Range Report Velocity Differences Help.

[ashland]

Last Wed. I started loading for a new rifle I got in 260. I loaded 42 grs of 4350 pushing a 139 Lapua seated at 2.80 OAL. It was averaging 2,830 fps. The temps were around low 70's. yesterday I shot those same loads and was averaging about 3,011 fps. Temps were aroun 68 with 60% humidity. I'm shooting over an el cheapo Chrony but why would the velocity change that much? About a month ago I shot some Black Hills 175s over that Chrony and the rounds were pushing about 2,650 fps. The next week I shot round from the same box that were about 100 fps faster than the week before. Temps were close to the same. Gonna try a different chronograph when I get the chance but was wondering if anyone has had this problem.

Thank you.

 

[MitchAlsup]

Re: Velocity Differences Help.

Try new batteries first.

 

[Victor N TN]

Re: Velocity Differences Help.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ashland</div><div class="ubbcode-body">... I'm shooting over an el cheapo Chrony but why would the velocity change that much? ... </div></div>

Look at the name brand of the equipment...

 

[jwoolf]

Re: Velocity Differences Help.

I've seen some cronos measure funky readings when you change the distance to the crono.

I saw one crono measure X fps at 12ft and X + 200fps at 16ft.

 

[flyboy]

Re: Velocity Differences Help.

Also look to make sure that the chrony is set up exactly the same each time. Alittle differnece in angle or level will change the total distance the round travled between the two eyes and throw the readings off.

I was messing around with mine in the back yard with the bow and arrow and trying different things to see just how much I could effect the numbers. With such a short sampling (12" to 24"), it doesn't take much to through it off.

 

[kraigWY]

Re: Velocity Differences Help.

Check the light too, I was working on loads while bright and sunny. It clouded up and through everything off.

 

[ashland]

Re: Velocity Differences Help.

I posted this thread in the Reloading section too. Really didn't know where to put it but I appreciate your comments. I'll keep working on it.

Thank you.

 

[former naval person]

Re: Velocity Differences Help.

Also, on the Chrony, there is a stop screw (or was on the one that I used to have) that locates the screen distances when opened. A little off on that distance when opening up the hinge can throw things WAY off. Got to be exactly flat out open. Found that out on my own.

 

[ashland]

Re: Velocity Differences Help.

Never heard of that. I'll check it out when I get home. Must look in my directions manual

 

[Grump]

Re: Velocity Differences Help.

Ummm, sorry to disagree but the pure mathematical fact is that a 1/4-inch discrepancy in screen distance over 24 inches is only 1.0416666... percent off. At 3,000 fps, that's 31.25 fps.

Does that set screw fudge your screens more than half an inch???

IME, angle and quality of light (Don't want the shadow of the gas cloud to trip the start screen now, do we?), and battery condition, are the first two things to look at. That reminds me--I don't have a spare 9v in my PACT box right now, and the older combo timer units eat batteries.

 

[ashland]

Re: Velocity Differences Help.

Well hell, I never paid that much attention to the angles of where I place the chrony or thought about the gasses tripping the system but I guess it would. Think I'll try it again and shoot over my friends chrono too.