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9mm Major Velocity difference


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Hi all, Been a while since I posted , question is - this load 6.8grs Win Auto Comp,124grMG JHP OAL 1.145(seated short for the Glock Mags), Small Pistol Magnum Primers. Originally made for my Carver Glock 17 Racegun with KKM Barrel & 4 port SJC Compensator. I Chronoed them thru the G17 on a day that was 75 degrees & this was the 6 shot string 1. 1391

2. 1376

3. 1390

4. 1395

5. 1388

6. 1374 AVG FPS- 1386 PF-171.8

Two weeks ago I shot my STI TruBor 9mm Major with T2 Comp & had some of these in my Bag & someone set up the Chrony after the match, so I went and grabbed a few rounds. My bag was sitting under an overhang all morning in the cool air ( 4 hours at about 55 degrees,these rounds were COLD) These are the numbers for 3 Rounds:

1. 1402

2. 1466

3. 1421 AVG FPS- 1430 PF-177

My question is-Can the Temperature(20 degrees Colder) have THIS MUCH influence on the load (same load ,different guns) or are the Guns utilizing the load differently ??? or is the STI pushing them out faster because of the deeper seating from the Glock Mags.My OAL's for the STI is around 1.155 to 1.165 no matter what tip I'm using. Any help shedding some light on this difference will as always be greatly appreciated..

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I am not famaliar with Auto Comp it could be reverse temperature sensitive. Another thing to consider is that your STI has a fully supported barrel and probably a tighter chamber along with a faster barrel. You could be getting a better seal around the brass in the STI along with the other variables resulting in the higher velocities. 44 FPS is not that much of an increase considering the differences in the guns.

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I have seen more difference between different guns, at close to the same temperature, no surprise there.

+1. Additionally, if you fire 30 rounds from the same gun on the same day,

you might find that the fastest 3 rounds and the slowest 3 rounds are quite a bit


Your sample size (6 and 3 rounds) is really too small to draw any conclusion. :cheers:

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Depending on what you are using to throw the powder, it may vary a couple of tenths from the beginning of the loading session when the powder holder is full and the end of the the session when it is closer to empty. That and the difference in barrels could produce the velocity change.

Hey, it still makes Major. It could be worse.


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I have seen differences in my own loads at greater extremes then that. tighten up to only 6 FPS in the same day? but they always made major and thats all that mattered at the time.

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Also, using two different chrono's can give very different results. That happened to a buddy of mine. We set up three different chrono's in series. Two of them had consistent results and one read consistently lower by as much as 5-10%. Unfortunately, he was using the lower one and his loads were too hot. He suspected something wasn't right, which led us to the experiment.

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