Burning question about speed

[river grant]

I'm not very new to reloading but have wondered about this for a while and I figured some of you "salty dogs" already have the answer.

Why does a parent case always shoot same weight bullets faster and tolerate heavier loads than necked down versions? Meaning, why does the .308 win shoot a 160 gr bullet faster than a 7-08 will? ...and why will the 7-08 shoot a 100 gr bullet faster than a .243 will? Obviously, the case capacity is basically identical before seating a bullet, so what gives?

[Eric in NC]

Smaller bore = more constriction and higher pressures (plus the longer bearing surface of a same weight bullet means more friction).

[efm77]

Exactly what he said. A 160 grain .308 is shorter due to the larger diameter than a 160 grain 7mm, therefore, it has less bearing surface on the barrel which equals less friction. It applies the same to other calibers too.

[BillPa]

Ackley breifly covers this in his books to some extent, but this is a better detailed explanation to the same question.

- For a 140gr bullet, the .260 muzzle velocity is listed as 2750fps.
- For a 140gr bullet, the 7mm-08 muzzle velocity is listed as 2860fps.


The only place you drifted off was in equating pressure to force. It is related, but is actually a compound unit of force per unit area. So pressure is in pounds per square inch, while force is just in pounds. 52,000 psi means that if you took any particular square inch of area inside the chamber and behind the bullet, it would have a total of 52,000 pounds force applied to by that pressure. Two square inches would have a total of 104,000 pounds force applied to it, etc.

So, your .260, including rifling, will have a bore cross-sectional area of about 0.054 in², while your 7 mm will have one closer to 0.063 in². At the moment of peak pressure that would be a force on the bullet base of 0.054 in² x 52,000 lb/in², while the 7 mm would apply a force of about 0.063 in² x 52,000 lb/in². In both cases the square inches cancel each other out and you are left with 2,808 lbs pushing the .260's bullet base, and 3,726 lb pushing on the 7 mm bullet base.

Your resulting velocities don't reflect that full pressure difference ratio, but that's because of other factors. The bullet's total acceleration force is the average pressure it sees during its whole time spent in the barrel, minus friction losses. The pressure peak occurs at just one moment in the bullet travel down the tube. Differences in powder charge size and powder burning rates also come into play in determining the lower-than-peak pressures during the rest of the time the bullet is in the barrel, and not just where it was when the pressure peaked. Then there is the pressure gradient that develops because the gas has to chase the moving bullet, and that results in a few thousand psi less pressure at the bullet base than a pressure graph taken at the chamber shows. So, its complex, but you get some idea of what is going on.

A detailed explanation, but in a nutshell, assuming everything is identical its easier to push a .308 caliber bullet than a 284 caliber of the same weight at the same pressure simply because there is more square surface of the bullet for the pressure to act upon much the same a holding a one foot square piece of plywood flat into a 10 mph wind then doing the same with a two foot square piece.

Bill

[river grant]

Thank you gentlemen! I had a feeling bore diameter had something to do with it but those explanations go far beyond my "feeling".

[Forester]

To extend the line of thinking though...that smaller diameter, same weight bullet should have a significantly higher BC. Thus it does not need to reach the same speed in order to have superior ballistics. Muzzle velocity is perishable but BC is forever.

[Dirk]

To extend the line of thinking though...that smaller diameter, same weight bullet should have a significantly higher BC. Thus it does not need to reach the same speed in order to have superior ballistics. Muzzle velocity is perishable but BC is forever.



I don't know who told you BCs are forever, 'cause it just isn't true! BCs are dependent on velocity. As a matter of fact that is it's definition....

"What is a Ballistic Coefficient?
: The ability of the bullet to maintain velocity, in comparison to a ‘standard projectile’. A high BC bullet can maintain velocity better than a low BC bullet under the same conditions. All measures of ballistic performance including drop and wind deflection are related to the bullet’s ability to maintain velocity. In short; the higher the BC, the better the all-around ballistic performance of the bullet will be."

In ballistic software, BCs are usually given at different velocity thresholds (but realize that the BC is decreasing as the bullet loses speed). Thus it is a constantly changing value.

Here is an example of a load I use out to 600 yards....

