique on other sites and was ridiculed for the effort.


Hm... It's good enough for Larry Potterfield:

He does it in this video, at 1:21
http://www.youtube.com/watch?v=OorpZlG28fI&t=1m21s
Quote:
Gunsmithing - How to Re-Crown the Muzzle of a Winchester Model 60 By Larry Potterfield of MidwayUSA

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