Same load. Different day. 100 fps difference.

This happened awhile back. Been thinking about it. Can't come up with anything conclusive, except maybe a chronograph error.

I was using a brand new Magneto Speed chronograph. I worked up a load to 3450 fps. A few months later, that same load chronographed at 3350. All prep work was the same. Loaded the same way. Environmental conditions were similar, although it was a bit warmer the second time. The powder was H1000, and while I think it was from the same lot (same can, even) I can't say for sure. There was never a hint of pressure issues, either time. I can't think of anything that would cause that much velocity loss, so I'm guessing the chronograph was reading different. It was set up exactly the same, also.

I've since ditched that particular load, but I'm curious about what may have caused it in case I run in to that again. I'd hate to think it was a brand new Magneto Speed. That would hinder my confidence in the unit moving forward.

What I've seen cause velocity changes:

Virgin brass vs. brass with a .002" bump to the shoulders.
Significant ambient temperature change.
Barrel that got 200-250 rounds down it.
New lot of powder.

So if you rule out all that, I say chronograph error. Cuz I got nuthin else.

Brass was all fireformed and bumped back.
It was warmer the second go round, which is when the velocity dropped.
That's probably close on the round count, total, but no rounds were fired in between sessions.
This is a possibility. I should have made a note of this. I was thinking it came from the same lot, but I'm not positive.

With more use, I'll probably be able to figure out if it's the chronograph or not.

What is the chambering? The 250 mark, on the round count is where lots of them jump in speed. I presume .223 something?

Chronographs are cool, but nothing will give you true velocity like 400 yards and beyond. And there's way more than that in Winters, Texas.

A couple more possibilities: a primer change? or maybe your scale was not performing the same for whatever reason...

A change in seating depth will can cause velocity to change also.

It's the 7 STW.

Jump up in speed? Or down? This went down, and overnight.

Primer was the same.

The scale is something I think could be a possibility. It's a GemPro 250. It seems like a good scale, and it gets good reviews, but maybe I didn't let it warm up properly, or something.

Seating depth was the same.

Once they get 250 round count the speeds go up. That is why I am thinking chronograph error. Or powder lot change.

Probably so.

I'm going to say chrono error. I've seen them read different speeds if they aren't set up properly or from differences in lighting.

The thing about the Magneto Speed is that it doesn't require light. I won't rule out operator error, but I had it set up the same way each time and in accordance with instructions.