The bigger the bullet, the less powder...

[Stalker62]

Just had a morning in the Cave, loading up some rounds for the AI.

I have used N140 in all the loads - bullets are 185gr, 175gr and 155gr

From Vihtavuori's web site I have loaded to the maximum for all bullets. What interested me was the lower recommended maximum charge weight for the heavier bullet.

I found this rather helpful explanation - which may (it may not) be informative to others.

​

It's not that the bullet is heavier, it's that heavier bullets tend to be larger. Larger bullets means it needs to be seated further into the case (due to max OALs for the cartridge design), thus resulting in lower case volume. This lower case volume means two things:

* less powder that can be put in the case (physically, unless you want to go into compressed loads)
* smaller initial volume in which ignition can occur in

Sure, you could try to use the same amount of powder from a lighter load in a heavier load but when considering those two above factors it causes one thing: significantly higher chamber pressures. Igniting the same amount of powder in a much smaller volume results in much higher pressures that eventually lead to (potentially) catastrophic results. That's why most SAAMI specs also list a maximum chamber pressure.

In layman's terms, and the analogy I've seen used and I've always liked is light a firecracker in your hand. One time, with your hand open, and another time with your hand squeezing it tightly. Which will hurt more? The open hand instance is with a smaller/lighter bullet with a larger chamber volume, and the closed hand is the larger/heavier bullet in a smaller chamber volume. Both have the same amount of powder, but depending on the space provided, the explosive reaction needs to go somewhere. That somewhere is your hand, or in the case of the firearm, the receiver. The receiver can only take so much pressure until it fails.

So when reloading specs are done, it's a balancing act (for the sake of this discussion) between the factors of case volume, powder burn rate (which we haven't discussed, but also why you see powder recommendations change depending on the weight/design of the bullet as there are tricks to using a slower burning powder so you don't see that huge pressure spike), and maximum case pressures to insure that there is some safety margin in which the firearm doesn't turn into an IED.

 

[Andy RV]

Based on that analogy, if the bullet was seated beyond the maximum COL to maintain the same case volume (without taking the effects of the lands into account) you could use the same charge weights?… you’d have to be brave!…

I’d recommend some more research as burn rate, and particularly the dynamic burn rate (vivacity) has a profound effect on peak pressures as the projectile weights increase, certainly more so than case volume.

The key points from me are that burn rate is proportional to pressure, peak pressure occurs while the bullet is within the first 1/4” of barrel, not still within the case, and that the bullet must accelerate at a sufficient rate to provide ample volume within the system to maintain a safe pressure level - See the Gas Laws.

Heavier bullets naturally generate a higher level of inertia which must be overcome, this leads to increased pressure levels and a greater burn rate as the volume within the system increases at a reduced rate to that of a lighter bullet.

Hence typically as bullet weight increases propellant burn rate should be decreased.

 

[borbal]

The firecracker example is not actually that good, unfortunately. The point about nitro powder is that the burning rate is proportional to the pressure. So, as the powder burns and turns into gas, the gas pressure increases. As the gas pressure increases, the powder burns faster. The faster the powder burns, the faster the pressure increases. The faster the pressure increases, the faster the powder burns.... You can see where this is going.

The rate at which the powder burns is controlled by the bullet moving forward in the barrel which then increases the volume. As the volume increases, the pressure reduces - or does not rise as fast as it would if the bullet were moving slower down the barrel.

For a given charge weight then, the maximum pressure reached will be lower for light bullets than for heavy bullets, because light bullets will accelerate faster down the barrel for a given pressure and so the volume will increase faster, and this will keep the rate at which the powder burns lower, which will in turn keep the maximum pressure lower.

Finally, as the bullet picks up velocity, there comes a point where the rate at which the volume is increasing equals the rate at which gasses are being produced as the powder burns. That is the moment of maximum pressure. After that, the bullet continues to accelerate and the volume increases even faster, and so the pressure starts to drop. However the maximum pressure reached will depend on the bullet weight and, very roughly, maximum chamber pressure increases as the square of the bullet weight.

Does that make sense...?

