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 Post subject: Re: Barrel crowning
PostPosted: Sun Aug 23, 2015 4:48 pm 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
The angle is not ideal but airguns don't care. In a firearm with high pressure and if looking for absolute best accuracy then I'd shoot for 11 degrees or whatever they claim is ideal. With an airgun the most important thing is that the crown is true so when the pellet skirt breaks contact with the barrel it's even all the way around. Picture an uneven crown; the pellet would tilt as one side was still in contact and the other has pressure pushing on it. The more pressure and lighter the pellet the more the issue. Many airguns have a crown cut off center, as if they used a lathe to cut the crown using the outside of the barrel as a guide, but the bore was not centered. I'm guessing that's what happened, but who knows what goes on in those factories. Things like the common brass screw method will make an offset crown smoother but it's still uneven. To fix it yourself at home you have two options I know of: One is cut and/or sand the barrel down to remove the old crown, and make the end as flat and true as possible, then cut a new crown by hand using whatever method. If you made the end true then the crown will be true. Well, usually it will but sometimes the bore is not parallel. It should be close enough tho.
The other way is similar to the brass screw method others mentioned, but instead you use a flat head screw. The screws shaft is wrapped in scotch tape to be a snug fit in the bore, which acts as a guide for the head. See attached pix. The trick is the screw needs to be fairly precision, most screws the head is not centered or even. I think the best odds of finding a good screw are brass, but you can also chuck a steel one into a drill and use a Dremel to make it more or less true. This works best if the barrel has been cut and/or sanded true as described above. Picture it as an alternative to cutting, or a bonus in addition to cutting.
The pix is an 8-32 wrapped for use in a 177 barrel. See the line in the tapered head? That's where I spun it lightly in a barrel I had crowned by hand to check if the barrel was true. Some magic marker on the shiny steel and the mark in the screw will tell you if everything is even. Brass will wear down fast so it's really best for touching up and/or verifying a crown. If using it to correct a crooked crown then it'll take several screws. Ideally I'd want a hardened steel screw but good luck finding one.
I don't know if airgun barrels are softer than firearm barrels, they both seem pretty soft to me.
How deep you cut the crown is up to you, but they don't need any cutting at all. The cut is really just to recess the delicate edge so it's less likely to be damaged.


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 Post subject: Re: Barrel crowning
PostPosted: Sun Aug 23, 2015 8:46 pm 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6845
Location: Rocky Mtn Hse Alberta
IMO very little tooling is required.

Single cut file to get it square
Polish that using file backed emery
With a strip of emery of 320 grit over the thumb twist and polish the bore, pad of thumb push into bore.
Use Quips to check for burrs.
Push pellets and check with a loupe for damage.

Typing this took more time than doing it.

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 Post subject: Re: Barrel crowning
PostPosted: Sun Aug 23, 2015 11:51 pm 
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Joined: Sat Jan 25, 2014 9:16 pm
Posts: 1271
Location: United States
Voltar1: Well, it sounds like he may have more serious issues but I'm not getting much feedback on the actual problem.
So what's that Voldar guy? And is there a Voltar and Voltar1? Because I don't remember the 1 before so now it appears to me there could be three of you or copycats? I dunno..


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