Why Different Powder Loads For Same Grain Bullets of Different Shapes ?

[daddyseal]

I Am Talking only about handgun loads and bullets here.
And Also...why does Cast, or Jacketed have different load data for sale caliber and grain weight?

[btroj]

Depends on bearing surface length, how much of the bullet is inside case, and material.
Lead deforms much easier than a jacketed bullet so it generally has lower pressure for same load.
A bullet with more length inside the case can increase pressure, this is more an issue with handgun cartridges.
Even amongst jacketed there are differences. The solid bullets like the older Barnes X bullet required a lower powder charge and the bullets makeup raised pressure. Bearing surface length makes a huge difference in jacketed bullets.
With cast in handguns much of the work by LBT was to get more weight out of the case. This reduced pressure some allowing more powder and more velocity.

None of this is an absolute. It is about balancing many, many factors to get velocity out of an acceptable pressure.

[429421Cowboy]

Different bullets/boolits of the same weight but a different profile often have different lengths of bearing surface, resulting in more or less pressure going down the bore, thus requiring different powder charges to maintain safe pressures in the load.

Cast boolits generate lower pressure for a given weight than do jacketed bullets, because of how much harder the copper jacket is compared to a lead boolit. Thus, i can drive a boolit of the same weight with less powder and presssure than a bullet of the same weight and similar shape. Part of the beauty of cast my friend!

[429421Cowboy]

btroj, you beat me by 3 minutes and posted a more complete answer than i did!

[btroj]

Luck of the draw?

I feel the pain of the OP. I remember years ago when I had the same questions. Some of the things that drove me nuts then seem so simple now. It is amazing what years of experience does for knowledge.

[daddyseal]

Depends on bearing surface length, how much of the bullet is inside case, and material.
Lead deforms much easier than a jacketed bullet so it generally has lower pressure for same load.
A bullet with more length inside the case can increase pressure, this is more an issue with handgun cartridges.
Even amongst jacketed there are differences. The solid bullets like the older Barnes X bullet required a lower powder charge and the bullets makeup raised pressure. Bearing surface length makes a huge difference in jacketed bullets.
With cast in handguns much of the work by LBT was to get more weight out of the case. This reduced pressure some allowing more powder and more velocity.

None of this is an absolute. It is about balancing many, many factors to get velocity out of an acceptable pressure.
I am only asking a bout handgun reloading here.
For a 9mm 124grain bullet...setting depth is different for cast than jacketed?
Please tell me how much?

[btroj]

How much isn't a set thing. Bearing surface makes a difference. So does powder and charge.
In a 9mm the seating depth is very critical. Small case means a tiny difference in searing depth can lead to a huge pressure difference.
Not all round nose bullets have the same ogive radius. I look far more at how much bullet is in the case than anything else. More bullet in the case means smaller combustion chamber, that leads to a higher pressure.

The lead bullet will generally give more velocity for the same load as it has much less friction and requires less energy to engrave the rifling.

You also need to make sure the loaded rounds feed thru the magazine so making them too long isn't an option.

Look at a Lyman manual. The differences in OAL for very similar bullet weights give you an idea of how critical this can be. 9mm cases are small enough that OAL is very critical. Deep seat even a tenth of an inch and you can ge into pressure troubles quickly.

[daddyseal]

Different bullets/boolits of the same weight but a different profile often have different lengths of bearing surface, resulting in more or less pressure going down the bore, thus requiring different powder charges to maintain safe pressures in the load.

Cast boolits generate lower pressure for a given weight than do jacketed bullets, because of how much harder the copper jacket is compared to a lead boolit. Thus, i can drive a boolit of the same weight with less powder and presssure than a bullet of the same weight and similar shape. Part of the beauty of cast my friend!


For a 147 grain 9mm bullet...over all length is 1.115 on jacketed...and 1.058 on cast in my Lyman manual. That's only 0.06 difference...I guess I'll need to calibrate my dial calipers!

Also, please tell me what the books abbreviations on bullets of "BC" and "SD" mean?

[429421Cowboy]

Seating depth is often controled by where the bullet begins to touch the rifling, therefore it could be more bullet/boolit shape than cast vs jacketed.

