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Balance in bore diameter???

 
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  #1  
Old 03-26-2006, 09:18 PM
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Balance in bore diameter???

After talking with a lot of diff people about wildcat cartridges I have seen the topic of bore diameter to powder volume come up over and over. While I understand the importance of this I do not completely understand the whole idea.

I know the 338 cal bullets are very efficent for bore diameter, but why??? 338 cal rounds come in a large amount of different powder volumes, from the new 338 federal cartridge to Kirbys massive 338AM and 408's hog leg Snipe-Tac. What makes this cal so effective???

What makes some calibers so a good balance between bore diameter and powder volume and some less effective??

This is something that I am completely in the dark on but I am very interested in????

Thanks
steve
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Old 03-26-2006, 09:35 PM
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Re: Balance in bore diameter???

yea no one knows! 6.5mm and 338 they just work!

haha

Im sure the smart guru's here know.
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Old 03-27-2006, 09:49 AM
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Re: Balance in bore diameter???

Lerch,

It basically has to do with bore volume and expansion ratios. Pretty much the two are interrelated.

If you take a certain case capacity, say, 100 gr of usible powder capacity and also use a fixed usible barrel length such as say 30".

Lets look at the bore volumes for the differnet calibers we commonaly use for long range hunting. This is a rough estimate considering the same chamber is used for all, same throat length and such. Just a theoretical comparision:

All are inches cubed:

257........1.556
6.5........1.642
270........1.808
7mm........1.900
30.........2.235
338........2.692
358........3.020
375........3.313
510........6.138

Lets compare a few things. If you take the 257 bore volume and compare it to say the 30 cal bore volume you see that with an increase of 0.051" in bore diameter you get an increase of 0.679 in3 in volume.

Now if you compare the 30 cal against the 358 cal bore volume. You have the same increase in bore diameter as you did with the 257 to 30 cal comparision but in this case, the bore volume increases by 0.785 in3.

The larger the bore diameter is the bore volume of that bore increases dramatically.

This is where expansion ratios come in. If you have a 100 gr powder charge, it will expand to a certain volume in the bore volume. In the larger bore diameters, this 100 gr of powder has the ability to expand to a dramatically larger degree then it does in the smaller bores.

This allows us to use faster burn rates of powder. On average, the faster burn rate of powder you can use for a given chamber and bore will produce the highest level of velocity.

With that said, it has to be a correct burn rate for the specific use. We can use WC872 in a 257 AM under a 156 gr ULD RBBT and get great results. If you put the same powder charge in say a 338 RUM you would get very poor velocity performance.

In comparision, if you took a top charge of say RL-22 in the 338 RUM with a 225 gr bullet, you would get very top end velocities, take that same powder charge and put it in the 257 AM and you would be picking pieces of rifle out of your teeth!!

The larger the bore diameter also effects the bore life to some degree. This has alot to do with the ability to handle faster burning powders. The faster a powder burns, again in general, the cooler the bore temps are. The cooler the bore temps, the longer the barrel will last, again in general.

SO really, its not a matter that one bore diameter is dramatically better then another. If you took all these bore diameters and chambered for a round that all had the same expansion ratio, they would all perform in a very similiar fashion. Especially if they all used bullet with similiar sectional densities.

What makes it appear that the 338 bore is so much more efficent is that its bore diameter will allow much heavier bullets over much larger powder charges to match the expansion ratio and sectional densities of the smaller chamberings.

DO not confuse this with ballistic performance though. Find a 338 round that will even match your 270 AM ballistically as far as bullet drop and wind drift, you will have to get into the 338 AM and 338 ST to find that level of performance ballistically!!

Hope I murkied the waters even more for you!!! [img]/ubbthreads/images/graemlins/grin.gif[/img]

Kirby Allen(50)
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Old 03-27-2006, 10:21 AM
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Re: Balance in bore diameter???

Thanks for the info Kirby

That is actually pretty much exactly what I was looking for. Now I just have to read it 6-10 times over before it starts to sink in!! [img]/ubbthreads/images/graemlins/grin.gif[/img] [img]/ubbthreads/images/graemlins/grin.gif[/img]

thanks
steve
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Old 03-27-2006, 11:06 AM
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Re: Balance in bore diameter???

See i told you that the OLE' Mighty one would know.
Chuckling here
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Old 03-27-2006, 01:10 PM
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Re: Balance in bore diameter???

What exactly is the 338AM and the 338ST , what is the parent case?
What velocity advantage over say the 330RUM do they have in a 30" barrel?
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Old 03-27-2006, 03:30 PM
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Re: Balance in bore diameter???

JD:

The 338 Allen Mag and the 338 SnipeTac are allmost twins. Kirby's is the AM and mine i call the SnipeTac. They come from the 408 chey-tac case necked down and body taper reduced, case capacity is around 165grns of H2o. They will push the 300gr Match Kings 3400+ fps. Around 147-150grns of powder is used. 32"bbl or longer is recomended for this.
A special action is needed to build one of these, the overall length of the cartrige is 4.140" base dia of .635

Dave
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