Explain Sudden Velocity Increase PLEASE

[Doc88]

So I'm working up a load for a 7STW RL 25 IN 0.5 grain increments. Get 3120 and 3123 fps between two half grain increments. Bullets pretty close in vertical. So there I am right. Load 5 rounds at 76.2gr. The above values were at 76 and 76.5. First two shots in same hole at 200 yds at 3120 and 3123. Piddled around a bit adjusted the scope for the final 3 shots probably about 5 min. Outside is mild 60 degrees. Fourth shot boom 3145 and 2 inches high. Fourth and fifth shot 3141 and 3130 but shots were half inch apart back down on the aiming point. Assuming all loads loaded exactly the same with same amount of powder etc. where did that 3rd shot come from?
On an aside I will be checking my RCBS chargemaster a little closer but it's regularly calibrated and rechecked. Rest of load info is 168 VLD at 3.640.
Any help would be greatly appreciated. Thanks.

 

[bsaoneshot]

Did you leave your rifle with rounds in the magazine or the rounds out I the sun? If they heated up any, it could attribute to the extra velocity.

 

[Wedgy]

The first thing I would suspect would be the scale, I've seen mine throw up to 0.2 over what's programmed, always over, never less as it doesn't stop until it hits the programmed amount. I check it on an FX120i so I don't have to deal with that anymore. Other things could be case volume, neck thickness/tension, annealing, flash hole, brass prep in general, bullet sorting/bearing surface....lots to look at as well as the chrono.
Two inches high for 20fps is a lot.

 

[Doc88]

Did you leave your rifle with rounds in the magazine or the rounds out I the sun? If they heated up any, it could attribute to the extra velocity.

No sir. Made the rounds and went right back to my range and shot. Range is behind my house.

 

[Doc88]

The first thing I would suspect would be the scale, I've seen mine throw up to 0.2 over what's programmed, always over, never less as it doesn't stop until it hits the programmed amount. I check it on an FX120i so I don't have to deal with that anymore. Other things could be case volume, neck thickness/tension, annealing, flash hole, brass prep in general, bullet sorting/bearing surface....lots to look at as well as the chrono.
Two inches high for 20fps is a lot.

I understand and I'm going to test a good bit more too. Just looking for a list of rule outs. This has been a picky gun. Thanks

 

[bigedp51]

Case capacity less?

I like going to the range when no one else is there, and then knowing I was the best shot at the range that day. And if I'm having a bad day at the range no one else needs to know about it.

How much coffee did you have to drink before shooting?

Sorry, I'm 67 and starting to get cataracts and shoot my best groups by accident.

 

[bsaoneshot]

Once fired brass vs new brass can show an increase as well.

 

[Doc88]

Once fired brass vs new brass can show an increase as well.

3X shot brass annealed FL bushing sized and trimmed. Bump shoulders .002.

 

[HARPERC]

....How much coffee did you have to drink before shooting?....Sorry, I'm 67 and starting to get cataracts and shoot my best groups by accident......

I've seen it discussed here, how the way the rifle is held can cause changes in MV. I don't know if it does or doesn't, but can't see 20 fps, adding up to 2" vertical change.

Respectfully, going with the shooter being the issue.

 

[C.O. Shooter]

What's your round count?

 

[Doc88]

I've seen it discussed here, how the way the rifle is held can cause changes in MV. I don't know if it does or doesn't, but can't see 20 fps, adding up to 2" vertical change.

Respectfully, going with the shooter being the issue.

I actually hope it is me being the issue. I do hold the forearm when I shoot as that's the way we shoot out of the deer stand. Good or bad I don't know. I'll prolly load some more and shoot holding only the rear bag.

Round count is about 125.

 

[cape cove]

I wouldn't consider 20 ft/sec as a great increase. I've had many 5 shot strings where there was a 20 ft/sec difference between the lowest and highest. Things change sometimes as you have more rounds down the barrel, like copper accomulation, heat etc. This can change velocity and pressure.

 

[HARPERC]

What is the accuracy range of your chronograph +/-. You may be just seeing the variation in the chronograph.

 

[LaHunter]

how far is your chrono from your muzzle? Muzzle blast can cause accuracy issues.