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Old 02-01-2020, 03:54 PM
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Default 9mm COA length

I have always been frustrated with the accuracy of my 9mm pistols.

My first 9mm, a mid 1990’s Tanfoglio P19 Combat (CZ75 alike) has a short leade from chamber into rifling. I have had the chamber recut twice, but some bullet profiles, including several factory loads, engage the rifling before the slide is closed. In my hands (but not all) it shot so low that I lowered the front sight to a fraction above the white dot. It now shoots to POA at 25 meters with 124 gn loads and I run plated flat point projectiles to avoid engaging the rifling too soon.

When I got my Kimber 9mm 1911 the only 10 round magazines available were 38 Super mags without a spacer. Because “standard” length 9mm rounds were misfeeding I loaded my ammunition “long”. Accuracy was just not up to the standard of my .45 1911’s.

When I finally got enough 10 round 9mm mags I went back to loading to “standard” COA length, which according to Hodgdon using both W231 and Tightgroup powders is 1.150”. These shot well out to 15 meters but beyond that there were always flyers. I can usually call bad shots due to my bad trigger control but these were often unexplained.

After some comments in a couple of other threads here I loaded 100 rounds with the COA length reduced to 1.025”. The changes were remarkable.

First, at 15 yards (I was using the falling plate range as my usual one was otherwise unavailable so it is marked in yards not meters) bullets were impacting at the top of the IPSC target. After some sight adjustments I fired 3 rounds with two hitting inside the 3 X 3 array of 3/4” black patches I was using as a consistent aiming point and the third just above and to the right. Mmmm.

Back at 25 yards my rounds were hitting high again, so after some more sight adjustments I found myself getting 4 out of 5 rounds in the A zone and the 5th just out in the C zone. That’s about as good as I can shoot on a good day.

Our falling plate range extends another 5 yards or so back from the 25 yard line with a plank seat across the whole width. Sitting down at about 30 yards I fired 3 rounds. 2 C zone hits and a miss.

This range faces directly east and it was 8:30 in the morning. I was looking into the sun at 11 o’clockwhich at the earlier distances had been shaded by trees along the backstop, and was having issues with the aiming point fading out. Grabbing my cap, which made the aim point a lot more visible as the sun was shaded from my eyes, I fired another 3 rounds. One Alpha, a Charlie that almost cut the line into the A zone and a second Charlie. The three hits were in a very flat triangle shape with the bottom two shots about 4” apart. Both C’s were to the left, as you could expect with the sun from that side.

I’m happy that reduced COA length has made this pistol shoot much more accurately in my hands. Next up I’ll load a few round to 1.012” and see how that goes.
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Last edited by Kiwi cop; 02-01-2020 at 10:18 PM.
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Old 02-01-2020, 04:46 PM
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"COA length reduced to 1.250"...."?
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Old 02-01-2020, 04:54 PM
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Take the barrel out of the gun . Let barrel / chamber determine the oal & crimp . You want casehead flush with the end of barrel hood . Hold barrel vertical & do the " plunk " test with your loaded ammo . Write your results down . Once it's been determined this will be Oal / crimp for that weight / brand bullet & that pistol only . Since you have several 9mm's you'll find they're all different . Part of the joy of loading for a 9mm , kind of like bleeding piles .
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Old 02-01-2020, 05:02 PM
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redacted........

Last edited by bigggbbruce; 02-01-2020 at 05:03 PM.
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Old 02-01-2020, 06:07 PM
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I have a 6906 and a Sig 226, which require different COLs. I reload using the shorter COL that fits both and don't look back. There's only a few thousandths difference on COLs between the two guns.
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Old 02-02-2020, 01:13 AM
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I did not catch the type of bullet..... FMJ or JHP/Plated you are using.

For a "Ball" FMJ type 124-125gr copper jacket, I have loaded these from
a short 1.09" all the way out to 1.169", with the pistols that I have on hand.
1.14" is my go to OAL for most of my target loads with no feeding problems at 945fps or higher in a 3".

A JHP can be tricky and if possible use the bullet makers load data, due to shape, length and Ojive.
Speer calls for 1.12" for it's Gold Dot but there are several brands of JHP bullets that can
start out at around 1.075" and go out to 1.13" if needed for the bulky Unique powder...........
if it will feed in your pistol and magazine.

The minimum OAL will help get the maximum FPS out of w231 powder but some of the powders that are a little slower will show good accuracy with a little longer OAL.

As mentioned the "Plunk test" in a stripped down barrel is a GREAT way to see what the maximum OAL can be, then back off a touch, for any odd bullets that might happen along.

My 3" likes a 124 Ball at 1.165" in a Military case with Bullseye at 945fps
for top accuracy.
However a 3" Kahr has problems swallowing a 147gr at 1.13"........
so you need to work with each weapon to get them to feed correctly, per slide speed, spring etc.

That barrel does have a standard 1:10 9mm twist, right ?

PS:
Hodgdon has 124 RN at 1.15" ......... JHP at 1.09"............ HAP at 1.069"
If you OAL is SHORTER................you may run into high pressures !!

Stay safe.

Last edited by Nevada Ed; 02-02-2020 at 01:28 AM.
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