Wild Velocity Readings

[loudandproud]

Hello Guys,

Im all sorts of confused.

I have chronographed my rifle on 4 different occasions. The rifle is a Savage 110 chambered in 7mm Weatherby Mag. I am shooting a 154gr Hornady SST over 69.0 grains of powder and igniting them with Federal Magnum Rifle primers.

The gun is very accurate. I have been consistently getting .5MOA accuracy out of this rifle.

My chronograph is a Competition Electronics Pro-Chrono. It seems to be very consistant. I usually chrono more than one rifle when I set it up, and I have not had trouble with it on any other rifle. This rifle, and this rifle only, is giving me wild chrono readings.

See below:

The rifle seems to be throwing different velocities with no rhyme or reason.
I changed the scale today to see if it will make a difference. Any other variables I should try to track? I wanna get to the bottom of this...

 

[stool]

TOO MANY UNKNOWN VARIABLES to answer.
do you trickle up your powder loads ??
what scale ??
what powder ?
same lot of powder ??
how far away is the first screen ?
what time of day
did someone shoot right next to you .
the three screen ohler will tell you when you have a suspect number.

 

[loudandproud]

-All the loads are trickled with .1 grains
-The first scale was a 1500 Hornady Electric scale, the new scale is a RCBS Chargemaster
-H4831SC
-All the same 8lb' bottle
-The chrono is set 10' away (muzzle to first screen) each time
-The time of day varies from each session
-Noone has shot around me

Gun and chrono was leveled each time.

Im icing down some ammo and trying again tonight to see if there is a dramatic difference in velocity.

 

[6BRinNZ]

My money is on the chrony. To get 100 - 200 fps more avg is gonna take a lot of powder - you would notice that.

Sure I understand that it seems fine with the other rifles.

Can you try another chrony?

 

[Old Navy]

Set the chrono out at 12 to 15' to see if that makes a difference. Later! Frank

 

[Outrider27]

Set the chrono out at 12 to 15' to see if that makes a difference. Later! Frank

+1.
I have used one of these over the years and for a 7mm Weatherby mag I definitely second Frank's advice.

Mike

 

[stool]

yep that is why i asked...
10 ft is kinda short for a mag.....i use 12, but 15 should remove it as a possible error.

quick load says at max coal that load is just under 3100fps....your first numbers seem right.

now since you changed the process with the chargemaster and velocity went up...
i suspect the charges are not/were not 69 gr.

 

[tjtjwdad]

Set the chrono out at 12 to 15' to see if that makes a difference. Later! Frank

++ 2.

In addition, make sure you're not set against a wall (either end of firing line) because turbulence bouncing off the wall can cause issues too. Barrel can temp makes a difference too and it doesn't take much.

In group 1, all are relatively close except #4 (look at that particular piece of brass if possible).
In group 2, all are relatively close except #5 (Same as above)
In group 3, looks like two are spread further apart but the temp is a little higher and the barrel is green.
In group 4, all are relatively close except #5 (check out that particular piece of brass)

For these type of rifles, if I can shoot 3 "quick' shots at .5 MOA at 200 yards from a cold barrel, that is a good load; sounds like you have a good load so the hard part is done.

With respect to high and lows within the groups I'd look at brass neck uniformity but in reality, none are that bad but again, it doesn't take much. The other thing I'd take in is the barrel temp "and" if you have a round sitting in the chamber longer than another. H4831SC is an temp stable powder (per Hogdon) but it still is subject to temperature swings; just less so, per Hogdon.

With respect to the velocity differences between the groups; distance and light come into play with chronographs (in my experiences).

 

[loudandproud]

Update.

Last night I took the rifle back out and set up at 12' instead of 10' and I check everything twice to make sure everything was good and level/square.

I brought a bag of ice and had 5 shell inside protected by a ziplock baggie. I measure the shell temperature with a laser temp gun.

The point of this test was to see if the temperature was causing the wild swings by isolating that variable. The ambient temp rounds and the "chilled" rounds were fired without changing the set up. The gun was allowed to cool two minutes between each firing. The gun was mounted in a vise, and shell were fed into the rifle and fired IMMEDIATELY. As soon as the bolt was shut, I ripped off a shot.

The results tell me that the rifle is not the problem. When the previous results are removed, the temperature sensitivity comes out to around .88FPS/degree. This seems probable...

Maybe the scale was throwing crazy loads or the chrono was picking up muzzle blast. I do have a Badger ordnance FTE muzzle brake and it is VERY concussive... like 50 bmg with a howitzer style brake concussive.

 

[stool]

i still think it is very simple..the scales and the operator.
test one beam scale..all low to expected...probably the scale is off or the powder is a slow lot.
test 2/3 are both over expected for the load...the scale is off or the powder is faster than expected.
( powder cannot be both slower in one case and then faster in the next...so it is the scale/operator)
notice that 4/5 are very consistent..but a little faster than expected.

the cold test is nice but not relevant to the first 5 tests.

bottom line the operator, the scale or something he changed and did not tell us...like primers from another lot, brass from a different lot. change in coal.

 

[loudandproud]

The scale was dropped and broken...hence the replacement. The previous scale can no longer be checked.