308 Velocity increase

[C.E.Smith043]

This may have already been mentioned but imo, I think its a combination of factors.

I think the round cooking in the chamber can be contributing to it along with the barrel itself being hotter and ever so slightly increasing the bores diameter resulting in less friction drag on the bullet, this increasing the velocity.

 

[C.E.Smith043]

So now maybe thinking when I was putting chrono up one of the sky screens diffuser popped of to the side, sun was to the side also. Think that's an issue?

This definitely could be an issue. The chronos with eyes that detect the bullet rely heavily on a consistent light source.

To my understanding, as the bullet passes over the eye in the screen the eye detects a change in the light, thus starting the clock and as the bullet passes the last eye it notices the light difference again and stops the clock. The calculation is then made to determine the speed based upon the time between both eye triggers and the distance between both eyes.

Sun at an angle is one of the hardest angles the chrono has to deal with. Ideally you want an overhead sun with the diffusers in place.

 

[Redrocket]

How many rounds through new barrel
Velocity increase could be durds, never have seen abbe to barrel break in process.

Yeah I though about that.<80rds, never seen a new bbl pop like that. In fact I have it's twin bb' on a Curtis Axiom action and it didn't do that.

 

[Ggmac]

What did the primers look like ? Any difference in extraction?

 

[charlie b]

Have you fired any more to see if the situation repeats?

 

[Doom]

What other info would be helpful. Glad to see you're a 4064 user also. These are all weighed charges. 44.5grs rem 9 1/2 norma cases with 155gr berger hybrid target bullets in a newish bartlien bbl. Not close to making max charge by what I see in some manuals. I work up from a lower velocity to normal range looking for accuracy. Wish I didn't have 2 1/2 jugs of this powder now. So now maybe thinking when I was putting chrono up one of the sky screens diffuser popped of to the side, sun was to the side also. Think that's an issue?

While I don't have the data to show it because I don't have anything to compare it with I found optical chronographs to be good for quick short term comparisons but they seemed to be sensitive to sun position. I'm sure that some are better than others. Could it explain all of your increase? Doubt it.

I would forget about this incident and make sure to work to keep the advice on storage and handling of rounds given here. As for 4064 temperature sensitivity it fairly flat on the upper end of the temperature range and more pronounced on the low end.

 

[charlie b]

FWIW, the reason I got a Labradar years ago was due to sun issues with my old chrono. I shoot early in the day and the low angle messed with the optical sensors. I tried all kinds of shades, optical slots, etc, etc. None were reliable. If it shaded the unit, there wasn't enough light on the screens to work.

 

[Doom]

FWIW, the reason I got a Labradar years ago was due to sun issues with my old chrono. I shoot early in the day and the low angle messed with the optical sensors. I tried all kinds of shades, optical slots, etc, etc. None were reliable. If it shaded the unit, there wasn't enough light on the screens to work.

I'll also add this. While the radar units cost more they are so much more usable requiring no downrange setup and will give extremely reliable data. There is no reason not to record the data for every shot taken.

Radar technology is vastly superior to optical. But one comment on accuracy that you see is the issue of the radar picking up the bullet somewhere downrange from the muzzle. This is no different than optical where the chronograph is downrange to avoid muzzle blast. It is very easy to estimate muzzle velocity from two velocities are known at known distances. This is especially true with modern bullets and most centerfire cartridges.

 

[Redrocket]

What did the primers look like ? Any difference in extraction?

All looked pretty good, think they were ~2grs under max load

 

[Redrocket]

I'll also add this. While the radar units cost more they are so much more usable requiring no downrange setup and will give extremely reliable data. There is no reason not to record the data for every shot taken.

Radar technology is vastly superior to optical. But one comment on accuracy that you see is the issue of the radar picking up the bullet somewhere downrange from the muzzle. This is no different than optical where the chronograph is downrange to avoid muzzle blast. It is very easy to estimate muzzle velocity from two velocities are known at known distances. This is especially true with modern bullets and most centerfire cartridges.

Yeah I got a lab radar but bought this unit to Chrono shotgun loads. It's so convenient compared to the older lab radar I find myself using it more often. One of the Garmin's or new lab radar is on the list

 

[Redrocket]

While I don't have the data to show it because I don't have anything to compare it with I found optical chronographs to be good for quick short term comparisons but they seemed to be sensitive to sun position. I'm sure that some are better than others. Could it explain all of your increase? Doubt it.

I would forget about this incident and make sure to work to keep the advice on storage and handling of rounds given here. As for 4064 temperature sensitivity it fairly flat on the upper end of the temperature range and more pronounced on the low end.

That's good to know

 

[Redrocket]

Have you fired any more to see if the situation repeats?

I'm going again Monday but won't be able to take 308. Will retest and report later. The reason or way I caught this is the new competition electronics chrony can present the data it a graph form and looking at it caught my notice more than a list of numbers. Of course you can always put you data in a spreadsheet and graph it but.....