whats up with my chrono????

[bteate]

I've got a Competition Electronics Pro Chrono Digital. and its starting to give me some velocities that are starting to irritate me. At the end of last year, the load I had worked up for my Sendero was averaging 3207fps with 140gr AB, 64.5gr IMR4350, WLRM primers<, and Fed.brass. Sounds good, so I took the same load this morning and avg was 3109! I used the same rifle and EVERYTHING. Whats going on?

I called the company this morning and that model requires no calibration, but I could send it back to check it out. I'm going to try new batteries in the morning and see what that does.

Does anyone have one of these models that have ever had this kind of trouble?

 

[hardpan]

Could be a lot of explanations for the variance. Bullet seating depth, heat/cold (outside temp), barrel temp, brass, etc... I doubt that the variance could be the powder charge, even though a lot of people look at that first. The 100 fps difference for a gun set to 300 yd zero is about 0.25 difference at 100 yds, in others 0.25 MOA. Another way of looking at it is that the zero went from 300 yds to about 287 yds.

I tend to trust the instruments more than the variables. Especially comparing the end of last year with now. Even if the Chrono is off, the accuracy difference is really small for hunting... good luck in tracking this down. But, I would check the other variables as well.
Hardpan

 

[bteate]

Its the exact same everything-brass, powder lot, seating depth, all of it. weather is the only variable, which is within 10 degrees of last time. I'm going to try some different things with it to see if I can find anything .

 

[hardpan]

Doesn't sound too easy. I have seen that much variance in my rifles within a single day... Especially a weatherby that is getting warm, or near max load. 10 degrees should not matter much on a cold barrel, but that is an opinion without any engineering work to back it up. And certainly the powder burn should not be very different at all within 10 degrees. Heck, i see that kind of temp swing in a single hour or two of range time in the early AM. You may try crimping the bullet... Sometimes the "grip" of the brass varies the velocity. And check the chrono just in case. Do you turn the necks? I tend to turn the outsides but not insides.
Hardpan

Its the exact same everything-brass, powder lot, seating depth, all of it. weather is the only variable, which is within 10 degrees of last time. I'm going to try some different things with it to see if I can find anything .

 

[bteate]

no, I don't turn necks. I just found one round left from the work-up last year. Its 64gr but I'm gonna check it in the morning also.

what style neck turner do you use, hardpan?

 

[DrMike]

Brandon,

Just out of curiosity, was your powder exposed to higher than usual temperatures? Is there any indication of decomposition (dust, acrid smell, colour change)?

 

[bteate]

Mike,

I keep my powder in a cabinet in the house, so it will be at a regulated temp, and it doesn't look or smell any different. I really think it's my chrony acting up. I've read the manual and I'm going to shoot the same load in the morning with one round painted black with a sharpie to see if its a lighting problem. I usually shoot about an hour and a half after sunrise to get ample lighting. It may be some weird shadows or something that I'm not recognizing.
When I say last year, it's actually in the spring before it got hot. I try to shoot when it's at or around 65* so it would be like a real warm day during deer season. I also take notes on temp and try to practice in temps down to freezing or a little below to note velocity changes. Down here in Ga, it doesn't get all that cold.

 

[DrMike]

Chronographs do tank! It has happened to me. Are you using the screens to give a background for the bullet to be shadowed against?

 

[bteate]

Yes sir, my manual says that I can take the screens off in certain lighting situations but its never ( I think ) been a problem before. Maybe I'll know more in the morning.

I want to thank you again for you're input on my .338 and all my other projects.

 

[hardpan]

no, I don't turn necks. I just found one round left from the work-up last year. Its 64gr but I'm gonna check it in the morning also.

what style neck turner do you use, hardpan?

I use the forster one. I have it set up and connected to a driver drill to turn the lathe...

BTW, definitely check the Chrono. Mine is several years old, but going strong.
Hardpan

 

[bteate]

Well, this morning I went out to shoot some and installed a new battery and made a sun shade for the sides. Bingo. Velocities were back to previous load, 3220 fps. The best I can tell, in the mornings, the sun cast shadows and bright spots giving false readings, or it could have been the battery. there's alot of trees around so I'll just have to pay closer attention to shadows and such. Either way, we're back in full force.