7mm-08 vs .308win / Same 150gn bullet weight / Different Velocities?

[Deleted member 107796]

Hey All,
Why is it that I'm seeing consistently higher 308 velocities vs 7mm-08, for the same 150gn bullet?
I figured with same max pressure, the velocities might compare more closely. regards

 

[elkaholic]

Hey All,
Why is it that I'm seeing consistently higher 308 velocities vs 7mm-08, for the same 150gn bullet?
I figured with same max pressure, the velocities might compare more closely. regards

It's because of your bore size to weight ratio. With a .308, you have more force being created on a larger cross section bullet.
Look at it this way; if you have a 10mph wind blowing against a 50 sg ft sail vs a 100 sq ft sail, which boat will go faster!
The downside is, it will start faster with a .308 but will lose velocity quicker (bc) so at some point, the 7 mm will take over with velocity, trajectory and energy. It just depends on your game as to which you want. In general, the 7/08 wins the competition at extended ranges

 

[memtb]

Also, for bullets of equal weight, generally the smaller diameter bullet has more bore surface area, thereby more friction ( bore drag) resulting in lower velocities with similar chamber pressures! memtb

 

[Deleted member 107796]

Thanks guys, makes perfect sense. It's nice to learn something new every day.

 

[alan.j]

For an even better example of this, check typical velocities of 338 Fed with various bullet weights!

 

[entoptics]

Though your question was answered for the most part, I figured I'd add a little math for a little more thorough explanation...

Area = πr^2

So, for the base of the bullet, being acted upon by 60,000 pounds per square inch...

π x (0.284/2)^2 = 0.0633 square inches x 60,000 lb/in2 = 3801 lbs
π x (0.308/2)^2 = 0.0745 square inches x 60,000 lb/in2 = 4470 lbs

So, even if you ignore differences in bearing surface friction, there's ~18% more force being applied to the bottom of a 308 bullet at the same pressure.

 

[LVJ76]

I shoot NRA Silhouette competition and in the late 80's we used the 308, it was the most commom cartridge used in this competition at the time, and then the 7mm-08 came along.

With the 308 we used the 168gr. Sierra MK, but the metal Rams at 500m sometimes did not fall down after you hit them, the bullet did not have enough speed and trajectory to knock them down.

So we tried the 168 gr Sierra MK with the 7mm-08 and it worked better, it got to the 500m target with a better trajectory, it still left less rams standing but less then the 308.

A few years latter my dad and I came up with the idea of using the 150gr Sierra MK on the 7mm-08 and Oh boy, no more rams stood standing after that. A couple friends tried the 150gr on the 308 and it did not work as good as the 7mm-08. The 7mm-08 shots flatter so the bullet has a more direct impact on the silhouette and not so much in an angle as with the 308.

Since then I am a true fan of the 7mm caliber. To this day I still use the 150gr Sierra MK, been thinking about trying the 160gr TMK or the 162gr ELDM, with the higher BC's they might work good. But why fix something that is not broken.

 

[Deleted member 107796]

My next 7mm-08 test loads will be 140 Berger VLD with IMR4350 and 150gn Berger Classic Hunter with either Varget or 4350.

 

[LVJ76]

My next 7mm-08 test loads will be 140 Berger VLD with IMR4350 and 150gn Berger Classic Hunter with either Varget or 4350.

I use 44.5gr of IMR 4350 with the 150gr Sierra MK, 6" groups at 500m. Load is a bit hot but no issues.

For hunting I use 41.0gr if IMR-4064 with the 139gr and 140gr bullets, 5" groups at 400m. I rarely shoot past 400m in hunting situations with the 7mm-08.

Hope this helps.

 

[Deleted member 107796]

Yes, all the input has been very helpful. I just loaded the 140gn Berger VLDH onto 45.5 and 46.0gn IMR4350 as initial tries. My attempt with Varget wasn't as good as my .308 loads with it so we'll see how the 4350 does. I'll check back with results. cheers, all.