BC Stands for ballistic coefficient and is defined as "the ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration —a high number indicates a low negative acceleration. BC is a function of mass, diameter, and drag coefficient. It is given by the mass of the object divided by the diameter squared that it presents to the airflow divided by a dimensionless constant that relates to the aerodynamics of its shape." Per wikipedia
The higher the BC the less drag a bullet has so less velocity loss, less wind drift.
SD stands for sectional density which is basically a measure of the bullets length to width, a higher SD will also help a bullet retain velocity better as well as penetrate well, which is why most long range riflemen shoot heavy for caliber bullets in rifles, with the 6.5mm calibers being very popular because of the high SD of their bullets.

[geargnasher]

For a 147 grain 9mm bullet...over all length is 1.115 on jacketed...and 1.058 on cast in my Lyman manual. That's only 0.06 difference...I guess I'll need to calibrate my dial calipers!

Also, please tell me what the books abbreviations on bullets of "BC" and "SD" mean?

If you're going to dink with cast boolits, forget every single thing you ever knew or thought you knew about jacketed. You can also pretty much ignore the OAL data with cast boolits except don't go too much shorter than the book says (ESPECIALLY on the 9mm) due to reducing the case capacity and spiking pressures. Always begin with starting loads anyway, just to be sure, use common sense, and watch for pressure signs. For example, if the pistol throws brass 25 feet and bulges the head with a starting load from a good, published source, your particular system is doing something to raise pressure and you need to figure out what it is.

For any cast boolit gun that has a fixed chamber, use the chamber for a case gauge. In other words, seat the boolit out until it touches something when it chambers. In auto pistols, we call this the "plunk test". Remove the barrel and start a boolit in an unprimed, sized, and expanded case. Keep testing and adjusting the boolit depth until it falls into the chamber on it's own, and is flush with the barrel hood. You might have to crimp it a bit to keep the bell mouth from interfering with this process. Seat it a few thousandths deeper for a margin of error and then make sure the round will go in the magazine and feed from the magazine. Same thing goes for rifles, although you can seat the boolits for a slight resistance when camming the action closed on bolt guns. There is no pressure increase by doing this like there is with jacketed.

Use cast boolit load data, not jacketed. There is a difference and a good reason for it due to the lower resistance of cast boolits.

Gear

[swheeler]

For a 147 grain 9mm bullet...over all length is 1.115 on jacketed...and 1.058 on cast in my Lyman manual. That's only 0.06 difference...I guess I'll need to calibrate my dial calipers!

Also, please tell me what the books abbreviations on bullets of "BC" and "SD" mean?

BC= ballistic coeffient, SD= sectional density

[TCTex]

The best example I can come up with from my experiences are Boat Tail and Plain Base bullets. In a rifle, If you shoot a 150gr BT and a 150 PB, (pick your favorite caliber and components,) the PB will show pressure signs before the BT will just because of surface bearing variable already mentioned.


I have also found, from tinkering with my long barreled TC handguns, that I can “push” a cast bullet to around 2000 to 2200 fps with less powder than its jacketed counter part.


Just my experiences so take them for what they are worth… LOL

[daddyseal]

If you're going to dink with cast boolits, forget every single thing you ever knew or thought you knew about jacketed. You can also pretty much ignore the OAL data with cast boolits except don't go too much shorter than the book says (ESPECIALLY on the 9mm) due to reducing the case capacity and spiking pressures. Always begin with starting loads anyway, just to be sure, use common sense, and watch for pressure signs. For example, if the pistol throws brass 25 feet and bulges the head with a starting load from a good, published source, your particular system is doing something to raise pressure and you need to figure out what it is.

For any cast boolit gun that has a fixed chamber, use the chamber for a case gauge. In other words, seat the boolit out until it touches something when it chambers. In auto pistols, we call this the "plunk test". Remove the barrel and start a boolit in an unprimed, sized, and expanded case. Keep testing and adjusting the boolit depth until it falls into the chamber on it's own, and is flush with the barrel hood. You might have to crimp it a bit to keep the bell mouth from interfering with this process. Seat it a few thousandths deeper for a margin of error and then make sure the round will go in the magazine and feed from the magazine. Same thing goes for rifles, although you can seat the boolits for a slight resistance when camming the action closed on bolt guns. There is no pressure increase by doing this like there is with jacketed.

Use cast boolit load data, not jacketed. There is a difference and a good reason for it due to the lower resistance of cast boolits.

Gear
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