Balance in bore diameter???

[lerch]

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

 

[Black Diamond 408]

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

haha

Im sure the smart guru's here know.

 

[Fiftydriver]

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!!! /ubbthreads/images/graemlins/grin.gif

Kirby Allen(50)

 

[lerch]

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!! /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif

thanks
steve

 

[Black Diamond 408]

See i told you that the OLE' Mighty one would know.
Chuckling here

 

[James Jones]

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?

 

[Black Diamond 408]

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

 

[goodgrouper]

I agree with Kirby's post.

I would also like to add that there are overbore 338's just the same as there are overbore 17's through the mighty 50. What you have to do is look at the bc's of the available bullets in your caliber, pick a target velocity window, see which case can give you that velocity with the smallest amount of powder, and then have fun with it. Every caliber now has bullets from the .2 bc's up to and past the .5's. The 338 caliber just has the sheer weight and momentum to maintain massive amounts of energy at long range and therefore makes it easier to kill and spot shots in the dirt.

 

[Bart B]

Fiftydriver's comments are pretty good and explain things well. Here's some observations I've made over the years.

For each caliber, there seems to be a few cartridges that seem to attain the best accuracy. These cartridges are the ones often used in competition and tend to produce the best scores. They all have one thing in common; their powder charge weight is at or very near 'bore capacity.'

OK, so what is bore capacity? Years ago a 'smith catering to the benchrest crowd explained it to me as follows. Bore capacity in grains of powder is equal to the cross sectional area of the bore in square millimeters; 1 grain of powder for each square millimeter. I did some checking and here's what I learned comparing the bore capacity for different calibers and examples of very accurate cartridges for them:

22 caliber, bore capacity = 26 grains; .22 PPC, .222 Rem.
24 caliber, bore capacity = 28 grains, 6mmPPC, 6mmBR
26 caliber, bore capacity = 33 grains, .260 Rem.
27 caliber, bore capacity = 38 grains, 7mm-08
30 caliber, bore capacity = 45 grains, .308 Win.

That 'smith also said that when cartridges at bore capacity are used, barrels typically last about 3000 to 4000 rounds of best accuracy. Double the powder charge over bore capacity for a given bore and barrel life drops to 1/4th as many; 750 to 1000 rounds.

Interesting stuff but it seems to be correct. There's a bunch of meanings/definitions for bore capacity, but this one makes the most sense to me.

 

[Fiftydriver]

Bart B,

What barrel length are you using for those Bore capacity figures?

So are you saying my 257 AM(95 gr powder charge) and my 7mm AM(107 gr powder charge) are a bit overbore?? /ubbthreads/images/graemlins/blush.gif /ubbthreads/images/graemlins/grin.gif

For pure accuracy and longest barrel life combinations I would tend to agree with your numbers for the most part.

Kirby Allen(50)

 

[Bart B]

[ QUOTE ]
What barrel length are you using for those Bore capacity figures?

So are you saying my 257 AM(95 gr powder charge) and my 7mm AM(107 gr powder charge) are a bit overbore?? /ubbthreads/images/graemlins/blush.gif /ubbthreads/images/graemlins/grin.gif

For pure accuracy and longest barrel life combinations I would tend to agree with your numbers for the most part.

[/ QUOTE ]Barrel length didn't matter for this 'bore capacity' stuff. As barrels typically wear out from the throat forward from highest pressure and powder burning temperatures, that's where it came from and was/is based on barrel life. What 'efficiency' rating one gives a barrel for a given powder's charge weight vs bore diameter AND bore length may well be something else. But there are typically a narrow range of powder makes/types that tend to give best accuracy for a given bullet weight. And there's another range of powder makes/types that produce the highest muzzle velocity. Sometimes these two are the same.

No, I'm not saying your two cartridges are a bit over bore. If I did say they were, I would use a word meaning a lot more than 'bit.' That aside, sometimes one chooses to go a humungus amount of over bore capacity to get the muzzle velocity desired for a given caliber's bullet weight. It's sometimes been said that a given caliber is accurate for a given amount of powder. Shooting more powder per shot means you'll run out of powder (wear out your barrel) faster.

Regarding barrel life, it's much like religion, life partners, politics and sex. Whatever makes you feel the best is what you're gonna go with. We all have different personal standards and opinions.

 

[Fiftydriver]

Ah, I see.

As you say its a compromise but not to the degree most think. I have punched several groups out of my AMs that will measure less then 3/4 moa out to 1000 yards. Many in the .1"s and .2"s as well at 100 yards, not uncommon.

I suspect proper receiver machining and barrel fitting have much more to do with accuracy potential then anything else.

That said, a small caliber, small capacity is very easy to get to shoot tiny groups, very easy. Conversely, its much more of a challange to get a big chambering to shoot well but it is surely possible, repeatable and practical to do so with the proper rifle building techniques.

In my opinion, a true test of a custom rifle builder is not how accurate they can build say 223 Rem or 6mm BR. What would interest me more is if they could build a highly accurate big gun, say 7mm RUM and larger. This is where close enough just don;t cut it and it shows on paper.

Many feel this is a result of to much capacity, in my opinion thats hogwash, and more then likely its loose machining that is the reasoning or unsquared receivers and barrel work.

When all is said and done, theres only one way to get a .800 BC or higher bullet to over 3300 fps, its with a big engine. Sure barrel life will be shorter then a 22 ppc but you don't drive a 1200 HP dragster as your everyday car either, only for special, specific purposes. Likewise, you don't ask a 22 PPC to reach out 800 to 1200 yards and hammer a deer size target either, for that matter even punching paper.

I think alot of smiths make a much bigger deal out of barrel life then needs to be said. Most shooters, even serious shooters never shoot their rifles enough to burn out a barrel. And those that do, will not waste a high performance rifles barrel life just plinking, more suitable rifles are used for general purpose in most cases.

For big game rifles, a 900 to 1000 round accuracy barrel life will last even serious big game hunters nearly a lifetime of use. The better the shots, the longer the barrels will last in my experience /ubbthreads/images/graemlins/grin.gif!!!

Kirby Allen(50)

 

[lerch]

And then you have idiots like me and Bill Bailey who buy these massive big game rounds and just have to **** much fun with them!!!

Seriously, barrels are a replaceable item, most shooters will never fire these big calibers enough to burn out a barrel, and ya cant take your money with ya to heaven so you might as well enjoy it while your here. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/wink.gif

"The better the shots, the longer the barrels will last in my experience !!! " Well I never thought being a bad shot could be so much fun /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif I am sure we will take it easier with this Big 7mm you are building us but for know I am just having to much fun with the 270AM

These guys at this shoot in Texas will probably crap themselves when they see me and Bill set up to shoot next to them. We will be surronded by guys shooting 6br's and 6xc'x and then BAMMMM!!! the big daddy 270AM roars, ha ha ha, win or lose it outta be a lot of fun

take it easy
steve