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PostPosted: Wed Oct 19, 2016 2:11 pm 
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Location: Tecumseh, Ontario in South Western Ontario
I've noticed, but don't understand why the preferred calibers for long range target shooting are 25, 30 and higher. I just learned from Bob's post topic74106.html that it's not because they are heavier. Accurate projectiles have a high SD and with the right shape can have a higher BC for a flatter trajectory and less wind drift. That sounds like accuracy is not caliber specific so why big bore guns?
Is it because the higher BC boat tailed bullets are only available in the larger calibers that they are used?

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PostPosted: Thu Oct 20, 2016 7:09 pm 
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Location: P.G. B.C.
1st of all, the larger the bore - at least with lead projectiles, the better the POTENTIAL accuracy as minor defects effect larger heavier projectiles less than they do smaller projectiles.

2ndly, with air rifles, longer bullets in any calibre size to produce higher SD's and BC's generate more internal bore friction - in order to get higher numbers in smaller bores, the bulelts become excessively long.

This is easier to "see" in extremes.

Thus, it is easy to obtain Sectional density of .238, just from a relatively short-for-.45cal. 350gr. bullet. The BC would be approximately .200.

To get these numbers in a .22cal. - well, you can't as the 'bullet' would have to be over 70gr. as at that weight, the SD is only .199 with a BC of .200 for a smooth sided jacketed bullet.

We do know, if the BC's are the same and the velocities are the same, the trajectories & wind drift will also be identical.

It is just easier to get higher, useable numbers in larger calibres.

FYI, In CF rifles, recoil comes into the equation, thus you will see .308 calibres about max. for long range accuracy. Thus, the current record at 1000 yard record is 5 shots in 1.403", shot with a .308 calire rifle.
http://bulletin.accurateshooter.com/200 ... 000-yards/

During the civil war and decades following, long range muzzleloading match rifles ran upwards of .60 calibre and today, lead bullet shooting muzzleloading rifles run to .69 calibre. These are mostly used for shooting from 100 to 300yards.

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PostPosted: Thu Oct 20, 2016 7:49 pm 
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Location: Kingston, ON
Here is an interesting website that explains a lot about ballistics.
http://www.dexadine.com/index.html

I found the explanation as to why we don't give units on BC particularly interesting as I always felt that BC should have units. It turns out it does, it's lb/sq in.
http://www.dexadine.com/qbcunits.html

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PostPosted: Thu Oct 20, 2016 8:37 pm 
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Location: Coalmont BC
If you look up SD in a Physics text, the formula is mass/area, and the units are lb/sq.in (psi).... However the area (of a cylinder) is cal^2 x PI/4, but in Ballistics we don't bother with the actual area, we "cheat" by just using cal^2.... Therefore, to express the SD in psi would be incorrect, which I suspect is why we don't....

The SD we can use in airguns is limited by barrel length and pressure, and for any given SD, larger calibers become rather draggy compared to smaller calibers.... IMO, the "best" calibers in theory are the smaller ones, eg. .224 or .257, rather than 7mm or .308.... unless you are running Helium at high pressures with long barrels, in which case you would be hard pressed to beat a .308 shooting a 200 gr. bullet, which can duplicate the ballistics of the .300 Whisper (AAC Blackout).... For us mere mortals, shooting air at 3000 psi, you are far better off with a .257 cal.... The .224 with a long bullet may be a better choice, again in theory, but the smaller the bullet the harder they are to cast, and it gets harder to find a barrel with a fast enough twist....

That is why the .257 ends up being the caliber of choice for most long range airgunners.... The current record is hitting a 1 meter square steel plate at over 1000 yards....

Bob

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PostPosted: Sat Oct 22, 2016 6:32 am 
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Joined: Mon Aug 24, 2015 4:56 pm
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Location: Tecumseh, Ontario in South Western Ontario
I see now that I didn't mention I was referring to airgun calibers and shooting up to 200 yards like in the EB.
I'm sure that a lot of the dynamics would be similar but in a different class considering the pressures and velocities involved.
At the EB one of the most popular guns is the FX Boss which only comes in 30 caliber. It doesn't mention the calibers of the other guns but the FX Impact can also be fitted with a 30 caliber barrel. I'm sure there are many factors involved with that result but I wonder why that caliber seems to dominate.
I'd like to get involved with the sport and choosing the best caliber is an important factor.

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PostPosted: Sat Oct 22, 2016 12:24 pm 
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When you said "long range", I immediately started thinking bullets, not pellets, as that is what is used in the 200 yard EBR.... The FX guns you mentioned are strictly pellet shooters, and do well at 100 yards and less.... My choice in calibers for 100 yards with pellets would be .30 cal, although there is no reason if you could find a barrel that shot well with the heavier pellets you could not do as well or better with a .25 cal, or even .22 cal or .177.... The trick is finding a combination that works with the (few) heavy pellets around in the smaller calibers....

When you are talking bullets, then my previous comment apply.... I prefer .257 cal on air, up to .308 cal on high pressure Helium.... Here is a visual that will help you understand what happens to bullet shape with the SD held constant.... Note they are all about the same length....

Image

The bullets in the above photo are all about the same SD of about 0.17.... Obviously the smaller calibers have a lower drag shape, and hence a higher BC....

Bob

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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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