Latitude loading auto vs revolver cartridges

[BondoBob]

I'm noticing that there is very little range in loads between min and max with 380 and 9mm. I see a range of about 3 or 4 10ths of a grain on average. While my revolver load data shows as much as 2 full grains of range. Before I start to attempt reloading autos, I wanted to check if I'm just looking at the wrong recipes or this is just the way it is. I see my Lee safety scale and a digital scale I got on Amazon for $30 disagree by abut 2 10ths some times. Would I need a much better scale before doing these loads in my autos? Or are the consequences of being under or over by a 10th of a grain not much of a big deal. Ultimately, I think I'd be loading the the exact middle weight. But in the beginning I know you have to start at the minimum.

[Unclenick]

The range is based on changing the charge weight by a percentage, not by some fixed number of grains. That is because different chamberings have different case capacities, so the number of grains that reduce the pressure a certain amount in one cartridge will reduce it by a different amount in another.

The most common practice is to reduce any maximum load by 10% to get a starting load. Though it varies with the powder characteristics, that commonly results in somewhere in the vicinity of a 20% reduction in pressure (pressure changes exponentially with powder charge). Reducing pressure to ensure a safe level is what the starting load is for. But this means a powder with a 1.5 grain maximum in 25 Auto would have a theoretical starting load of 1.35 grains, or 0.15 grains reduction, which is a 10% reduction from 1.5 grains. That calls for more resolution than a lot of powder scales have. Also, note that Western Powders actually recommends reducing rifle loads by 10% from maximum to find a starting load, but recommends reducing handgun loads by 15% from maximum to find a starting load. I'm sure part of that is the problem with scale resolution, but it is a good practice with the exception of a few slow magnum powders which may not be safely reduced much without risking squibbing out and leaving bullets stuck in the barrel.

To reduce a load by 10%, multiply the maximum charge by 0.9 to get your 10% reduced load.

To reduce a load by 15%, multiply the maximum charge by 0.85 to get your 15% reduced load.

[rclark]

Case volume is the difference. In 9mm, .45 Auto, .380 .... have little volume, so a 0.2g of powder can make a big difference in pressure. Where-as a .357 cartridge has much more case volume than a 9mm, so 0.2 makes little difference. Also, remember, a semi-auto cartridge has to have enough power to cycle the slide, so the minimum load has to be higher than a .357, or .45 Colt, or.... Hope that makes sense to you.
{ I see above says what I said after I 'reread' it ... }.