Generic OAL question

[Bench]

With two reloads being exactly the same (bullet/weight 124 FMJ RN, brass head stamp, Mid range powder drop, 5"barrel) except for the OAL, what factors will be different between the two cartridges?

Longer OAL: better accuracy?

                       lower PF?

Shorter OAL: 'less' accuracy?

                        higher PF?

What else?

 

Thanks All!

Edited September 27, 2018 by Bench

[Hi-Power Jack]

4 hours ago, Bench said:

 what factors will be different between the two  ?

 

It frequently depends on the specifics - the gun and the load.

 

Slight differences in OAL frequently mean NO difference in

accuracy or velocity.

 

Obviously, the shorter the OAL, you will tend to get higher

pressures and velocity, but they are not too obvious until

the difference in OAL gets a little larger.   Unless, you are

shooting loads that are near the top of their range - then

a small difference can make a big difference.

 

Where you can run into real problems with OAL is if you

have heavy bullets and fast powder, and you're at the top

of the power chart - then, especially if you get some

bullet setback, you can have an explosive situation.

That's what makes .40 Major a potential problem.

 

BUT, if you have 2.7 gr Bullseye in a .38 Special case,

I can't imagine there being any difference by shortening

the OAL.  There is so little powder and power, and such

a huge case, that OAL would have almost no effect     

 

In my BHP, I did get better accuracy with the longer

OAL, but had to cut it back to enhance feeding.  But,

I doubt that is universal - probably depends on the

actual load and bullet and gun.

[stick]

Jack nailed it!

[Bench]

5 hours ago, Hi-Power Jack said:

In my BHP, I did get better accuracy with the longer

 

OAL, but had to cut it back to enhance feeding.  But,

I doubt that is universal - probably depends on the

actual load and bullet and gun.

Thanks Hi-Power Jack. I was thinking that the accuracy bit might be right because of being closer to the lands (?) and PF can be adjusted as needed with the load (later as needed). I was working on OAL for a new 9mm bullet (124 FMJ RN) and just amazed at the change of OAL from my usual load (124 CRN coated) which will not plunk any longer than 1.1. The new bullet plunks at 1.150 and runs through the mag but that's pushing it I think. I loaded up some at 1.130 too and wondered what variations I might see between the two new bullet OAL's. Range time is tomorrow to see how it all works together live fire.

[Bench]

5 hours ago, stick said:

Jack nailed it!

Thanks for the confirmation, it all makes sense.

[noylj]

I find that, in rifles and pistols, that for almost all of them, if I set the COL at the start to be long enough length so the bullet is touching the lede/rifling (that the case head is practically touching the breech face), I get best accuracy. Never had a pressure issue because I start loading at the start load and start at the long COL at the beginning for testing, and reduce COL from there, as  needed.

You need to approach north of 0.050" COL variation to have any pressure changes of any significance.

The big concern is NOT the COL your press provides, but the COL after chambering due to ANY bullet set-back. Always do a "push test" on seated bullets.

[Bench]

53 minutes ago, noylj said:

You need to approach north of 0.050" COL variation to have any pressure changes of any significance.

The big concern is NOT the COL your press provides, but the COL after chambering due to ANY bullet set-back. Always do a "push test" on seated bullets.

Thanks for the setback reminder!!!

[superdude]

6 hours ago, noylj said:

 

You need to approach north of 0.050" COL variation to have any pressure changes of any significance.

 

Data?

 

I sometimes see improved accuracy after seating the bullet deeper.

Edited September 27, 2018 by superdude

[noylj]

> I sometimes see improved accuracy after seating the bullet deeper.

 

And I often find that working up the load at near max COL gives me the best accuracy. Guns are fickle and I was writing about pressure/velocity worries for small COL variations well within normal.

[lgh]

On 9/27/2018 at 10:03 AM, noylj said:

You need to approach north of 0.050" COL variation to have any pressure changes of any significance. 

 

Why do you say that?  Powder type/charge, bullet weight, case volume, and a bunch of other things will dictate what COAL change will do to pressure change.

[noylj]

lgh: yes, but I assume that one has worked up the load already from the start load and is not just throwing different powder/charge, bullet weight, case volume and other things into the mix randomly.

