9mm + 124gr MG CMJ - Lower than expected velocity?

[Steve_P]

So I wanted to try out some CMJ bullets in my shadow.

I had been running 124gr MG JHPs, loaded short to fit my shadow chamber.

My load with those was 4.2gr N320, Fed SP Match, 1.085" OAL. I was getting about 132 PF thereabouts.

I would get very random feed failures, the bullet nose would impact the ramp and just stop. I could always bump the back of the slide and it would continue forward into battery.

So, I loaded up a batch of the 124gr CMJ, same charge and primer, OAL of 1.125".

Fed just fine, but my velocities only averaged 1015 fps. SD was very low at 5

This data seems a bit lower than what I was expecting, based on what I have read others use here.

I will probably bump up a tenth or two and see what that gets me.

Does anyone else here have anything to add?

Thanks in advance,

Steve

[CZ85Combat]

FPS went down do to changing the OAL from 1.085" to 1.125"

[Steve_P]

I definitely expected the drop with increased oal.

I guess what i am slightly confused at is most reports i read here with this load at about the same oal have people making 130-132ish pf. I am barely making minor.

Does anyone here load in the 4.4gr range with a 124?

[mpeltier]

One thing to consider is with the bullet not seated as much, if you did not add a touch more crimp there may not be as much case tension gripping the bullet. You are only talking a 49 fps differance. And you already were at 132 with your jhp load seated deeper. Not sure why you expected to be closer to 132.

[Steve_P]

I guess i was trying to figure out why my observed readings are a bit slower than other folks here have seen with basically identical loading conditions.

As you said, crimp could be one variable that i am not taking into account.

No biggie, I'll probably just chalk it up to chrono variations.

I made up a few more test batches, some at 4.3 and some at 4.4

i will re chrono them tomorrow after the match.

Steve

[stick]

FPS went down do to changing the OAL from 1.085" to 1.125"

That's what I was thinking. Longer bullets=lower pressure

[bajadudes]

loading conditions may be identical but are pistol and environmental conditions the same? I notice that even in the same model guns the same loads will crono differently.

[Justsomeguy]

Some barrels are slower than others, and in my experience MG bullets do run a little slower than others, especially lead or moly. With an SD of 5 you are getting consistent ignition, so that is not the problem if indeed there is any problem at all. It just might be a slower barrel, and it might get a bit faster with age anyway. Like bajsdudes is saying it could be your elevation or temperature that is making some difference also. If the cases are sealing the chamber and there isn't a lot of soot on the case mouths, then just kick it up a tenth or so and shoot on..!