[JamesT]

Been loading straight wall 9 mm for a couple of years and now starting rifle bottleneck (.223).
All other things being equal....With pistol, shorter OAL generally increases pressure due to less case volume; but, with bottleneck, it is the longer OAL that increases pressure even with more case volume. Cannot find answer as to why the difference in books that I have. What am I not getting?

 

[fredj338]

Been loading straight wall 9 mm for a couple of years and now starting rifle bottleneck (.223).
All other things being equal....With pistol, shorter OAL generally increases pressure due to less case volume; but, with bottleneck, it is the longer OAL that increases pressure even with more case volume. Cannot find answer as to why the difference in books that I have. What am I not getting?

In pistol rds, especially small volume ones, you are reducing case volume by seating deeper. In rifles, even small cases like 223, the fastest powder is far slower than the slowest pistol powder & your bullets are closer to the rifling in most rifles than most handguns. The further from the rifling the more time the pressure has to build before it meets resistance.

 

[steve4102]

This is from John Barsness of Handloader Magazine.

It decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.

Handgun powders are much faster-burning, and even regressive, meaning pressure peaks when they're first ignited. Thus seating them deeper allows more time for initial pressure to build.

This effect is slightly exaggerated in revolvers. The initial, quick pressure rise is relieved somewhat when the bullet passes the cylinder gap, allowing some gas to escape. When bullets are seated deeper it takes them longer to pass the cylinder gap, giving pressure more time to rise.

 

[steve4102]

Here are a couple charts that may help.

This first one is a typical 30-06 load with 180gr bullet and IMR 4350.
Note that "peak" pressure is not achieved until the bullet has traveled 1,5 inches down the bore. In other words the bullet has long left the case before pressure reached Max.

This next chart is a typical 45 ACP with 230gr bullet and 231.
Note it reaches Peak pressure after only .2 inches of travel down the bore.

If we select an even faster powder for the 45 ACP like Clays, we can see the Peak (max) pressure is achieved in .15 inches of travel down the bore.

 

[JamesT]

Gentlemen, I thank you for the excellent replies! I think that I an now closer to "getting it".

I do have one further question: in mid-range loads, how much change in OAL does it generally take to affect velocity in pistol and rifle cartridges. (I thinking about my own question, perhaps there is no standard answer due to difference in powder burn rates...but I will ask anyway).
Thanks again
Jim

 

[steve4102]

Gentlemen, I thank you for the excellent replies! I think that I an now closer to "getting it".

I do have one further question: in mid-range loads, how much change in OAL does it generally take to affect velocity in pistol and rifle cartridges. (I thinking about my own question, perhaps there is no standard answer due to difference in powder burn rates...but I will ask anyway).
Thanks again
Jim

now starting rifle bottleneck (.223).

What are you loading for, bolt action or AR type semi-auto?

 

[JamesT]

AR type semi-auto
Jim

 

[steve4102]

AR type semi-auto
Jim

In that case,
Seat to mag (2.260) length or shorter.

Make sure you have enough bullet seated into the case for adequate neck tension. A general rule of thumb is, at least one (1) bullet diameter seated into the case. In the 223 that would be about .224 inches of bullet into the case.
Make sure your OAL Feeds reliably.

Seat to mid cannelure if your bullet is so equipped and you are going to crimp.

 

[JamesT]

Thanks Steve. The load data I am using seats the Hornady cannelure bullet such that the OAL is 2.200. However, the cannelure is then all enclosed by the case. That resulted in my original question in that I was wondering if I could seat the bullet less deeply w/o adjusting the powder charge. In other words, at what point does an OLA change affect the pressure/velocity.

 

[fredj338]

Thanks Steve. The load data I am using seats the Hornady cannelure bullet such that the OAL is 2.200. However, the cannelure is then all enclosed by the case. That resulted in my original question in that I was wondering if I could seat the bullet less deeply w/o adjusting the powder charge. In other words, at what point does an OLA change affect the pressure/velocity.

In rifle, only when the OAL puts the bullet into the rifling or really close to it. In an AR, I doubt you can load much past mag length & get anywhere close to the rifling.

 

[timmay]

Good thread. Thanks for the lesson Fred and Steve.

 

[JamesT]

As Timmay said above, "great thread".... and thanks for the schooling!
Jim

 

[steve4102]

Thanks Steve. The load data I am using seats the Hornady cannelure bullet such that the OAL is 2.200. However, the cannelure is then all enclosed by the case. That resulted in my original question in that I was wondering if I could seat the bullet less deeply w/o adjusting the powder charge. In other words, at what point does an OLA change affect the pressure/velocity.

Yes, adjust your OAL to whatever works for you. As Fred pointed out, loading Long and into the lands is the only OAL that may net you unwanted "High" pressure. In an AR, any OAL that fits the mag will not even come close to the land/rifling.