 

[Smellydog]

It's all bollocks really.
The loading data is written so because s62's rounds tomorrow could be in the Sahara desert.
For someone to write a ditty suggesting a catastrophe could happen because someone didn't reduce a charge for a heavier bullet is a knobed!
If that was true how come rifles don't explode at the proof house whereupon they are subjected to about 70% more than top service pressure?
People that read this nonsense must near poop their pant every time they pull a trigger!

How come these numpties never show any evidence derived from tests to demonstrate their scare mongering theories?
Ever likely the general public are **** scared of firearms!

Yes, a heavier bullet relatively to a lighter one of the same calibre will take longer to swage into the rifling.
Will it push the same powders constitution beyond its performance envelope? I doubt it. Why? Because no one makes a range of rifle powders that goes from good to out right dangerous on one chart of load options.
Firecracker in the hand my arse!

 

[Foxyboy43]

Hmmm
“From Vihtavuori's web site I have loaded to the maximum for all bullets”.
Maybe I have misread your statement but in the spirit of friendship I trust you have followed basic reloading safety procedures and worked up to the maximum looking for pressure signs throughout and that cartridge specs have been adhered to?
Just checking…

 

[Stalker62]

The firecracker example is not actually that good, unfortunately.

@borbal

To be fair, I did recognise that the analogy was aimed at the rather less 'well educated' folk (me), who usually pick up their learning from these books...




Thank you for taking the time to post a 'proper' explanation.

 

[Stalker62]

Why? Because no one makes a range of rifle powders that goes from good to out right dangerous on one chart of load options.
Firecracker in the hand my arse!

You make an interesting point.

Sadly, it caused me to spray my early morning espresso over my keyboard...

 

[Acm]

It's all bollocks really.
The loading data is written so because s62's rounds tomorrow could be in the Sahara desert.
For someone to write a ditty suggesting a catastrophe could happen because someone didn't reduce a charge for a heavier bullet is a knobed!
If that was true how come rifles don't explode at the proof house whereupon they are subjected to about 70% more than top service pressure?
People that read this nonsense must near poop their pant every time they pull a trigger!

How come these numpties never show any evidence derived from tests to demonstrate their scare mongering theories?
Ever likely the general public are **** scared of firearms!

Yes, a heavier bullet relatively to a lighter one of the same calibre will take longer to swage into the rifling.
Will it push the same powders constitution beyond its performance envelope? I doubt it. Why? Because no one makes a range of rifle powders that goes from good to out right dangerous on one chart of load options.
Firecracker in the hand my arse!

Agree to disagree on this one SD. I’ve been guilty of some rather sloppy reloading in the past where I had a fabulous load for a nosler 50grn bt in .222 using reloader 7, I lazily used the same charge and seating depth for the same bullet in 55 grn. And blew the extractor claw on my rifle to bits and caused head separation on the case.
Worth noting the 50grn load was well within max load and the same charge was less than 10% over max for the 55.

 

[borbal]

It's all bollocks really.

I think you know that it is not 'all bollocks' really....

Why? Because no one makes a range of rifle powders that goes from good to out right dangerous on one chart of load options.

Not sure what you mean by this. Reloading 'charts' are supposed to show safe loads and so by definition do not show 'dangerous' loads.

Reloading entrepreneurs who fancy they know a thing or two about reloading can be tempted to push the envelope on what are regarded as safe loads and see if they can get away with it. Most times they will. But, speaking from experience, things can go very non-linear very quickly once you start pushing the limits...

 

[Stalker62]

... things can go very non-linear very quickly once you start pushing the limits...

Beautiful example of 'understatement'.

 

[Smellydog]

Agree to disagree on this one SD. I’ve been guilty of some rather sloppy reloading in the past where I had a fabulous load for a nosler 50grn bt in .222 using reloader 7, I lazily used the same charge and seating depth for the same bullet in 55 grn. And blew the extractor claw on my rifle to bits and caused head separation on the case.
Worth noting the 50grn load was well within max load and the same charge was less than 10% over max for the 55.

How many times had that case been reloaded?
Was it over book for the 50gn?
How many grains over book was the load for the 55g Nosler?

