9mm OAL Question....


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mcdonl
November 24, 2010, 10:06 PM
Hi there, I have a general question about over all length....

I have been using the following 9mm load for about a year now:

Powder: UNIQUE
Powder Wt: 4.6g
Bullet: LRN 125gr (Cast from W/W's with Lee Mold....)
Primer: Federal SP
OAL: 1.119 (I used this because it was the length of a WWB 9mm load and it was under the 1.169 that is shown on the Lee 9mm dies, it functions well and I stuck with it....)

There is no load data in the Lyman 49th manual for this particular projectile so I have been using the following load data as my basis for the above load:

Powder: UNIQUE
Powder Wt: Start-4.5 Max-5.0 (Because of the way this powder meters in my Lee Powder Measure I chose to use 4.6 as it is the 5cc Lee scoop)
Bullet: 125gr JHP
Primer: Federal SP
OAL: 1.075

Someone on here who I respect suggested that I use the load data for the 120gr LRN as the 5gr is negligible... So I used the following load as a basis:

Powder: UNIQUE
Powder Wt: Start-4.0 Max-5.0
Bullet: 120gr LRN
Primer: Federal SP
OAL: 1.065

From that load data I ended up with this:

Powder: UNIQUE
Powder Wt: 4.6g
Bullet: LRN 125gr (Cast from W/W's with Lee Mold....)
Primer: Federal SP
OAL: 1.075

So, to the point about the OAL.... Until I loaded up a batch of ammo based on the 120gr LRN from the lyman book I never gave the OAL much thought. All of my loads were 1.119

Well, today I brought two boxes... 50 of the 1.065 and 50 of the 1.119 I was shooting them all at 75 yards through a Hi Point 995 carbine...

The longer OAL outperformed the shorter OAL on every target. Why? How does OAL impact the performance? The gun functions the same with both loads... any thoughts?

Thanks and Happy Thanksgiving!

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Xfire68
November 24, 2010, 10:18 PM
The OAL will cause pressure changes and can cause some difference in groups and finding the right combo for a bullet weight and type along with the powder and primer combo in a gun is why some of us hand load. Why did I write that all in one sentence?:o

rfwobbly
November 24, 2010, 10:23 PM
It has to do more with the composition of the bullets and the probably the length of the bullet itself. The longer lead bullets are probably flying faster. To get a definitive answer you need a chrono.

bds
November 24, 2010, 10:38 PM
Typically, longer the OAL, sooner the bearing surface of the bullet will engage the rifling, bullet base will bump (obturation) better for improved bullet to barrel seal, and more consistent chamber pressure/powder burn will result - which all translates to improved accuracy and tighter shot groups.

Shorter the OAL, more the bullet will "jump" from the case neck to engage the rifling, during which time more hot gas may escape around the bullet, decreasing the chamber pressure, which will affect the bullet base bump (obturation) - this all translates to more inconsistent powder burn and decreased accuracy. If you are shooting lead bullets, shorter OAL will increase gas cutting of the bullet and increase leading at chamber end.

I typically load 9mm 115/124/125 gr RN to 1.125" OAL, but longer OAL that will reliably feed/chamber in your pistol/barrel will generally shoot better at mid-high range load data.

Hondo 60
November 24, 2010, 10:38 PM
There are a number of factors involved in accuracy.
Speed of the bullet, shape, pressure etc, etc.

You'd need a PhD in physics to be able to discern all the variables.

Its just easier to figure out what works in your gun & "don't worry, be happy".
(at least that works for me) :D

mcdonl
November 24, 2010, 11:21 PM
Hondo, trust me... I was happy with those groupings :)

BDS, that was great info. I may just try a little longer OAL for the next batch.

To get a definitive answer you need a chrono.

Oh my, something to buy!!

Seedtick
November 25, 2010, 12:00 AM
oh my, something to buy!!

Ho! Ho! Ho!
:D

ST

:)

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