PS: COL is not dictated by case volume or powder or charge.

If you have a safe load, it takes at LEAST a 0.100" change in COL for typical handgun cartridges to go so over-pressure as to injure the gun or you.

Heck, the industry reports that for typical bottleneck rifle cartridges, the case volume is so huge that shortening the COL can actually LOWER the pressure as it gives the slow powder a longer to produce pressure and the bullet more velocity to squeeze through the lede. Pressure can go UP as you lengthen the COL.

 

[Hi-Power Jack]

2 hours ago, noylj said:

 

If you have a safe load, it takes at LEAST a 0.100" change in COL for typical handgun cartridges to go so over-pressure as to injure the gun or you.

 

I don't have any specific data, and I agree that this is "USUALLY" true, but I would think that

with a heavy bullet and very fast powder, you just might be able to hurt yourself with a smaller COL

in specific situations.

[IDescribe]

6 hours ago, noylj said:

Heck, the industry reports that for typical bottleneck rifle cartridges, the case volume is so huge that shortening the COL can actually LOWER the pressure as it gives the slow powder a longer to produce pressure and the bullet more velocity to squeeze through the lede. Pressure can go UP as you lengthen the COL.

Right.  Most action pistol shooters only see this when the bullet is loaded right into the lands.

To expand on what noylj described:

Smokeless powder burns faster the more pressure it's under, and faster it burns, the faster it generates pressure, so we have a positive feedback loop.  Anything done to increase pressure at the beginning of the burn will increase pressure geometrically through to the peak, and anything done to decrease pressure at the beginning of the burn will have the opposite effect.  Such is the nature of shortening or lengthening OAL.

As that regards shortening OAL in rifle cartridges lowering pressure (to a point) because of giving the bullet more velocity to pass into the rifling:

One of the things that will increase pressure at the beginning of the burn is reducing the rate of acceleration or delaying the movement of the bullet.  With the bullet at an OAL that keeps it away from the lands, the bullet picks up a little velocity, and that velocity and momentum helps the bullet pass into the rifling more easily (with less of a hitch or studder in the acceleration).   But if the bullet is loaded right into the lands, it sits for a moment before pressure builds enough to start the bullet moving.  This delay in bullet movement for a bullet loaded into the lands increases pressure significantly more than does the studder in acceleration for a bullet loaded away from the lands.

With rifle powders, the powders burn slow enough, and the acceleration of the bullet before hitting the rifling is slow enough, that the hitch/studder in acceleration as the bullet passes into the rifling still has a significant effect on pressure.  So with rifle powders, if you look at pressure with the bullet loaded into the lands, then .003 out of the lands, then .006 out of the lands, then .010 out of the lands, and so on, and so on, you see pressure go down because as the OAL gets shorter, the bullet is building more velocity before engaging the rifling, and thus passing into the rifling more easily.  This isn't to say shortening the OAL doesn't also have a pressure increasing effect in terms of decreasing the initial size of the combustion chamber, only that the pressure decreasing effect of allowing the bullet to gain more velocity before hitting the rifling outweighs it.  Keep in mind, thought, that all of this is tied to how close the bullet is to the rifling, not to the OAL difference itself.  If your two OALs are .001 out of the rifling and .011 out of the rifling, that .010 difference will have a measurable impact on pressure, but if you're looking at .051 out of the rifling and .061 out of the rifling, the .010 difference will amount to diddly.   And of course, the slower the powder, the further away you can be from the rifling and changes in OAL matter.  And conversely, the faster the powder's burn rate, the closer you have to be to the lands for differences in OAL to matter, which brings us to pistol powders:

With pistol powders, the powders burn so fast that once you're out of the lands, there's no further reduction in pressure by getting further away from the lands.  This is why we talk about not loading into the lands, or loading into the lands causing pressure spikes.  Really, it's about delays in acceleration, but with pistol, because of the fast burn rates of the powders, the only significant delays in acceleration come when you load right into the lands.



 

[lgh]

7 hours ago, noylj said:

but I assume that one has worked up the load already from the start load and is not just throwing different powder/charge, bullet weight, case volume and other things into the mix randomly.

