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PostPosted: Sun Jan 04, 2015 6:29 pm 
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Location: Hanover, On
I'm interested in getting a sub-30fpe airgun to shoot the .177 caliber, 15.74 gr H&N Rabbit Magnum IIs. If someone could share the mathematics for figuring out the correct rate of twist to stabilize these pellets, that would be really great. Also, is there a way to mathematically determine kinetic energy losses from the increased rate of twist ? There is a youtube video of a Russian man measuring the diameter of the 3-bands of these pellets. He's using a dial caliper to do this, but he consistently is measuring 4,64, 4,65 and 4.66 mm. So given these pellet's measurements what kind of land and groove measurements would be ideal for the rifling, to best stabilze these non-waisted pellets ? If the frictional losses from the increased rate of twist cancel out the benefits of these pellet's high ballistic coefficient then I will forsake the idea of using these. But if the losses are negligible then I believe that, for my own purposes, the effort and expense to get my rifle able to shoot these, accurately, is well worth it.


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PostPosted: Sun Jan 04, 2015 7:38 pm 
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Interesting.
Not that far off the .17 HMR for projectile length...
I don't know if the bore diameters are the same.
Lotsa food for though there.
Will be watching this...

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Doc Sharptail

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PostPosted: Sun Jan 04, 2015 8:00 pm 
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I've never measured them, but I could never get them to shoot in any of the .177 airguns I tried them in.... A 6" scatter (I won't call it a group) at 20 -25 yards was typical.... If they are that large, that could be the problem, as a .177 barrel is typically about 4.52-4.54mm.... They aren't that long, so I don't think twist is the problem.... but I have some here and will measure the length and do a twist calculation for you....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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PostPosted: Sun Jan 04, 2015 8:54 pm 
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Thank you for your replies! Yes, I haven't found a single person that is getting these to group tightly, as everyone seems to be using the stock barrels. I believe that most barrels come with 1:18" twist, with a few exceptions. The centerfire .17 Remington barrels are generally 1:9" twist, but that bullet is 25 grains. If your measurements agree with the Russian's measurements, then yes we'll need a slightly larger bore, as well.


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PostPosted: Sun Jan 04, 2015 9:19 pm 
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I'll look up the bore specifications for the .17 HMR. That just might be the barrel that we need to get these .177 Rabbit Magnum IIs to stabilize. That would really be nice, and relatively inexpensive, if that would work.


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PostPosted: Sun Jan 04, 2015 9:30 pm 
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Location: Lindsay Ont Area
most 17 cal barrels I'm seeing are like 4.36mm groove diameter... I don't think a 17hmr Barrel is gonna work...

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PostPosted: Sun Jan 04, 2015 9:42 pm 
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The .177 cal. Rabbit Magnum IIs that I have, which are a few years old, measure 0.177-0.1775" at the front band and 0.1775-0.178" at the middle band.... and they measure 0.183-0.184" at the rear band (skirt).... Those are very normal measurements for a .177 pellet.... They are 0.320" long, and when I run that through the Kolbe Twist Calculator it recommends an 18" twist at 500 fps, 16" at 950 fps, and 14" at Mach 1 to get a Stability Factor of 1.5.... Inputting a 16" twist, it gives Stability Factors of 1.9 at 500 fps, 1.7 at 800 fps, 1.5 at 950 fps, and 1.4 at 1050 fps, all of which are just about perfect, IMO.... The SF drops to 1.2 at 1100 fps and 1.15 at Mach 1, which is pretty marginal, but if you keep it under 1050 fps with a 16" twist there should be no problem.... 16" should be the optimum twist....

0.177" is 4.50 mm, 0.178" is 4.52 mm, and 0.184" is 4.67 mm.... It is quite normal for .177 pellets to have heads from 0.173-0.178" and skirts from 0.182-0.188".... Here is a (partial) list.... http://www.airgunhome.com/pages/pelletspecs.html

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: Sun Jan 04, 2015 10:53 pm 
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Google-Fu didn't yield much in the way of concrete fact.

Groove diameters on the p.b. 17's run the gamut from 0.172" to 0.174".
From what I'm seeing, it would be doable if a resizing die could be adapted to pellet use...
It ain't that far to size down.
Twist rate for the r.f. .17's seems to be optimized at 1-9".

