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Need some advise

 
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  #15  
Old 04-02-2010, 09:50 AM
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Re: Need some advise

One must never forget one little thing about long skinny bullets.

RPM and twist rates, although not the same thing, share a symbiont relationship

You calculator geniuses should make pretty quick work of this. Lets wave a magic wand and speculate that a 338 or 378 squished down to 7mm will dabble in the high 3000 to low 4000 fps velocity figures. Any "lawn dart" weight 7mm bullet is going to require a pretty aggressive twist rate to stabilize properly.

At these kinds of velocities the bullet will be buzzing along somewhere in the neighborhood of 300,000 rpm.

It's a tall order for the jacket of any bullet to tolerate this kind of centrifugal force. A guy can slow the twist a bit to compensate but I think the gun becomes a bit more fussy as to what it'll shoot well.

Just something to consider before whippin out that checkbook.

Good luck.

Hey Jan, go shoot your gun!
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  #16  
Old 04-02-2010, 12:17 PM
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Re: Need some advise

Quote:
Quote:
Originally Posted by groper View Post
im not sure if its possible, but could you do a 7mm-378??? just neck down the existing 30-378 cases, would be the same everything just new barrel, sizing die and projectiles...

the heavy 7mm projeciles are much better (higher SD and BC) for LR work than the 30cal offerings...
Well... I'm sure it could be done. The question is, do you really want to do it? The only 7mm bullets that I'm aware of that will give you an advantage over the 30 bullets are the Berger 168 and 180, and the 162 Amax. With the Bergers, you would have to go with the target bullets because the "hunting" bullets would not hold up. The 30 cal GS HV 177 would run ballistically with the 180 Berger and it is a hunting bullet, but a little spendy. Also, the 180 E-tip or 200 AB out of the necked down 378 would be more than adequate for a 500 to 1K shot. My choice would still be the 30 cal. I don't think that there is all that great of selection of real high BC 7mm bullets and the GS HV 177 erases that advantage and is a much better all around hunting bullet.
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  #17  
Old 04-02-2010, 12:24 PM
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Re: Need some advise

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Originally Posted by MontanaRifleman View Post
Well... I'm sure it could be done. The question is, do you really want to do it? The only 7mm bullets that I'm aware of that will give you an advantage over the 30 bullets are the Berger 168 and 180, and the 162 Amax. With the Bergers, you would have to go with the target bullets because the "hunting" bullets would not hold up. The 30 cal GS HV 177 would run ballistically with the 180 Berger and it is a hunting bullet, but a little spendy. Also, the 180 E-tip or 200 AB out of the necked down 378 would be more than adequate for a 500 to 1K shot. My choice would still be the 30 cal. I don't think that there is all that great of selection of real high BC 7mm bullets and the GS HV 177 erases that advantage and is a much better all around hunting bullet.

+1 good advise.
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  #18  
Old 04-02-2010, 04:07 PM
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Re: Need some advise

Chad, how fast would that 7mm-338LM with a 168 gr smk?
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  #19  
Old 04-02-2010, 04:39 PM
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Re: Need some advise

I pulled the BC from Sierra's website.

Above 2800fps they call out for a BC of: .488

I have an old version of the NightForce ballistic software called Exbal.

Unfortunately it does not have a simulator for load data. So I'll be making some rather BOLD ASSUMPTIONS. That means a big salt shaker as all of this will be blue sky BS.

I guessed a velocity of the bullet. I know with 125 grain Nosler Ballistic Tips in my 300-338 LM the muzzle velocity is almost 4400fps coming out the gate.

I'm going to GUESS that a 168 bullet will be in the 4100 range.

So, with that BC here's the numbers:

"Drop" figures are in inches, not MOA.


@100: 3871fps Drop: 0 (zeroing distance)
@500: 2988fps Drop: -32.
@750: 2517fps Drop: -81.
@1K: 2092fps Drop: -165.


Again, none of this is really verifiable with the software I have.

Good luck.

C
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  #20  
Old 04-02-2010, 05:02 PM
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Re: Need some advise

Quote:
Quote:
Originally Posted by Chas1 View Post
Chad, how fast would that 7mm-338LM with a 168 gr smk?
Chas,

The Sierra .284 175 would have a lot better BC than the 168 and would be my choice out of a super charged magnum. Out of the same parent cartridge, the GS 177 could be launched 300 fps faster, or more, with as good or better a BC and it's terminal performance is both better and more reliable, not to mention an expansion velocity of 1600 fps.

A rule of thumb I use to compare 7mm calibers to the 30's is a 7mm will launch 160 gr bullet about the same speed as a 30 will launch a 180 bullet. i.e., 7 WSM vs 300 WSM or 7 RUM vs 300 RUM, etc.,. That's just a very general rule of thumb for rough comparisons.

The 7mm high BC bullet selection is very limited IMO. The top end of the commercially available bullets, which is the 180 Berger is velocity limited unless you go with the "target" version. ATM, my top choice for a 7mm hunting bullet is the 150 E-Tip, followed by the 160 AB.

For deer size game, the 7mm is a great cal. But for elk, it's a very good idea to get as much of an edge as you can. The 7mm can get the job done, the 30 cal can do it better, especially @ 1K.

For the OP's requirement of a 500-1K elk cartridge, the 30-338 LM fits very nicely in his Mark V action, IMHO and the barrel will last twice as long as a 7mm-378.
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  #21  
Old 04-02-2010, 07:23 PM
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Re: Need some advise

Blue sky or not I thought it would be really really fast, man it's got to be a barrel burner. I agree in not saying a 7mm couldn't get the job done on elk at 1k but, I like insurance too, 30-338LM sounds good.
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