Why do heavier bullets require LESS powder?

[jake2far]

It took 23 posts, but somebody on this thread finally used the right word: inertia. Not bullet mass - its inertia.

For CHemist: I know its easy to use 20/20 hindsight, but you will note that I even avoided "mass" until pretty late, and someone else ventured to display the term. I was gonna go with "inertia" at that point as well, but there's (in my mind at least) some value in remaining in layman's terms (like the hardboiled egg thing).

Perhaps you should study Newtons 2 Law, it uses Mass not inertia, Mass is the correct term, so before you proclaim people got it right after 23 posts you should understand Mass.

try this:
The inertial mass of an object determines its acceleration in the presence of an applied force. According to Newton's second law of motion, if a body of fixed mass M is subjected to a force F, its acceleration α is given by F/M. A body's mass also determines the degree to which it generates or is affected by a gravitational field.
Mass - Wikipedia, the free encyclopedia

And yes I can read, the term inertial mass is a term used, but mass is what matters.

In common usage the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context. The term "inertia" is more properly understood as shorthand for "the principle of inertia" as described by Newton in his First Law of Motion; that an object not subject to any net external force moves at a constant velocity. Thus an object will continue moving at its current velocity until some force causes its speed or direction to change.

On the surface of the Earth inertia is often masked by the effects of friction and gravity, both of which tend to decrease the speed of moving objects (commonly to the point of rest). This misled classical theorists such as Aristotle, who believed that objects would move only as long as force was applied to them.[2]

Since this discussion was about the increase in bullet weight and why lower powder charges are needed (read the original post) Mass is what is being dicussed.



Jim

The Greatest Obstacle to Discovery is not Ignorance, it's the Illusion of Knowledge.

 

[Spitpatch]

Okay. one more reason(s) I used mass instead of Chemist's inertia.

In Chemist's defense, however, I will focus on jake's reference to friction adding to inertia, and point out (as some others did) that usually, a heavier bullet (in the same caliber) will have more surface area contacting the rifling, resulting in more friction (increased inertia). This is another factor (other than the mass itself) that requires us to reduce our charges for a heavier bullet. Please note that the OP did not phrase his question, "why would the weight ALONE require us to reduce our powder charges?"

Others (very rightly) pointed out that sometimes a heavier bullet MIGHT be seated to an increased depth (which has nothing to do with mass or inertia), and in some cases (such as .300 BLK) such invasion of the case may be a partial reason to reduce charges as well.

I still prefer avoiding either for a venue such as this, and speak of jelly beans and hardboiled eggs.

 

[Varmit]

Okay. one more reason(s) I used mass instead of Chemist's inertia.

In Chemist's defense, however, I will focus on jake's reference to friction adding to inertia, and point out (as some others did) that usually, a heavier bullet (in the same caliber) will have more surface area contacting the rifling, resulting in more friction (increased inertia). This is another factor (other than the mass itself) that requires us to reduce our charges for a heavier bullet. Please note that the OP did not phrase his question, "why would the weight ALONE require us to reduce our powder charges?"

Others (very rightly) pointed out that sometimes a heavier bullet MIGHT be seated to an increased depth (which has nothing to do with mass or inertia), and in some cases (such as .300 BLK) such invasion of the case may be a partial reason to reduce charges as well.

I still prefer avoiding either for a venue such as this, and speak of jelly beans and hardboiled eggs.

I am completely satisfied with your wrap-up SP. I am also glad that you restated what the original question was as I wasn't sure anymore....

 

[jake2far]

Okay. one more reason(s) I used mass instead of Chemist's inertia.

In Chemist's defense, however, I will focus on jake's reference to friction adding to inertia, and point out (as some others did) that usually, a heavier bullet (in the same caliber) will have more surface area contacting the rifling, resulting in more friction (increased inertia). This is another factor (other than the mass itself) that requires us to reduce our charges for a heavier bullet. Please note that the OP did not phrase his question, "why would the weight ALONE require us to reduce our powder charges?"

Others (very rightly) pointed out that sometimes a heavier bullet MIGHT be seated to an increased depth (which has nothing to do with mass or inertia), and in some cases (such as .300 BLK) such invasion of the case may be a partial reason to reduce charges as well.

I still prefer avoiding either for a venue such as this, and speak of jelly beans and hardboiled eggs.

Man you can throw some BS, you proclaim loadly that after 23 posts someone finally got it right, inertia is what we are dealing with, now you flip flop and run with mass. If I change bullet construction from lead too depleted uranium the size of the bullet will go down, the MASS will go up and a smaller charge of powder will be needed, even though the friction bearing surface went down, the size of the bullet went down THE MASS went up. If ya can't keep your story straight I suggest you sit down and shut up.
Mass and friction both contribute to inertia, or resistance to movement, leave the technical stuff to people who can figure it out and go play with your jelly beans and hard boiled eggs, they seem to make more sense to you than physics.

Jim

 

[PX4WA]

I think all the technical stuff lost sight of the op question...

The reason less powder is required has to do with safety and pressure..

The heavier bullet creates a higher pressure with the same charge..

If you used the same powder you could explode uour barrel...