 

[Smellydog]

Agree to disagree on this one SD. I’ve been guilty of some rather sloppy reloading in the past where I had a fabulous load for a nosler 50grn bt in .222 using reloader 7, I lazily used the same charge and seating depth for the same bullet in 55 grn. And blew the extractor claw on my rifle to bits and caused head separation on the case.
Worth noting the 50grn load was well within max load and the same charge was less than 10% over max for the 55.

Note the tiny over all length change.
Note same charge weight for reloader 7 in 222 50&55 GRN bullet

Alliant Powder - Reloader's Guide

www.alliantpowder.com

 

[Smellydog]

Not sure what you mean by this. Reloading 'charts' are supposed to show safe loads and so by definition do not show 'dangerous' loads.

As far as I am aware I have not seen a chart for a given rifle calibre with a range of suitable powders that give optimum recognised performance parameters for that cartridge and on the same chart give data that with one change, like bullet weight will make for a very dangerous situation.
You may well see one powder type dropped from the chart as bullet weight goes up because that in itself is suggesting a danger can arise but if a powder is listed across the weight of bullet range the provider is confident their suggested data is safe within an envelope of shall we say ' experimentation'.


In our friends relation to a failure in 222 we are not given all the information. 222 data is conservative due to some weaker actions out there for it. The brass however is the same thickness of 223. 222 has been pushed by thousands in strong actions by those that know. Not to 223 levels but somewhere between.
My 222 mostly used h322 and bl-c2.
The cases would tell me via the primer pocked and or a ring forming past the case head when they were passed any more use. But I can tell you I was pretty much over book on what ever bullet I used and never had an ' incident'. In fact the only incident I did ave in a 222, out of three rifles was with a factory round!

 

[Heym SR20]

Two things not mentioned

1) heavier bullets are longer and thus will have a larger surface area in contact with the bore. This will increase friction. So for the same level of pressure (effectively force on the base of the bullet) it will accelerate more slowly down the barrel.

2) Powder Burn Rate - as a bullet moves down the barrel the volume increases so pressures go down, unless the powder burn rate can keep up. If you are using a fast powder with a big bullet, the additional friction may be enough so that the burn rate is too fast and thus pressure increase faster than bullet can accelerate.

With bigger bullets you often want to use a slower burn powder to take into account the friction and inertia of a bigger bullet.

 

[borbal]

Have a play with my P-Max internal ballistics simulator where for a given charge weight of powder, you can see what happens to the pressure as you increase the bullet weight, without any fear of blowing yourself up as you push the limits...!

 

[Smellydog]

Take my wee 30/30 for example.
Switching from a 150 gn to 170 gn bullet with one suitable powder from Alliant the difference is around .8 of a grain! Another suitable powder from the same manufacturer the difference is 1.5gn!
Now, I know that with just a charge weight of half I can get another powder of theirs to give me around 3/4 the performance of standard service.
So I could reason that just a little more will get me more performance but be way more frugal!
But wait.... Alliant don't list the powder I am referring to in 30/30 data! So there is the clue to NOT to push it!
But what if I got those tiny changes the wrong way around with the suitable powders in their data, mis read the data, nothing, nothing catastrophic anyway. The manufacturer's are fully aware of idiots like me!

 

[Heym SR20]

With most rifles there is probably little need to run at max load / velocity. Throttle it back and you probably get a more accurate load and you lessen the stress on the rifle, your shoulder, the barrel and your wallet.

Doubt any deer or fox will notice the difference. You might have add a couple of clicks if dialling in for long range.

 

[johndeere]

start [Gordons Reloading Tool Community]

grtools.de

have a play around with Gordons tool

 

[Smellydog]

With most rifles there is probably little need to run at max load / velocity. Throttle it back and you probably get a more accurate load and you lessen the stress on the rifle, your shoulder, the barrel and your wallet.

Doubt any deer or fox will notice the difference. You might have add a couple of clicks if dialling in for long range.

Yep, shot my longest fox with a 222 just pootling along at 2850 FPS. A 52 gn bullet at 350yds plus.