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PostPosted: Tue May 02, 2017 9:30 am 
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Location: Central West River Nova Scotia
I understand not all air gun rifling is created equal, but for those that may be like one of mine, I have a theory. I have a crosman 24" .22 cal that has very shalow rifling. With light pellets and low power, IE , under 700 fps. the accuracy is exceptional. However with heavier pellets, 18 to 24.5 gf, not so much. So my theory is with heavier pellets, the extra weight causes the pellet to resist spinning and skip over the rifling. Some spin is imparted, but not as much, and therefore not as accurate, or inconsistent accuracy. Some pellets might travel 6" to 10" or more before starting to spin. Like turning a very large wheel versus a small wheel. It takes more effort, or friction on the outer surface, to get it started.

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PostPosted: Tue May 02, 2017 11:16 am 
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Location: Coalmont BC
Easy way to find out.... fire into water, or a pail full of crumpled up grocery bags.... and recover the pellet.... It it has skidded through the rifling for any distance, there will not be any sharp, clear rifling marks on the pellet.... it will be smooth and undersize, like what you see with an FZ Smooth Twist barrel....

IMO, the skirt blows out to completely fill the rifling, getting an incredible "bite" the instant the air hits it.... If the rifling starts at the chamber, the pellet simply "spins up" as it accelerates.... Once it strips through the rifling, in an oversize barrel.... it will be much less likely to every reach full RPM, skidding progressively more as it accelerates down the bore....

Pellet inspection is the key to proving, or disproving, the theory of the pellet skidding....

Bob

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PostPosted: Tue May 02, 2017 12:17 pm 
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Location: Winnipeg, Canada
I think most Crosman .22 bbls are 12-16 groove r.h. I have yet to see this rifling engage the heads of any pellet I've tried~ including the 21 gr silver point by H&N. Rifling marks on the skirts of pellets shot through these can be hard to find, but are there. I don't think the current shallow rounded button grooving provides a lot of bite to begin with...
I haven't had accuracy problems with the 18.1 gr JSB heavy out of these bbls. I'm not driving them that fast though- my fastest crosman .22 is in the mid 700's range. Personally, I can't really find a logical reason to drive them any faster. Yes, it can be done. The grouse and rabbits I take with my air rifles can't really tell the difference between 7 and 900 fps(which is getting into the overdriven range for a pellet).

I have 2 Brass Crosman bbls here- one each of .177 and .22. Both are 6 groove r/h twist. Rifling is on the old ballard styled square cut groove and shoulder system. The .22 is exceptionally accurate at mid 5's to low 6's range- which is as far as I wish to push this oldster.
The .177 is just as accurate, if not more so at the mid 7's range. Pellet weight is not as important as a uniform rounded proper diameter skirt, with these brass bores. Again, rifling marks on the skirts can be hard to find, but are there.

Incidentally, I get the same slight rifling marks on skirts from the lone L.W. bbl. I have in .177.

-D.S.

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PostPosted: Tue May 02, 2017 1:35 pm 
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Location: P.G. B.C.
At the very low velocities of air rifles, I find it hard to believe the rifling does not grip the bullets perfectly, even if pure lead. I suspect your rifle simply does not like heavier pellets - or the rate of twist is not conducive to producing good accuracy with them.

my .17 Remington calibre rifle barrel has rifling that is .002" deep - yes - that is it - 2 thou, yet spins the bullets perfectly in it's 9" twist at speeds to 4,400fps and produces sub 3/4" groups at 100 meters.

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PostPosted: Tue May 02, 2017 3:43 pm 
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Joined: Mon Aug 24, 2015 4:56 pm
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Location: Tecumseh, Ontario in South Western Ontario
Daryl wrote:
At the very low velocities of air rifles, I find it hard to believe the rifling does not grip the bullets perfectly, even if pure lead. I suspect your rifle simply does not like heavier pellets - or the rate of twist is not conducive to producing good accuracy with them.

X2

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PostPosted: Tue May 02, 2017 3:54 pm 
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Location: Bradford
The only pellets I've seen with consistent rifling marks on the heads (other than some flat head types) are what I call the "factory deformed" ones....Crosman Destroyer and Crosman Piranha.

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PostPosted: Thu May 04, 2017 7:48 am 
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joe hickey wrote:
So my theory is with heavier pellets, the extra weight causes the pellet to resist spinning and skip over the rifling.


It would make sense, for powder burner, there are many charts/references as far as barrel twist vs bullet weight VS speed.
I don't see why it would be any different for airgun.


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PostPosted: Sat May 13, 2017 4:35 pm 
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Location: Halifax,Nova Scotia
https://airgunaccuracy.wordpress.com/po ... -velocity/

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