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PostPosted: Sun May 28, 2017 8:58 pm 
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Posts: 8989
Location: Coalmont BC
At my request, NOE have run sizing dies 0.001" smaller than previously available for airgun barrels.... You can now get bushings in 0.215", 0.248", and 0.298" in addition to the larger sizes previously available.... The 0.248" are listed in the .243 cal section, the others in the .22 and .30 cal airgun sections....

http://noebulletmolds.com/NV/index.php? ... lb8g1efpi6

I also asked Al to make sizing bushings in 0.241", 0.242", 0.336", 0.337", 0.354", 0.400", 0.450", 0.451" & 0.457".... some airgunners are finding that they are better off for accuracy (and velocity) sizing just under the groove diameter, rather than just over.... NOE listened.... 8)

Bob

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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
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PostPosted: Tue May 30, 2017 7:22 am 
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rsterne wrote:
some airgunners are finding that they are better off for accuracy (and velocity) sizing just under the groove diameter, rather than just over.... NOE listened.... 8)

Bob


This is the kinda stuff that keeps me reading. Watch for the above to become a basic rule of thumb for a/g bullets. Have heard of horror stories from south of the border involving over-bore bullets in a/g's....

-D.S.

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PostPosted: Tue May 30, 2017 5:37 pm 
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Some guys are using about 0.001" over the groove diameter, some groove size, and others about 0.001" under.... I guess it's like anything else in airguns, you can't tell your barrel what to like, you have to listen to it.... Some also state that as little as 0.0002" change in diameter makes a big difference.... I'm too much of a novice in the cast bullet field to really know.... :roll:

I have done some velocity testing, however, and you can get up to a 10 fps difference with a 0.001" change in diameter within those ranges (20 fps between a thou over and under).... I also know from experience that if the bullet is too large, it can stick in the chamber and resist being fired at all.... requiring driving out from the muzzle end.... It doesn't take much, a bullet that is 0.227" can jam in a 0.224" barrel at 3000 psi.... If you can't close the bolt without hammering on it with your fist.... use a smaller bullet....

I have no idea regarding the opposite case of too small a bullet losing velocity from blow-by, but I'm guessing that isn't an issue as long as it's big enough to engage the lands.... Accuracy may be another matter entirely, though....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Wed May 31, 2017 11:45 pm 
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Joined: Wed Jan 05, 2011 12:35 pm
Posts: 6082
Location: P.G. B.C.
rsterne wrote:
Some guys are using about 0.001" over the groove diameter, some groove size, and others about 0.001" under.... I guess it's like anything else in airguns, you can't tell your barrel what to like, you have to listen to it.... Some also state that as little as 0.0002" change in diameter makes a big difference....
Bob



Not in CF, it doesn't. A thou and make or break you, though. That being groove diameter less .001" can amount to a serious problem, especially with too hard a bullet to oburate & fill the grooves.

Most CF rifles like .002 to .003" OVER groove diameter., just saying and probably has nothing to do with air rifles.

The lower the breech pressure, the worse the problem becomes. This should be all applicable to air rifles as well, to some point. Testing various options is the only method I know of to obtain real answers.

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