Cartridge velocities - Garmin - what is happening?

[Websurfer]

Hello all,
Can someone help interpret what is happening with my cartridge velocities?
Some background: I'm a short-range benchrest competitor. Just got a Garmin table-top chronograph, set it up at a match, and was surprised at some of the data from it.
My average velocities increased significantly (well, it seems significant) from the start of the match. Here are the data:
1st target: 3043 FPS average (mean)
2nd target: 3053 FPS
3rd target: 3079 FPS
4th target: 3083 FPS
5th target: 3080 FPS
The lowest velocity recorded was my very first shot, a fouler, at 3013 FPS. The highest velocity was during the 4th target, at 3090 FPS.
Temperatures ranged from a low of 74 degrees to a high of 79 degrees. We shoot from covered benches, so there was full-time shade.
All rounds were loaded at the same time, LT30 powder, thrown from a Harrells measure, then weighed on an electric jewelers scale, and powder trickled in, as needed. Visual review before seating indicated all rounds had equal amounts of powder. Primers are Fed Match, same lot, same box.
Thoughts?
Many years ago, Jackie Schmidt posted a thread in which he expressed surprise at the velocity differences in his rifle (I believe a 6PPC) which shot more or less in the .1's-.2's. He was wondering if the velocity difference made any actual difference on target, which if you're getting .2's or .1's, maybe it doesn't.
Just because there is a measured difference along one line of data, doesn't mean that the difference is relevant.
Anyways, any thoughts on these seemingly large velocity ranges which my little Garmin recorded? Any ideas as to why there was such a difference? Why the velocities increased so much over time?
THANKS!

 

[RabbitSlayer]

Now with the garmin i am collecting a lot more data. My fouler on a clean bore in 6 creed is slow about 50 fps.

 

[foxguy]

How did it group? Humidity Changes thru the day?

 

[Homerange]

Dwell/heat soak time for each load in the chamber ?

 

[Websurfer]

I was shooting an NBRSA score match. In order, from first target to last target:
50 4x
50 4x
50 4x
50 3x
50 5x
Humidity started at 84% and fell to 67% by the end of the competition.
Time in the chamber could be a factor, I’ll have to try and track that…
Interesting, rabbitslayer, that your fouler was really slow, too.
Thanks for all the responses so far…

 

[Someone online]

How many rounds down the barrel so far? Is it a barrel speedup from a new barrel?

 

[WyleWD]

I was shooting an NBRSA score match. In order, from first target to last target:
50 4x
50 4x
50 4x
50 3x
50 5x
Humidity started at 84% and fell to 67% by the end of the competition.
Time in the chamber could be a factor, I’ll have to try and track that…
Interesting, rabbitslayer, that your fouler was really slow, too.
Thanks for all the responses so far…

With scores like that ..... just keep doing what you're doing in the reloading area and at the bench. The data you collected is not that unusual for averages IMHO. The slow first shot is pretty normal for my rifles with cleaned barrels. Great scores. WD

 

[William Vogel]

My velocity always climbs with increasing shots unless I rest the rifle in a climate controlled environment for long enough for it to get cold again. I store and travel in air conditioned spaces and my shooting bench is always warmer until winter. Barrel temp and barrel shot seasoning is what I recognize as the velocity rise. My rifles except one all become more accurate and precise after about 8 rounds. The one rifle shoots dead nuts up to about 12 rounds and then opens. It’s a Remington 700 Police 308 factory rifle with a Triggertech diamond and it shoots less than 1/2 moa at 700 yards and makes 1 hole at 100 until 12 shots. I rest it for 30 minutes and it’s back to perfect. The other guns are as good or better accuracy but the warmup is annoying because I have to shoot them into accuracy and require more targets. I pull out that raggedy looking 700 and people at the range are amazed at how it puts splats on the center of the 6 inch steel at 700 yards. I love it.

 

[dgeesaman]

My barrels speed up as they get warm. Of course the ammo tends to be getting warmer too.

 

[6.5 forever]

Now with the garmin i am collecting a lot more data. My fouler on a clean bore in 6 creed is slow about 50 fps.

Same here

 

[Doom]

First off you need to look at this in relation to what you are shooting. I'll speculate your shooting 100yd and 200 yds and something in a 6mm although its not very cartridge specific. Your drop at 200yds with a 100 fps velocity difference is only around 0.6" so your 40 fps is about 0.25" which is getting to be pretty hard to find on target.

Your barrel took about 10 rounds to stabilize. Now put the Garmin away and go shoot.