Smaller bullets, more powder. Why is that?

Gunnels · December 20, 2013, 01:21 PM

I have been reloading 9mm mostly in 147gn and 124gn. Recently I was experimenting with reloading smaller bullets in 9mm. I was reloading 100gn bullets. I noticed that, based upon the load data I have from several sources, the minimum and maximum recommended powder weight was more for smaller bullets. Sometimes much more. Just as an example, Lyman recommends a maximum load for a 95gn jacketed bullet in Bullseye to be 5.2gn. For a 147gn jacketed bullet they recommend 3.9gn.

I was expecting just the opposite to be true.

PawPaw · December 20, 2013, 01:41 PM

Pressure. It takes more powder to get to a certain pressure level with a lighter bullet. The effect is even more pronounced in rifle cartridges.

SL1 · December 20, 2013, 01:41 PM

What you are looking at is the maximum load and a start load which is usually just 90% of whatever the maximum loads is. A true minimum load is not provided by most manuals, although the Hornady manual often showes minimums.

So, what you are seeing is the maximum amount of powder that can be loaded under each bullet without the peak pressure exceeding the same number (SAAMI peak pressure limit for that cartridge). When a lighter bullet is loaded in a case to the same overall cartridge length as a heavy bullet, it takes-up less room inside the case, thus leaving more space for the powder. That lowers the peak pressure if the same charge is used as for the heavier bullet. In addition, the lighter bullet accelerates faster than the heavier bullet, so by the time the peak pressure is reached, the lighter bullet has also provided more additional room for the powder gases, which also lowers the peak pressure if the same charge is used. So, it is possible to put a lot more powder under a lighter bullet and stay within the peak pressure limit.

Now, because a little knowledge is a dangerous thing, I should warn you that there are other important factors at play. One is that the rate at which smokeless powders burn depends strongly on the pressure at each instant that the powder is burning, plus a lot of things like grain shape and coatings on the powder's surface. So, you can't just use the Perfect Gas Law to figure out how to adjust a powder charge for a different bullet weight.

What really happens when a cartridge is fired is a race between (1) the powder trying to burn faster because it is raising pressure in a confined space as it releases more gas, and (2) the bullet's motion trying to make the powder burn slower by givng those gases more room and thus lowering the pressure. Basically the powder wins up to a point, which is when the peak pressure occurs, and then the bullet wins and the pressure starts going down.

SL1

Nick_C_S · December 20, 2013, 01:57 PM

SL1 kinda nailed it.

Perhaps an oversimplification, but one way to look at it is that lighter bullets give the effect of having a slower powder (conversely, heavier bullets give the effect of having a faster powder). For the reasons explained by SL1.

If you look at load data for a given bullet, you'll see that your slower powders usually call for higher powder charges. Well it's the same with lighter bullets - they usually call for higher powder charges too.

(Somehow, that makes sense in my head

)

BigD_in_FL · December 20, 2013, 02:03 PM

It also holds true even when loading shotgun shells; lighter payloads need more powder to get pressures up for effective powder burn and performance.

steve4102 · December 20, 2013, 02:07 PM

It's all about Newton's Second Law.

Gunnels · December 20, 2013, 02:16 PM

Thanks all for the detailed explanation. I was getting nervous with how much powder I was putting in the shell and still staying within the recommended limits.

Btw, is there some place is can check to determine the total volume in cc for the various caliber shells?

Wreck-n-Crew · December 20, 2013, 02:17 PM

Is this the way you thought it was ?:

A pick up needs more power to haul heavier loads. Picture the cartridge as the truck, the charge as the engine and the load as the bullet weight. The problem with this analogy is the "engine" part of this equation and Pressure is your engine and not powder charge.

The pressure drives the bullet and pressure needs resistance. Resistance is higher in heavier bullets and less powder is needed to build up pressure in heavier bullets and more I lighter bullets where resistance is much lighter.

