More powder charge on lighter bullet than heavier, why?

[wjc]

I'm new on reloading. When I read the receipt on .40S&W, suggested using Unique powder on:
8.5 gr on 135 JHP bullet
6.4 gr on 180 JHP bullet

Could you tell me the reason why the lighter bullet need more powder charge than heavier bullet?

If I charge the bullet as suggested receipt, the recoil is much sharper on lighter bullet than heavier bullet? More powder more recoil is that true?

Could I lower the charge on lighter bullet, e.g. 6.4 gr on 135 JHP bullet?

Thanks! :)

[Hunter]

The lighter bullet can use a heaver charge (because the lighter bullet can be propelled faster than a heaver bullet) but does not have to use the heavy charge. The lighter bullet will also have a heavier max powder charge which will propel the bullet faster than the heaver bullet. Since it takes less energy to move the lighter bullet more energy can be applied than a heavier bullet (before an equilibrium is reached with pressure that is moving the mass). A light bullet moving fast can make the same energy as a heaver bullet moving slow. Example a 155gr .40 caliber bullet moving at 1000fps (about middle of the road velocity) is making 344 ft lbs of energy and a 180gr 40 caliber bullet moving at 900fps is making 324 ft lbs and that same bullet at 950 fps is making 361 ft lbs . Even though the bullet does not weight the same energy can be made up in the lighter bullet by increasing it's velocity. I am not great at explaining (or understanding) physics but I hope this helps. I would suggest getting a quality loading manual and consult that as far as powder charge and begin at the light end of the scale and work your way up watching for pressure signs (explained in the front of your loading manual) and never exceed the max charge listed in the manual.

[Joni Lynn]

One more thing about a lighter bullet. It's shorter, so when loaded to about the same overall length you will have more capacity in the case which all other things being equal would give less pressure. The deeper the bullet is seated, the higher the pressure, of course that is with all other things being equal.

[Woodman]

Physics simplified.

Say you're loading 230gr .45ACP and 185gr .45ACP rounds. You keep everything else the same except the bullet itself. Powder, seating depth, etc are all identical.

If you use the same powder charge, the .230gr bullet will travel faster due to it's heavier initial weight resisting the explosion behind it trying to make it move. Because of that increased resistance due to it's heavier weight, it takes less powder to build up the necessary pressure. (I'm sure the longer bullet length factors into this as well, but I don't think it matters as much as the weight of the bullet).

The 185gr bullet will need more powder to build more initial pressure because it's lighter weight allows it to be pushed by the explosive force easier.

Please remember, I'm not a physicist, I only play one on the internet. If I am mistaken, please correct me so I can apologize for being an idiot. :)

[LeeCreekKid]

Woodman has said it the way I was taught when learning to reload. The lighter bullet takes less pressure to move it so a heavier powder charge is necessary to develop the same velocity.

[TheGerk]

I'm new on reloading. When I read the receipt on .40S&W, suggested using Unique powder on:
8.5 gr on 135 JHP bullet
6.4 gr on 180 JHP bullet

Could you tell me the reason why the lighter bullet need more powder charge than heavier bullet?

If I charge the bullet as suggested receipt, the recoil is much sharper on lighter bullet than heavier bullet? More powder more recoil is that true?

Could I lower the charge on lighter bullet, e.g. 6.4 gr on 135 JHP bullet?

Thanks! :)
QUOTE=wjc]I'm new on reloading. When I read the receipt on .40S&W, suggested using Unique powder on:
8.5 gr on 135 JHP bullet
6.4 gr on 180 JHP bullet

Could you tell me the reason why the lighter bullet need more powder charge than heavier bullet?

Like Woodman said, It’s really all about pressure. Your .40 cal operates at a SAAMI Maximum Average Pressure (MAP) of 35,000 P.S.I.
You can stoke the lighter stuff hotter because the lighter bullets generate less pressure.
Pressure is also effected by actual bullet diameter, actual land and groove diameter, powder burn rates, load density, powder charge weights, temperature, altitude, case crimp, case construction, case length, seating depth, bullet shape and type (there is a lot going on with pressure)

If I charge the bullet as suggested receipt, the recoil is much sharper on lighter bullet than heavier bullet?
It’s not always linear; sometime it’s caliber specific, or gun specific. If I load my 500 magnum up to max load with a 350gr bullet it is no were near the recoil of the max load with a 400gr bullet even with less powder.

More powder more recoil is that true?
Depends on caliber, bullets, guns

Could I lower the charge on lighter bullet, e.g. 6.4 gr on 135 JHP bullet?
Be safe and use the recommended minimum charges for your combination. In the early nineties after the .40 came out there was some issues of case failure on the .40 it was discovered that the .40 experienced radical pressure spikes when minimally or under loaded. In many load manuals you will see a warning not to go below recommended start weights.
Good Luck
Thanks! :)[/QUOTE]

[Woodman]

Regarding the recoil on a heavier bullet vs. a lighter bullet...

Recoil is a function of mass. The effort it takes to move a heavier object will have a stronger effect (recoil) than the effort to move a lighter object.

That's why a lighter bullet typically has less felt recoil, the bullet is lighter, so all the Newtonian physics say that every action has an equal and opposite reaction, thus the action that moves a 200gr lump of lead will have a lesser opposite reaction (recoil) than the action that moves a 230gr lump of lead.

As the Gerk mentioned though, there are other factors involved than just the amound of lead leaving the barrel. How the gun handles the recoil and ejection of leftover gasses has an effect as well, so the effect will vary by some degree between different styles of gun.