Strange chrono results on virgin barrel

[Nostradumbass]

Hey guys,
These are results from a rock creek barrel chambered in 408 cheytac. I know what Mike says, but I did a a 20 round break in regiment anyways.
I chrono'd every round and it left me scratching my head. The first round ran at 2744fps, and gradually got faster after every shot. I cleaned after each shot, then every other, etc. so I don't think the results were due to fouling. The gun shot progressively faster until it settled out at an average of 2962fps with the load I'm running.
I thought it strange that the barrel gained over 200fps in the first 20 rounds.

The gun grouped <.5 MOA after the break in, so I'm happy bout that.....just wondering why the bullet speed got faster. Maybe this is common, I don't know....have y'all seen this before?
Thanks!
Phillip

 

[Temp9]

How was your chrono set up?

 

[Nostradumbass]

10ft from muzzle

 

[Tango down]

Nostradumbass
I found this thread written in 2009, may be it can help some.
Let's see if others have more suggestions.
Ombre noire

EFFECTS ON BALLISTIC

2. Temperature and Trajectory

A change in temperature can affect the trajectory or ‘flight path’ of the bullet in two well-known ways:

So long as altitude, barometric pressure and humidity remain constant, an increase in air temperature will cause a flatter trajectory due to a lower air density (less collisions with ‘air particles’ per unit length of flight path).

The same increase in temperature also causes the nitro cellulose based powder inside the cartridge to burn at a higher rate, producing approximately four times the Point of Impact (POI) shift than just air temperature alone.

Just how much does an increase in temperature affect the powder burning-rate? Some powders are more susceptible to temperature effects than others and will burn faster than others. Some powders will experience a burning-rate

increase of 3.5 feet per second (fps) for every 1 °C (1.8 °F) increase. Others will be more resistant to heat and may only have an increase of 1.5 fps/1 °C.

The .308 Win. Federal Premium cartridge with a 175 grain (gn) Sierra Matchking (Gold Medal Match) fired from a Blaser R93 Tactical 2 Sniper Rifle will show a muzzle velocity increase of approximately 2.5 fps for every 1 °C

increase in cartridge temperature. If there is a 10 °C (18 °F) temperature increase, this will equate to a muzzle velocity increase of 25 fps. This increase in velocity will change the POI at 100 yds by approx 3 – 4mm (1/6 inch), not counting the lower air density.

Notwithstanding these observations, there are also other factors that cause a much greater change in the POI at this range with changes in ambient temperature.
Acknowledgments

Would like to thank Sgt G. Roberts Western Australia Police TRG Sniper Cell Maylands WA 6051(Author), Senior Constable Clive Roberts of the Western Australia Police Forensic Ballistics Section, members of the Western Australia Police Tactical Response Group Sniper Cell and finally Dr Alexander Krstic MD, Spectre Ballistic Solutions Pty Ltd, Adelaide, South Australia.

 

[ssatt68]

All barrels speed up to about the 200 round mark and then gradually slow down usually as a % of velocity.

 

[Rem40xb1]

sounds like a crono problem

 

[Nostradumbass]

Yeah, I'm beginning to believe the chronograph may be suspect.
I'm going to try another chrono this weekend.

 

[Win_94]

10ft from muzzle

I place my chronograph 10 yards beyond the muzzle so muzzle blast will not effect velocity readings.

 

[swampbuck]

Most cut rifled barrels need about 100-200 rounds to level off and be fully boken in. This is normal. They start to clean easy after a few rounds but as your chrono tells you it is still breaking in. Button barrels usually don't take as long.