If your gun is overbuilt you can use the same powder charge in different weights within reason... I use the same charge for 115g and 124g fmj. But i get different fps results. Both within safe tolerances

 

[Spitpatch]

I'm sorry jake2far confused me with Chemist. It was not I, but Chemist who originally said "It took 23 posts..."

I would politely refer him back (post #29), at which point I am confident he will acknowledge the mixup. (I did quote Chemist later, in bold print to define it. I acknowledge my failure to use quotation marks as I should have: my immediately-attached address to Chemist, I felt was distinction enough. For some, apparently it was not.)

I can address any contributions I have made, and how they might be interpreted as "BS", but I simply am not empowered to do the same for things OTHERS said.

Contrary to Jake2far's challenge, I have adhered to my contention that mass is the central factor. I did give credit to Chemist for mentioning inertia (which can be a combination of mass and friction). I also gave credit to (and learned from!) Jquirit's contention that case volume can also be another factor.

Sitting down and shutting up are things perhaps suited to persons who are confused as a result of assigning statements to the wrong people. Persons who participate in civil discussions and honest exchange of information and experience would most normally avoid hostile retorts (especially when mistaken as to who said what). It saves embarrassment: if not for the person engaging in it, for others in the discussion who might be embarrassed for him.

 

[Jamie6.5]

I would say the principle reason is friction. Especially when one considers that the bullet must obdurate (conform to the shape of) to the lands and grooves.
Consider how difficult it is to drive a squibbed slug from a barrel (overcoming the friction of a bullet that has already taken on the contour of the barrel's interior), vs. just tossing it in the air.
Now consider starting that bullet in the throat (leade, freebore etc.) and driving it manually into and through the barrel. It is closely akin to the force required to final size/swage a cast bullet, or swaging a jacketed bullet. Only more, due to the radical change in the contour of the bullet's surface..

Think of the cartridge/casing as a combustion vessel, and the barrel as the expansion chamber for the force to be directed into. Now consider the bullet. It essentially becomes a relief mechanism that allows the force to be released in a controlled fashion down the expansion chamber.
The initial spike in pressure is overcome by the pressure achieved as the bullet enters the lands/grooves, and begins the obduration phase of it's journey. This is the point of highest pressure generation.
As it progresses down the barrel, the expansion chamber behind it grows, just like pushing a piston in a cylinder.
The larger the bore of the barrel (cylinder) the faster the pressure drops, due to the additional space the larger bore supplies per inch of bullet travel.
That is why for instance, if we compare 3 cartridges based on the '06 case, the .25-06, the 30-06, and the .35 Whelen (roughly .050" between each), the .35 Whelen will allow the fastest powder, and the most of it when a bullet of mid weight for each is considered, and still allow safe pressures, even though the Whelen will have the heaviest bullet.

Because it has the greatest amount of room for expansion, per inch of bullet travel, it allows for the greatest pressure relief (space for the pressure to expand into) as the bullet progresses beyond obduration.
But it also has the least amount of obduration, when measured as a percentage of its overall diameter, allowing the slug's material to spread the force required over a greater volume.

 

[Spitpatch]

To clarify (and to reiterate the OP's question), Jamie, are you saying the principle reason we reduce our powder charges for larger (heavier) bullets is friction?

I can demonstrate that is not the principle reason, almost as easily as I demonstrated that the principle reason was not case volume invasion. (Jake2far gets credit here, as do some others.):

A .270 Nosler Ballistic tip of 130g weight, due to its all copper base and hollowpoint (polymer tipped), will have nearly the identical bearing surface of a 150g conventional lead-core soft point. Similar friction surfaces.

I still have to reduce my powder charge for the heavier bullet. I don't get away with the same charge because the bullet engages the rifling the same amount during its travel.

We could take this further, and use a Barnes all-copper bullet. I would venture to guess that this 130g bullet actually engages the rifling MORE than a 150g conventional bullet. MORE friction. I still have to reduce the powder charge for the 150g projectile.

Now: Friction IS a factor to a degree (because it is part of inertia, along with mass). Here's where jake2far agrees with Chemist (and me). Two bullets of the SAME weight, one a Barnes all-copper 130g, and the other a conventional lead-core spitzer of 130g weight: We need to reduce our powder charge (a moderate amount: nothing of the degree when we change weights) to allow for the higher pressures to be developed as a result of the Barnes bullet's greater length engaging the rifling to a greater surface area while it is traveling in the barrel (friction).

Nosler Ballstic Tips and Partitions will nearly always show a milder charge listed than for a conventional bullet of same weight for this very same reason.

But: It cannot be ignored that given two bullets OF THE SAME DESIGN, but of differing weights, the heavier bullet WILL create more friction, by its longer surface area engaging the rifling. My contention is that the weight itself is more of a factor toward reasons to reduce powder charge.

I don't believe friction in any scenario can match the significance of mass (weight) as the reason we reduce our powder charges for heavier bullets (because if we equalize friction between two different weight bullets, we still have to reduce the charge). Just as when we equalize case invasion by the two bullets, we still have to reduce our charge.

