How does a 20" barrel affect start/max loads?

[Nik S]

All powder manufacturer's load data that I've seen is for a 24" barrel.
When developing a load for a 20" barrel, should I look at Starting/Max powder load differently? Should the Start/Max loads be more, less, or the same?

I guess the confusing bit for me on the published data is whether the 24" barrel is simply used to estimate velocity of a given load, or whether there is more going on inside a shorter barrel that fundamentally affects Start/Max loads.

 

[straightshooter1]

All powder manufacturer's load data that I've seen is for a 24" barrel.
When developing a load for a 20" barrel, should I look at Starting/Max powder load differently? Should the Start/Max loads be more, less, or the same?

I guess the confusing bit for me on the published data is whether the 24" barrel is simply used to estimate velocity of a given load, or whether there is more going on inside a shorter barrel that fundamentally affects Start/Max loads.

It's not the length of the barrel that makes the difference for starting/max powder load, it's the chamber configuration that will make a difference, if there's a difference in the chamber configuration.

 

[Supersubes]

Treat it exactly the same. Slightly shorter barrel makes no difference with regard to pressure.

 

[918v]

It's not the length of the barrel that makes the difference for starting/max powder load, it's the chamber configuration that will make a difference, if there's a difference in the chamber configuration.

That and case capacity. Don’t use max loads developed in lightweight brass with your heavy brass which has less capacity.

 

[Nik S]

This is good to know.
Thank you.

 

[Alaska]

Suggest WW brass with most capacity

 

[Doom]

One of the benefits of slower powders is they hold their pressure longer so the pressure is maintained further down the barrel. Hence the pressure at the muzzle will be higher with slower burning powders to achieve around the same velocity. This affects recoil, report, and muzzle flash. Typically this isn't a noticeable issue but sometimes this can become noticeable. I have observed this with 2000MR in a 20" 308 bolt gun. I have also observed it with some other shorter barreled rifles on our range.

This doesn't necessarily mean there is no benefit but the benefit comes with a price. I tend to stay away from the slower burning powders unless maximizing velocity is my main goal as those powders can often produce higher velocities.