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Posted: 7/24/2013 8:52:49 AM EST
In a different thread, writer stated going from 2.26" OAL to

2.24" would actually REDUCE pressure. How can this be?

Shorter OAL equals lower capacity, which I understand to mean

higher pressure with same load. Isn't lower capacity the reason

we reduce load with mil spec brass, especially in 7.62?
Link Posted: 7/24/2013 8:55:14 AM EST
There seems to be a lot more misinformation, half-truths, and general guessing going on in this forum as of late.
Link Posted: 7/24/2013 8:59:30 AM EST
He was probably speaking along the lines of taking the bullet off of the lands of the rifling.
If a bullet is loaded right on the lands, it will generally have higher pressure than one that is loaded so there is a gap.

All generally speaking of course.
Link Posted: 7/24/2013 9:06:00 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By tlw613sp:
He was probably speaking along the lines of taking the bullet off of the lands of the rifling.
If a bullet is loaded right on the lands, it will generally have higher pressure than one that is loaded so there is a gap.

All generally speaking of course.
View Quote



Yup. THIS! Typically a non issue in most factory barrels with a 5.56 chamber.... there's a lot of leade already... so going shorter would give a little more pressure, I'd guess.
Link Posted: 7/24/2013 9:13:46 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By tlw613sp:
He was probably speaking along the lines of taking the bullet off of the lands of the rifling.
If a bullet is loaded right on the lands, it will generally have higher pressure than one that is loaded so there is a gap.

All generally speaking of course.
View Quote

Link Posted: 7/24/2013 9:23:45 AM EST
Just look at the freebore in a Weatherby, that's how they can get such higher velocities. Freebore = longer throat.
Pressure goes down with a longer throat IN RIFLE ONLY
Try that with a shorter freebore, and you have problems.
Look up freebore
Link Posted: 7/24/2013 9:32:18 AM EST
Link Posted: 7/24/2013 1:06:54 PM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By dryflash3:
You are confusing straight sided pistol rounds and bottle neck rifle rounds as acting the same.

The rules are different for each.

Generally, with straight wall pistol rounds, pressure increases with a shorter OAL.

Bottle neck rifle rounds with much greater internal capacity will be the opposite.





View Quote


And This from John Barsness of Rifle/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.


Here are a couple graphs as well.



Link Posted: 7/24/2013 1:38:28 PM EST
[Last Edit: 7/24/2013 1:40:37 PM EST by splunkinoob]
Read the last few paragraphs on Hornday's internal ballistics page.

Freebore

I have a manual of theirs from the 70's with the same pictures, guess the concept hasn't changed.

In general, the deeper the oal on pistol rounds the more pressure, the opposite is true with rifle rounds to an extent.

Link Posted: 7/25/2013 6:21:44 AM EST
Great info, thanks everyone. Now I understand the OAL/ leade/pressure relationship.
Link Posted: 7/25/2013 6:48:50 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By johndm1967:
Great info, thanks everyone. Now I understand the OAL/ leade/pressure relationship.
View Quote


Remember, this "relationship" pertains to Bottle Necked rifle rounds and not straight walled pistol rounds.
Link Posted: 7/25/2013 9:33:25 AM EST
[Last Edit: 7/25/2013 9:37:24 AM EST by CCW]
Originally Posted By johndm1967:
In a different thread, writer stated going from 2.26" OAL to

2.24" would actually REDUCE pressure. How can this be?

Shorter OAL equals lower capacity, which I understand to mean

higher pressure with same load. Isn't lower capacity the reason

we reduce load with mil spec brass, especially in 7.62?
View Quote


It depends on who you talk to.

For a constant jump to land distance (free bore flight distance) the smaller the case volume for a given amount of powder, the higher the chamber pressure. For a constant boltface-to-land distance the smaller case volume translates into a longer free bore flight or jump to land and may translate into a lower pressure because there is no rifling resistance in the free bore travel of the projectile.
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