Varying Chrony Results With 3n37

[alzo]

I am relatively new to chronographing rounds and to reloading 9mm. In early spring, I clocked a load of 5.9gr of 3N37 with a 124gr Berry's plated RN (OAL=1.15) at 1130fps. It was about 50 deg F and dry.

A week or so ago it was about 80 deg F and very humid. The same load averaged only 1086fps.

Can anybody help me understand why I am seeing this difference ? I am shooting a CZ-75B-SA.

Thanks.

[warpspeed]

Is it the same batch of powder ?

I've seen results wider than that between 2 lots of powder before.

What is the load for BTW ? For a non-comp gun, 3N37 is a little slow for my liking.

[alzo]

Yes, it is the same batch of powder. The rounds were even loaded the same day, just fired months apart.

I do not compete. I was just trying to come up with a load that was the rough equivalent of factory 124gr ammo. I was happy with the 1130fps.

The only thing I can think of that was different between the two days was the temperature and humidity. I'd be surprised if those things account for the difference in velocity, but I don't know for sure. For all I know, that's typical.

[GuildSF4]

Temperature (powder dependent), humidity (more = slows down faster), and altitude (higher = less air resistance) can all affect velocity.

You are not looking at a huge difference in the velocity either (44 fps).

[MoNsTeR]

What were your sample sizes?

[alzo]

What were your sample sizes?

<{POST_SNAPBACK}>

20 rounds on the first outing at 50 degrees.

40 rounds on the second outing at 80 degrees.

SD's on both days were about 20.

[ipscron2000]

What were your sample sizes?

<{POST_SNAPBACK}>

20 rounds on the first outing at 50 degrees.

40 rounds on the second outing at 80 degrees.

SD's on both days were about 20.

<{POST_SNAPBACK}>

I can get that out of my gun and loads in the same day. I wish mine were only 44 some days. I even load slow and accurate.

[coldchar]

Like you, with 3N37, I have seen major velocity differences with the same loads shot a few months apart at different times of the year.

With 7.5 grains and XTP HP bullets in the winter I have seen velocities vary 50-65 FPS with a 5" standard barsto as compared to shooting here in Phoenix in the summer. The hotter it is the hotter the loads are which is the opposite you are experiencing. Go figure.

What I have not figured out is the pressure does not seem to dramatically climb.

I use Tite Group for my steel shooting and get much more consistency with temperature variations.

I guess it is just temperature sensitive or maybe it is the nature of the powder to be inconsistent. I still like it but it makes me a little cautious on the hotter loads.