Friction, therefore cannot be the principle reason to reduce powder charges for larger (heavier) bullets.

 

[deadshot2]

All this can simply summed up as "You need to work up a new load whenever you change components".

It's just like in Math. Theorems, once proven, are accepted as rules from there on. Change bullet weights, change powder charges. simple as that. Ignore it and you could be introduced to the The Law of Momentum Conservation.

If you're unfamiliar, then follow the advice of my Dad (who would have been 110 next year) and "Look it Up".

 

[Spitpatch]

I have a hound dog that practices The Law of Momentum Conservation. I think it was a major factor in my life as a teenager (especially at 7AM) too.

 

[iusmc2002]

I always that that you reduced charges between different weight bullets was because the heavier bullet is going to move a little slower down the barrel, allowing a more complete powder burn. Obviously, you could move that heavier bullet at the same speed as the lighter bullet if you used extra powder, but like a couple people already stated, the heavier bullet has to overcome it's own weight, maybe a crimp, then get into the lands correctly, and deal with a (usually) larger/longer/more bearing surface, all without causing over-pressure and blowing up the gun and individual body parts.

I know some people are way more smarter than me, and this discussion has been very informative, but I also see that the more people think they know, the more condesending they usually are. Thanks Spitpatch and deadshot2 for your deeply informative posts. You're a funny SOB, Spitpatch

 

[Jamie6.5]

I never said that friction was the only factor involved. Only that it was/is the principal reason.

I offer as proof, given identically constructed bullets of the same materials, the difference in the amount of force required to roll a 150gr, or a 200gr bullet 1 foot across a smooth surface, vs the amount of force required to drive them down a rifled barrel that is properly sized to cause obduration/engraving from the rifling(lands). The most significant of these will be the percentage of increase.

Regarding solid copper bullets, overcoming their additional friction entails building the bullets with driving bands, that decrease the amount of material volume that must fully obdurate in conforming to the contours of the rifling. This is due to the additional hardness of copper over lead, despite the fact that they are lighter, or that copper is less dense.
If friction were of such small consequence, Barnes could build solid copper bullets without driving bands, that were identical in dimensions to lead bullets, and have the advantage of reduced weight.
There is one reason they don't, and that is friction. The greater amount of friction that is/would be created by the copper's greater resistance to obduration, (hardness) that makes driving bands necessary in their design.

Using the analogy in my previous post above, and the 30-06 cartridge case, the .35 Whelen is capable of pushing a 200 grain bullet over 2800FPS (Hodgdon's data) while the 30-06 is only capable of achieving the 2550FPS mark with a 200gr bullet, it often needs much higher pressure(s) in the effort.
Despite the fact that in nearly all load data, the .35 Whelen recipes call for faster powders. Powders that if/when used in the '06, at similar quantities, with a 200gr bullet, would generate dangerously and possibly catastrophically high pressures.
Similar results are seen with the .338-06 vs. the 30-06, using bullets of the same weight, type and brand.

Not inconsequential in this equation is the fact that two bullets of identical construction, by the same company (say, Nosler partitions for instance) lengths and bearing surfaces are significantly longer in the '06, generating more friction.

There are many factors involved in this phenomenon, like the "expansion chamber" explanation I used above. Also is the difference in shoulder dimensions when the case mouth is expanded to accommodate the larger diameter bullet.
But if the weight/mass of the bullet was the principal reason, I believe the velocity capabilities would be much closer, given that they are both based on the same cartridge case.

ETA: Perusing Hodgdon's data for the '06, I also noticed that the data for the Hornady 165gr GMX, a banded solid copper bullet, vs the Hornady 168gr HPBT match bullet, using the 4350 powders (H and IMR), that the 168gr lead HPBT with it's shorter bearing surface is listed as achieving over 100 more FPS, at lower maximum pressure, while employing MORE powder to do so.
Considering that the HPBT is slightly heavier, there must be a contributing factor regarding the difference(s) in friction.

If you can think of another reason, I'm happy to listen. (read)

 

[Spitpatch]

And, I held you to your premise ("principle"), allowing and acknowledging your claim (and my claim) that other factors are in play.

I'm not sure that comparisons of different calibers are helpful to narrow down a question of why we reduce powder charges for larger (heavier) bullets. I took the OP's question as regards to such varied bullets within the same caliber. Perhaps my narrow interpretation was incorrect. But when we introduce different case shapes or bore sizes (even minute ones), we open up an entirely larger can of worms, and it becomes more difficult to eliminate variables.

I would also note that the original Barnes copper bullets had no "driving bands", or "relief bands", and that not only were they added for friction reasons, but for reduction of fouling (a major complaint, along with accuracy for the original bullets).

I maintain that mass (weight) is the primary reason we should reduce powder charges for larger (heavier) bullets within the same caliber. Chemist's "inertia" is more comprehensive, in that it gives complete consideration to both friction AND mass. Equalize friction between two bullets of significantly different weight, and the powder charge still needs to be reduced significantly for the heavier bullet. Equalize weight while increasing friction (longer bullet, same weight), and the necessary reduction of powder charge for the longer bullet is present, but not dramatic.