Why are bullets for the 7mm-.08 so "small"?

[Wannashootit]

Trying to understand why, the .308 handles bullets well over 200 grains....

Yet, neck it down to shoot the higher BC 7mm bullets, and the 175 grains seems to be the limit, with the 162 Amax having the highest BC.

Identical case. So why can't the 7-08 push heavier bullets- like VLD's in the 180-200 class (hopefully having a higher BC)?

Does bullet design/stability prevent the additional length required to make them the equivalent weight of a .308?

Just trying to get a better grasp on external ballistics...

I was noticing the same thing on the 6.5-06. I was thinking, man- you could launch a big, heavy 6.5 bullet at mind-blowing velocity with all that case capacity- yet, it ain't so.

 

[flashhole]

Re: Why are bullets for the 7mm-.08 so "small"?

I think Nosler used to make a 190 grain bullet in 7mm. The 175 grain bullets are already quite long and you need a reasonably fast twist barrel to stabilize them. I shoot 175 grain bullets in my 7mm Rem Mag with a 1:10 twist barrel and they are very accurate. They chew up quite a bit of powder room too when properly seated. Anything longer will leave even less room for powder. I would be looking to shoot bullets on the order of 150 - 160 grains in a 7mm-08 due to the powder issue.

 

[Wannashootit]

Re: Why are bullets for the 7mm-.08 so "small"?

I'm shooting 150 SMK's with the factory 10 twist now, and want to get an 8 twist for the 162 Amax.

I guess it comes down to the bullet shape in general- and the location of the ogive. Bullet design would seem to preclude just seating a "given" amount in the case (to maximize powder capacity) and cutting a longer chamber to accommodate the bullet...

 

[1lnbrdg]

Re: Why are bullets for the 7mm-.08 so "small"?

The longer the bearing surface of a bullet, the higher the pressure goes when firing. There are some limitations on how much powder you can burn.

 

[skog]

Re: Why are bullets for the 7mm-.08 so "small"?

Same weight in small diameter means longer bullet, so it is either reducing case capacity by seating the deep to mag feed or seat the long and the don't feed. 243 win is the same casehead and 115 grain is the biggest in know while the 338 federal is also the same casehead can run 250 gainers and 300 gainers.

 

[tigerfan_9]

Re: Why are bullets for the 7mm-.08 so "small"?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 1lnbrdg</div><div class="ubbcode-body">The longer the bearing surface of a bullet, the higher the pressure goes when firing. There are some limitations on how much powder you can burn.</div></div>

this....you can go bigger but not in a 7-08 case otherwise the pressure goes up to much to keep velocity up or you must increase the case to a magnum to get the velocity up.

 

[Wannashootit]

Re: Why are bullets for the 7mm-.08 so "small"?

Understood- but there's no way to even further lengthen the secant ogive-adding length and weight- without increasing the bearing surface?

Guessing not- there must be a mathematical formula- and a physical limitation of bullet design- that's already got the VLD's at their max in this area- I'm supposin'...

 

[JGorski]

Re: Why are bullets for the 7mm-.08 so "small"?

Um, you dont need a 8 twist to shoot the 162AMAXs, 9 twist works just fine.

 

[Wannashootit]

Re: Why are bullets for the 7mm-.08 so "small"?

Even if a 9 will work, isn't an 8 twist more suitable for long-for-length, VLD bullets? What's the downside?

 

[jdmartin]

Re: Why are bullets for the 7mm-.08 so "small"?


Guessing not- there must be a mathematical formula- and a physical limitation of bullet design- that's already got the VLD's at their max in this area- I'm supposin'... [/quote]

There is an optimal maximum weight for each caliber.This runs about 90 gr. in 22;about 115 in 6mm;about 140 in 6.5mm;about 180 in 7mm; and about 230 in 30 cal.
A 7mm at 175 gr.is near it's practical maximum,plus as mentioned above you run out of room for powder in a 308 based case with the long bullets.I think it takes a bigger case to get the 180s to their optimal velocities,though I think the 7mm 08 is a fine cartridge.

 

[shoot4fun]

Re: Why are bullets for the 7mm-.08 so "small"?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wannashootit</div><div class="ubbcode-body">Even if a 9 will work, isn't an 8 twist more suitable for long-for-length, VLD bullets? What's the downside? </div></div>

I have great results in 7WSM with Berger 180 VLD with a 1 in 9 twist. My 7MM08 is a 1 in 8.75 and I shoot the 168 SMK there. I am sure I could have gotten good results from the 168 with a 1 in 9 twist.
I think the factory twist for Remington is 1 in 9.25 for 7MM08.

 

[JGorski]

Re: Why are bullets for the 7mm-.08 so "small"?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wannashootit</div><div class="ubbcode-body">Even if a 9 will work, isn't an 8 twist more suitable for long-for-length, VLD bullets? What's the downside? </div></div>
In my Berger booklet it suggests a 9 twist for their 180VLD. Ive fired the 162s out of my 700 9 twist, btw.

 

[Helmet]

Re: Why are bullets for the 7mm-.08 so "small"?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wannashootit</div><div class="ubbcode-body">Even if a 9 will work, isn't an 8 twist more suitable for long-for-length, VLD bullets? What's the downside? </div></div>If you lived somewhere that got real cold then you would need a twist faster than 1-9. The most difficult bullet to stabilize that I can find is actually the Berger 175 gr XLD at 1.535" long. If a bullet is light and long it is even harder to stabilize than a bullet that is heavy and long so the Berger 175 gr actually requires more twist than the Berger 180 gr bullets which aren't any longer. In spite of that the JBM stability chart shows that a Berger 175 gr XLD should be stable in a 1-9 twist if shot at 2600 fps as long as the temperature doesn't drop below 40f.
I just bought a Criterion barrel in a 1-9 twist that should be here in a couple days so I have been doing alot of checking of different bullets on the stability chart. The only way a 162 Amax isn't going to stabilize in a 1-9 twist is if the temperature drops under -35F. The Savage factory twist for 7mm08 is 1-9.5 and it will stabilize a 162 Amax.
In answer to your question of a downside for using more twist than you need, it just means that any imperfections in the bullet will be magnified. Just like a fan will a piece broken off on one blade. If the fan is on low speed you don't notice, but if it's on high speed the fan wobbles. Same with a bullet. If a bullet isn't perfect it will have a greater negative effect the faster it is spinning. If you want to have a 1-8 twist barrel I doubt you would ever notice a negative effect. A 162 Amax in a 1-8 twist is still spinning much slower than a 168 SMK spins out of a 1-10 twist in 308win. On the other hand I seriously doubt you could find a bullet to shoot that a 1-9 twist wouldn't stabilize unless you plan on leaving Florida with the rifle.

 

[Nortex]

Re: Why are bullets for the 7mm-.08 so "small"?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wannashootit</div><div class="ubbcode-body">I'm shooting 150 SMK's with the factory 10 twist now, and want to get an 8 twist for the 162 Amax.

I guess it comes down to the bullet shape in general- and the location of the ogive. Bullet design would seem to preclude just seating a "given" amount in the case (to maximize powder capacity) and cutting a longer chamber to accommodate the bullet... </div></div>

My 1:10 7mm-08 stabilizes the 162 Amax with no problems.