Why does .38 need more powder than .380 for same bullet weight?

[collards]

I've got a ruger LCR and I bought a thousand 100 gr wadcutter bullets I thought to use as light shooting practice loads.

My load manual is recommending a starting load of 5.0 gr of unique for this bullet in a .38 special. However, the same weight bullet in .380 has a recommended weight of 3.1. I've loaded and shot some 5.0 loads and they are fairly hot, hotter than what I'm looking for anyway. What is the danger in downloading the load significantly? Pressure spikes?

[kneelingatlas]

Should be a problem, load 'em down.

[atbarr]

The .380 has less case capacity than the .38 sp, so you need less powder.

[MikeRush]

You could have inconsistent ignition with that much air space. I think a squib would be more likely than a pressure spike though Trail boss would be a great powder for what you are looking for, much bulkier and makes a nice soft 38 load. Pleasant even in a 15 oz J frame.

[Youngeyes]

I've got a ruger LCR and I bought a thousand 100 gr wadcutter bullets I thought to use as light shooting practice loads.

My load manual is recommending a starting load of 5.0 gr of unique for this bullet in a .38 special. However, the same weight bullet in .380 has a recommended weight of 3.1. I've loaded and shot some 5.0 loads and they are fairly hot, hotter than what I'm looking for anyway. What is the danger in downloading the load significantly? Pressure spikes?

Not sure what you are asking here. The difference between a 38spl and a .380? IMO you should not be comparing the two for load data. Way apples and oranges here. If you load down to a .380 level load for a 38 spl you may get a dirty gun from not fully burning the powder. You may get some breech face erosion from blowback from the primer not fully sealing. If you load low enough you may even get a squib rd. What you won't get is pressure spikes. Are you loading .380 bullets into a 38spl gun? The .380 bullet is a hair smaller than a .38spl. BTW, Where did you find load data on a 100 gr wadcutter with unique powder in a 38Sspl?

[collards]

I've got a ruger LCR and I bought a thousand 100 gr wadcutter bullets I thought to use as light shooting practice loads.

My load manual is recommending a starting load of 5.0 gr of unique for this bullet in a .38 special. However, the same weight bullet in .380 has a recommended weight of 3.1. I've loaded and shot some 5.0 loads and they are fairly hot, hotter than what I'm looking for anyway. What is the danger in downloading the load significantly? Pressure spikes?

Not sure what you are asking here. The difference between a 38spl and a .380? IMO you should not be comparing the two for load data. Way apples and oranges here. If you load down to a .380 level load for a 38 spl you may get a dirty gun from not fully burning the powder. You may get some breech face erosion from blowback from the primer not fully sealing. If you load low enough you may even get a squib rd. What you won't get is pressure spikes. Are you loading .380 bullets into a 38spl gun? The .380 bullet is a hair smaller than a .38spl. BTW, Where did you find load data on a 100 gr wadcutter with unique powder in a 38Sspl?

You've answered my question, and I appreciate it. I'm not really comparing .380 to .38 special, just wondering what kind of issues the increased .38 headspace can cause.

I think the answer here probably is some trail boss powder. unique is just not going to be useful for this application.

[njl]

The .380 has less case capacity than the .38 sp, so you need less powder.

Huge difference in case volume. PV=nRT, or P=nRT/V. So, pressure is inversely proportional to volume. Double the case volume, and the same amount of gas is under half the pressure. It's a lot more complicated than that in cartridges since the volume starts expanding while the powder is converting to gas, but that's the simple answer as to why you need a lot more powder in .38spl to drive the same bullet as a .380acp.