Wreck-n-Crew · December 20, 2013, 02:19 PM

Three responses before I typed out a few lines!....see how fast these guys are willing to help?

xcracer160 · December 20, 2013, 04:58 PM

i learned something new already

57K · December 20, 2013, 05:18 PM

There's a little recoil geometry involved as well. To make the slide function properly, a lighter bullet must have higher velocity to generate a similar slide velocity. You can easily determine that with a calculation of Power Factor that is simply Bullet Weight X Velocity / 1000. Say you're shooting IDPA and want a Minor power factor load with just enough extra power for a safety cushion at say 130 PF. In 9mm with a 147 gr. bullet, velocity will need to be 884 FPS. With a 124, 1048 FPS. With a 115, 1130 FPS and with a 95 gr. bullet you'll need 1368 FPS.

The comments on pressure and physics are good and each bullet weight will have it's own pressure peak for that weight and the powder used. While all of the above mentioned 130 PF loads can probably be achieved with Bullseye, there are better powders that are more pressure stable toward the medium burn rate area, or closer to it, anyway. Unique isn't a bad choice if it's available. I like spherical powders at a similar burn rate or slightly slower like WSF and particularly True Blue and Silhouette, but there a number of good choices and with light bullets like 95 grs., some typically used 9mm powders can be a bit too slow in burn rate like say AA#7.

December 20, 2013, 01:21 PM	#1
Gunnels Member Join Date: March 23, 2013 Location: Dallas, TX Posts: 76	Smaller bullets, more powder. Why is that? I have been reloading 9mm mostly in 147gn and 124gn. Recently I was experimenting with reloading smaller bullets in 9mm. I was reloading 100gn bullets. I noticed that, based upon the load data I have from several sources, the minimum and maximum recommended powder weight was more for smaller bullets. Sometimes much more. Just as an example, Lyman recommends a maximum load for a 95gn jacketed bullet in Bullseye to be 5.2gn. For a 147gn jacketed bullet they recommend 3.9gn. I was expecting just the opposite to be true.

December 20, 2013, 01:41 PM	#2
PawPaw Senior Member Join Date: December 24, 2010 Location: Central Louisiana Posts: 3,137	Pressure. It takes more powder to get to a certain pressure level with a lighter bullet. The effect is even more pronounced in rifle cartridges. __________________ Dennis Dezendorf http://pawpawshouse.blogspot.com

December 20, 2013, 01:57 PM	#4
Nick_C_S Senior Member Join Date: March 21, 2013 Location: Idaho Posts: 5,523	SL1 kinda nailed it. Perhaps an oversimplification, but one way to look at it is that lighter bullets give the effect of having a slower powder (conversely, heavier bullets give the effect of having a faster powder). For the reasons explained by SL1. If you look at load data for a given bullet, you'll see that your slower powders usually call for higher powder charges. Well it's the same with lighter bullets - they usually call for higher powder charges too. (Somehow, that makes sense in my head ) __________________ Gun control laws benefit only criminals and politicians - but then, I repeat myself. Life Member, National Rifle Association

December 20, 2013, 02:17 PM	#8
Wreck-n-Crew Senior Member Join Date: March 8, 2013 Location: Ohio Posts: 1,820	Is this the way you thought it was ?: A pick up needs more power to haul heavier loads. Picture the cartridge as the truck, the charge as the engine and the load as the bullet weight. The problem with this analogy is the "engine" part of this equation and Pressure is your engine and not powder charge. The pressure drives the bullet and pressure needs resistance. Resistance is higher in heavier bullets and less powder is needed to build up pressure in heavier bullets and more I lighter bullets where resistance is much lighter. __________________ If you ever have to use a firearm, *you don't get to pick the scenario!*

December 20, 2013, 02:19 PM	#9
Wreck-n-Crew Senior Member Join Date: March 8, 2013 Location: Ohio Posts: 1,820	Three responses before I typed out a few lines!....see how fast these guys are willing to help? __________________ If you ever have to use a firearm, *you don't get to pick the scenario!*