Thanks for clarifying.

[IDescribe]

Regarding how much shortening OAL increases pressures, I tested it a few years ago.  Obviously, I did not measure pressure itself, as I have no access to a piezo device.  What I did was measure velocity differences with the same charge weight, bullet, gun, where only OAL changed, with the assumption that a velocity increase with the same charge weight and bullet MUST mean an increase in pressure.

I used a Montana Gold 124gr JHP with 3.6gr of N320, OALs of 1.04 through 1.14.  Gun was an HK VP9 and all cartridges were with Starline brass.  Results:

OAL 1.14      Depth  .186  |  Avg fps - 925  fps |  SD-13  | ES-42
OAL 1.12      Depth  .206  |  Avg fps - 922  fps |  SD-19  | ES-59  | -3
OAL 1.10      Depth  .226  |  Avg fps - 941  fps |  SD-15  | ES-51  | +19
OAL 1.08      Depth  .246  |  Avg fps - 962  fps |  SD-11  | ES-34  | +19
OAL 1.06      Depth  .266  |  Avg fps - 979  fps |  SD-12  | ES-34  | +17
OAL 1.04      Depth  .286  |  Avg fps - 994  fps |  SD-12  | ES-32  | +15

 

The number out on the right is the difference in velocity relative to the load above it.  Notice that the first OAL change from 1.14 to 1.12 resulted in a velocity decrease of 3 feet/sec.  This is because that bullet hits the rifling in that gun around 1.13.  That 1.14 OAL cartridge was loaded into the lands.  But from 1.12 down to 1.04  there was a reasonably consistent increase in velocity as OAL got shorter, and that increase in velocity is more or less the same change I get with that bullet and that powder when I keep OAL the same and increase charge weight by a tenth of a grain.  With that powder and bullet, a change of .02 in OAL is worth about the same as a change of .1gr in charge weight -- in terms of velocity, and I assume pressure is somewhat correlated.

I have never done a full test like I did above with a different bullet/powder combo.  I have, however, in load development, seen the results MANY MANY times of changing OAL by .01/.02/.03 without changing charge weight, and the resulting changes in velocity are consistent with the above test.  With powders in the burn rate vicinity of N320, loading to 9mm minor velocities, and regardless of bullet weight, a change in OAL of .02 is worth about the same in velocity as a change in charge weight of .1gr.  

Changes in pressure (observed indirectly through changes in velocity at the same charge weight), are significant enough to be seen even at modest changes in OAL.  This does NOT mean such changes are dangerous or likely to cause catastrophic failures -- that would be largely dependent on where the pressure was to begin with.  In the above test, I cut the OAL by .1 with no ill results, and was in no way concerned, given that I was operating down in "starting load" territory, but if I'd been up to 4.3gr at max standard pressure and THEN reduced OAL by .100, THAT could be a very bad day.

If you're loading WSF with a 124gr bullet for 1040 feet/sec, changes in OAL aren't going to affect pressure too much.  But if you're loading a 147gr bullet with Ramshot Comp to 915 feet/sec, already well into +P range, a change in OAL might violently disassemble your pistol for you.
 

[PhillySoldier]

I previously did an varying OAL test. OAL range was 1.06" to 1.15" for a total extreme OAL spread of 0.09". I was shooting 10 round groups (two groups each per seating depth) @ 25 yards from a ransom rest. Shot group difference was about an inch from worst to best. 
"

[Bench]

1 hour ago, PhillySoldier said:

I previously did an varying OAL test. OAL range was 1.06" to 1.15" for a total extreme OAL spread of 0.09". I was shooting 10 round groups (two groups each per seating depth) @ 25 yards from a ransom rest. Shot group difference was about an inch from worst to best. 
"

So @ 25 yds. what was the best group size and the worst group size? 1" to 2" etc.

[PhillySoldier]

1.6 to 2.5". I actually had one shot group at the lowest OAL that was 2.9" but am considering that may of been a fluke and didnt match close with all the other shot groups. Even without that though I think thats a pretty significant increase in shot group size; all for a difference of 0.09" OAL spread