Gun / Ammunition : 6 mm B.R. Norma
Bullet : .243, 80, Sierra SPBT Blitz 1515
Bullet weight : 80 grains or 5.18 Grams
Muzzle velocity : 3179 fps
Crosswind speed : 1 Mph
Ballistic Coefficient(s) (G1):
C1=0.313@V>2800 fps;
C2=0.305@V>2200 fps;
C3=0.290@V>0 fps;

As you can see, the BC is highest greater than 2800 FPS and decreases as the bullet get further from the muzzle and is slowing down.

[Forester]

Wow, thanks for the education. You would not need those extra velocity "steps" if a proper reference projectile was used (i.e. G7 BC numbers) Those steps are a ham handed way to overcome the problem with G1 BC numbers. If marketing departments would get out of the way maybe more manufacturers would give useful BC numbers for their bullets

Velocity is irrelevant to the point of the phrase. That higher BC bullet is ALWAYS going to be a higher BC bullet, just because the BC is changing does not change that fact.

[Dirk]

Forester,

I agree. The higher BC bullet is always going to perform better than a lower one, at any speed and whether one uses the G1 or G7 calculation. The marketing departments are not likely to use the G7 because it makes their products look worse. Whether one uses G1 or G7, the BC's are just relative things. If one bullet has a G1 BC of .500 and another has a G1 BC of .450, the higher one is the better performing bullet. That same G1 bullet with a BC of .500 might only have a G7 BC of .470 and that doesn't look good to the consumer! ;)

[tammons]

At some point you get into overbore,
where the cartridge is just not that efficient,
but a 7mm-08 compared to a 308 it is not the case.

A few reasons.

A 7mm bullet of the same weight is longer and takes up more room in the case.
As mentioned above a longer bullet has more bearing surface.
A larger diameter bullet is more efficient, IE the piston effect.

[river grant]

I was looking at the sierra website and found a bullet that had a higher BC the more it slowed. The BC is a good point though Forester. Quite honestly, I was thinking apples to apples and that's just not the case when one bullet has a superior BC.

Now on a different note, what type of bullets keep the highest BC throughout their flight? I know the poly tipped bullets seem to have higher BC's at muzzle velocity but competitive shooters seem to really favor hollow points.

[hunter2]

And - does the BC really matter that much once the bullet goes subsonic? Just curious....

[Forester]

I was looking at the sierra website and found a bullet that had a higher BC the more it slowed. The BC is a good point though Forester. Quite honestly, I was thinking apples to apples and that's just not the case when one bullet has a superior BC.

Now on a different note, what type of bullets keep the highest BC throughout their flight? I know the poly tipped bullets seem to have higher BC's at muzzle velocity but competitive shooters seem to really favor hollow points.


The VLD types seem to typically have a higher BC across the board. They do come with their own challenges regarding seating depth and magazine feeding though. (No free lunch huh?)

Edited to Add: I think the issue with poly tipped bullets for competitive shooting is that the tips are not all that consistent. HP bullets can be made very consistent and have the meplats uniformed if that is not enough.

[Forester]

And - does the BC really matter that much once the bullet goes subsonic? Just curious....


I would have to believe the higher BC bullets would have a smoother transition when they go subsonic. But that is just a guess on my part. That said, holding much accuracy past that transition point is a pretty large question mark.

[Forester]

Forester,

I agree. The higher BC bullet is always going to perform better than a lower one, at any speed and whether one uses the G1 or G7 calculation. The marketing departments are not likely to use the G7 because it makes their products look worse. Whether one uses G1 or G7, the BC's are just relative things. If one bullet has a G1 BC of .500 and another has a G1 BC of .450, the higher one is the better performing bullet. That same G1 bullet with a BC of .500 might only have a G7 BC of .470 and that doesn't look good to the consumer! ;)


I wish the numbers were that close, the problem is G7s are often in the neighborhood of half the value of a G1, so it looks really bad to those that do not understand the difference.

Publishing and teaching about G7s is one reason Bergers are always the first bullet I try in a new barrel. I respect the fact that they are giving their buyers the best information at the risk of losing a few un-informed buyers. Plus they flat out shoot in most guns I have tried them in, and perform great on game.

[Dirk]

Forester,

I see we agree completely. I was only using numbers in an example. G7 BCs are like you say about 1/2 of G7 BCs.