Regards,

Doc Sharptail

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PostPosted: Sun Jan 04, 2015 11:15 pm 
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.17 cal PB bullets are wayyyyyy longer, hence the 9" twist rate.... That is way too fast for the Rabbit Magnum, IMO.... Spin a bullet too fast and every tiny imperfection is magnified.... The SF at 900 fps with a 9" twist is 5.0, and anything over 3.5-4.0 can be problematic....

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: Sun Jan 04, 2015 11:56 pm 
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Location: United States
Don't need to do the math, I can tell by looking at it. The twist of a generic airgun will be fine, but those pellets suck so I wouldn't bother. The Silver Points aren't very good either. The best heavy from H&N is the Barracuda Match, but they now also make a longer/heavier Barracuda Magnum, which I haven't tried, but it sounds like a great idea. Looking at the pix, however, it appears it might be pushing the limits of the rifling. Normally I would assume it'll be fine because why would H&N release it if it's unstable, but they do have other dud pellets so who knows... The JSB 13.43gr are excellent, so if you haven't tried them I'd check 'em out.
Also, I didn't see a mention of the type of airgun, just <30ftlbs. Springers lose power with heavies, and the heavier you go the more they lose. PCP however are the opposite. Just fyi, because some people think they can buy a generic springer like a Crosman or Gamo rated at 1200fps and put Rabbit Mags into it thinking they'll get that velocity, but in reality they get closer to 600.


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PostPosted: Mon Jan 05, 2015 12:10 am 
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Chevota wrote:
The JSB 13.43gr are excellent, so if you haven't tried them I'd check 'em out.


Those are a new one on me. I've tried the JSB 10.3 and they don't stand up to the crosman premier ultra mags in the crosman bbls I've tried them in.

Which Chevota is it? Dakota 350 4 bbl? :wink: Or a chevette with a 2 litre 'yote under the hood?

Regards,

Doc Sharptail

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PostPosted: Mon Jan 05, 2015 11:31 am 
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Doc Sharptail wrote:
Chevota wrote:
The JSB 13.43gr are excellent, so if you haven't tried them I'd check 'em out.


Those are a new one on me. I've tried the JSB 10.3 and they don't stand up to the crosman premier ultra mags in the crosman bbls I've tried them in.

Which Chevota is it? Dakota 350 4 bbl?How about a 360, aka 5.9L... :lol: :wink: Or a chevette with a 2 litre 'yote under the hood?

Regards,

Doc Sharptail

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PostPosted: Mon Jan 05, 2015 4:01 pm 
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Toyota 4x4 mini truck with a 400 small block Chevy.


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PostPosted: Mon Jan 05, 2015 5:49 pm 
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Location: Hanover, On
Thanks to all for sharing your findings. That's great news about this projectile's measurements and that the standard .177 barrels dimensions will handle these, with an increased rate of twist. Are not the older BSA barrels mostly offered in 1:16" twist ? So, the .17 HMR barrels are specced wrong.. thanks for that info! @chevota thanks for your input. The 13.4g JSB is a fine pellet, I'm sure, but I will continue to pursue finding out how to get the Rabbit Magnum IIs to shoot accurately until I'm either successful or have exhausted every possibility. Speaking of other pellets, H&N has bought out the people that created the "piledriver" pellet and are in the process of setting up the tooling, in Germany, to have these on dealers shelves by April 2015. The .177 version is over 21gr and the .22 version is over 30gr. These are supposed to be incredibly accurate in everything, even the UK's sub 12 fpe guns. But again, like the JSB 13.4gr, these don't exibit the particular ballistics characteristics that I'm looking for.
@rsterne Thank you very much for all of that detailed information that you shared. It is very helpful! I really do think that these pellets will fly true to the mark if we can figure out exactly what they need to do so. Is your rate of twist calculator something that I can find online and download. I'd like to see the various results from trying different rates of twist. If I were wealthy I would just order barrels with all possible twist rates from 1:10" to 1:16". But I'm not wealthy, so I want to be 100% certain that I'm exactly right before placing my order. I'm going to place an order for some Rabbit Magnum IIs and start with seeing how bad these shoot in a standard 1:18" twist barrel. Then I may look for an older BSA barrel, I do believe that these were 1:16", I'll research it more. If the 1:16" barrel then shoots perfectly, then I'll stop there. If it only shows a marked improvement, then perhaps an even faster rate of twist may be in order.


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PostPosted: Tue Jan 06, 2015 12:02 am 
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http://www.geoffrey-kolbe.com/barrel_twist.htm

Also of interest....

http://www.geoffrey-kolbe.com/drag.htm

Bob

_________________
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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