December 20, 2013, 01:41 PM	#3
SL1 Senior Member Join Date: November 8, 2007 Posts: 2,001	What you are looking at is the maximum load and a start load which is usually just 90% of whatever the maximum loads is. A true minimum load is not provided by most manuals, although the Hornady manual often showes minimums. So, what you are seeing is the maximum amount of powder that can be loaded under each bullet without the peak pressure exceeding the same number (SAAMI peak pressure limit for that cartridge). When a lighter bullet is loaded in a case to the same overall cartridge length as a heavy bullet, it takes-up less room inside the case, thus leaving more space for the powder. That lowers the peak pressure if the same charge is used as for the heavier bullet. In addition, the lighter bullet accelerates faster than the heavier bullet, so by the time the peak pressure is reached, the lighter bullet has also provided more additional room for the powder gases, which also lowers the peak pressure if the same charge is used. So, it is possible to put a lot more powder under a lighter bullet and stay within the peak pressure limit. Now, because a little knowledge is a dangerous thing, I should warn you that there are other important factors at play. One is that the rate at which smokeless powders burn depends strongly on the pressure at each instant that the powder is burning, plus a lot of things like grain shape and coatings on the powder's surface. So, you can't just use the Perfect Gas Law to figure out how to adjust a powder charge for a different bullet weight. What really happens when a cartridge is fired is a race between (1) the powder trying to burn faster because it is raising pressure in a confined space as it releases more gas, and (2) the bullet's motion trying to make the powder burn slower by givng those gases more room and thus lowering the pressure. Basically the powder wins up to a point, which is when the peak pressure occurs, and then the bullet wins and the pressure starts going down. SL1

December 20, 2013, 02:03 PM	#5
BigD_in_FL Junior member Join Date: December 20, 2012 Location: The "Gunshine State" Posts: 1,981	It also holds true even when loading shotgun shells; lighter payloads need more powder to get pressures up for effective powder burn and performance.

December 20, 2013, 02:07 PM	#6
steve4102 Senior Member Join Date: December 23, 2005 Location: Minnesota Posts: 2,955	It's all about Newton's Second Law.

December 20, 2013, 02:16 PM	#7
Gunnels Member Join Date: March 23, 2013 Location: Dallas, TX Posts: 76	Thanks all for the detailed explanation. I was getting nervous with how much powder I was putting in the shell and still staying within the recommended limits. Btw, is there some place is can check to determine the total volume in cc for the various caliber shells?

December 20, 2013, 04:58 PM	#10
xcracer160 Member Join Date: December 19, 2013 Posts: 32	i learned something new already Last edited by xcracer160; December 20, 2013 at 05:04 PM.

December 20, 2013, 05:18 PM	#11
57K Junior member Join Date: March 2, 2013 Location: Heart of Texas Posts: 788	There's a little recoil geometry involved as well. To make the slide function properly, a lighter bullet must have higher velocity to generate a similar slide velocity. You can easily determine that with a calculation of Power Factor that is simply Bullet Weight X Velocity / 1000. Say you're shooting IDPA and want a Minor power factor load with just enough extra power for a safety cushion at say 130 PF. In 9mm with a 147 gr. bullet, velocity will need to be 884 FPS. With a 124, 1048 FPS. With a 115, 1130 FPS and with a 95 gr. bullet you'll need 1368 FPS. The comments on pressure and physics are good and each bullet weight will have it's own pressure peak for that weight and the powder used. While all of the above mentioned 130 PF loads can probably be achieved with Bullseye, there are better powders that are more pressure stable toward the medium burn rate area, or closer to it, anyway. Unique isn't a bad choice if it's available. I like spherical powders at a similar burn rate or slightly slower like WSF and particularly True Blue and Silhouette, but there a number of good choices and with light bullets like 95 grs., some typically used 9mm powders can be a bit too slow in burn rate like say AA#7.