How do you know if your chronograph is accurate?

[5MilesBack]

I've mostly used my chronograph for archery, and it's been pretty spot on compared to a few other chrono's that I've compared to (shops, shoots, etc). But used it on two rifles this week, and wondering how accurate these numbers might be.

Rifle 1 - Tikka D-18 .243 with 22" barrel and 1:8" twist -

Hornady Precision Hunter 90gr ELD-x - 10 shot average 2989fps, box says 3150fps.
Unknown Munitions 108gr ELD-m - 10 shot average 3036fps, UM says 2980fps.

Rifle 2 - Seekins Precision PH2 6CM with 24" barrel and 1:8" twist -

Federal Premium 105gr Berger OTM - 10 shot average 2947fps, box says 3025fps.
Hornady Black 105gr BTHP - 10 shot average 2848fps, box says 2960fps.

To me, these all seem fairly reasonable given manufacturer's probably listing best case scenario speeds with 26" barrels etc. But the UM loads surprised me a little bit (the only ones higher than listed), and I'm wondering if my numbers are accurate. I will check it against dope and longer ranges, but I'm not going to be able to do that before a late season antelope hunt right around the corner. Should I use my numbers or UM's? I don't anticipate needing any shots past about 400 yards. What say you?

 

[The Guide]

I would use your numbers and verify via drops at 600yds. You should have time on a hunt to shoot at a rock at distance to verify dope before you spot a target animal and fire on live game. Sub 300 yards you shouldn't need to dial dope anyway.

Jay

 

[Mule3006Elk]

Shoot at distance and verify drops. If they are spot on your mv is probably good.

If not, there are 2 variables at play assuming it's not the system (gun, scope, etc.) or the shooter.
Bad mv data or incorrect b.c.

I have a Kestrel which takes the guessing out of the 2 variables. If Kestrel dope isn't working I really at my system and me the shooter.

 

[hereinaz]

Which chronograph?

I ran Magnetospeed and Labradar next to each other and they were off maybe 15% off the time and at most by 2 mph.

I ran Magnetospeed and Garmin and same thing.

 

[The Guide]

by 2 mph

Which equals 2.9333... fps.

Jay

 

[SDHNTR]

Use your numbers and then verify drops. Box numbers are nearly useless. Every barrel is different.

 

[hereinaz]

Which equals 2.9333... fps.

Jay

Crap, meant fps… lol. I had my head somewhere else.

 

[The Guide]

Crap, meant fps… lol. I had my head somewhere else.

A fun curiosity is that a bullet with a mv of 3000 fps has a mv of 2045.454545... mph and the commonly accepted minimum terminal velocity of 1800 fps is 1227.272727... mph. Gives another way to visualize how a bullets fragmentation at velocity can cause so much damage in tissue.

Jay

 

[Legend]

Shoot at distance and verify drops. If they are spot on your mv is probably good.

If not, there are 2 variables at play assuming it's not the system (gun, scope, etc.) or the shooter.
Bad mv data or incorrect b.c.

I have a Kestrel which takes the guessing out of the 2 variables. If Kestrel dope isn't working I really at my system and me the shooter.

Add a third. Environmental variables such as elevation and air density.

 

[5MilesBack]

Which chronograph?

It's a cheap Prochrono.

I'm going to try and get to the 600 yard range next week and check the drops with these numbers.