Why is load data higher for a lighter boolit?

[BHuij]

When I first started casting, it was for 9mm. I have made and fired thousands of the Lee 9mm 124gr RN TL boolits out of my M&P. After doing a little testing, my best loads are 3.9 grains of Bullseye or 4.0 grains of W231.

I'm now experimenting with some different boolits in this gun, and yesterday loaded up a handful of Lee 105gr SWCs to test out. My Lyman cast bullet handbook (4th ed) doesn't have load data for that exact boolit, but it does have data for a very similar 102gr RN, so I went with that. The loads for Bullseye range from 4.6 grains to 5.1 grains of powder.

I guess I'm a little confused by this. The reason I wanted to try this bullet was because it's lighter and possibly more accurate than my 124gr RN. I'm not trying to use it for defense, hunting, or any other purpose that requires me to hit specific velocities, I'm only punching paper with it. I would have figured a lighter boolit requires LESS powder, not more? I followed the load data, so I have 10 rounds loaded up with 4.6 grains, but if anyone has any insight as to why I couldn't just load up like 3.5 grains of Bullseye and have a nice, soft, target load, I'm curious to hear.

[trails4u]

Pressure. A heavier bullet is more resistant to movement, so a heavier powder charge behind a heavier bullet would dramatically increase pressure. The same powder charge behind a heavier bullet would also increase pressure, as the object is heavier(the bullet) thus more resistant to movement.

[BHuij]

Makes sense why I couldn't put 5.1 grains of Bullseye behind a 124gr boolit then, but is there some minimum amount of pressure I have to hit that requires me to have at least 4.6 grains behind the 105gr SWC?

[trails4u]

I don't know that you have to hit any certain minimum pressure, other than what would be required to cycle the slide, and obviously enough to ensure you don't squib something and leave a barrel obstruction. Try the search function for 'mouse fart loads' for 9mm. Many others have experimented with low pressure/low velocity stuff.

[country gent]

Several reasons for the lighter bullets requiring more powder to reach "peak" operating pressures.

1) the lighter bullets are shorter and take up less case capacity. In the smaller cases this is more pronounced.

2) the lighter bullets move easier ( as mentione above), less resistant.

3) Lighter bullets often have shorter driving lengths on the bullet lowering friction and resistance

Most powders are meant to operate in a certain pressure range to low and it may burn dirty or incomplete, to high and pressure spikes or erratic velocities. When in the range it performs as desired. Some powders are more prone to this. Start low and work up with a given combination. Pay attention to accuracy, fouling levels, not just leading but powder fouling also. And functioning of the firearm. Watch for "sooty" cases ( not enough pressure to seal the chamber and gasses are getting around the case between case and chamber). Watch primers for set back flattening and cratering.

You should find a very good load with the 105 grn that is accurate clean shooting and functions the firearm easily. Start low and work up to the level you want to be at as far as accuracy, velocity and function.

[Mr_Sheesh]

If you use a faster powder you can sometimes get the required pressures with less of a faster powder; Do beware of double charges of course...

Powder Speeds, Fastest to Slowest
1 NORMA R1
2 Winchester WAALite
3 VihtaVuori N310
4 Accurate Arms Nitro 100
5 Alliant e3
6 Hodgdon TITEWAD
7 Ramshot Competition
8 Alliant Red Dot
9 Alliant Prom
10 Hodgdon CLAYS
11 Hodgdon Clay Dot
12 IMR, Co Hi-Skor 700-X
13 Alliant Bullseye
W-231 falls at # 29 on that powder speed list so it's not the faster powder.

Best to look for published loads, the 'mouse fart' or I've also heard some name like 'cat sneeze' loads somewhere, should help.

[Texas by God]

Let us know how it turns out. I have the same reloading project in the works.

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