Chrony Question

[Firecat]

Hello, Its been a few since I've posted on here but have a simple question. I just purchased a new chronograph. For those of you with experience, how many shots do you fire through your chrony to get adequate data for any given load/gun combination.

The calibers under consideration are:

223 Rem - 50 Grain Speer TNT
220 Swift - 52 Grain Sierra Matchking
6mm Rem - Under Development
25-06 Rem - 100 Grain Barnes TSX
270 Win - Under Development
7mm Rem - 140 Grain Barnes TSX
30-06 Spring - 168 Grain Sierra Matchking
300 Win - 180 Grain Sierra Gameking

 

[Mikecr]

Kinda depends on the type of data you want..
Do you want Hot MV, or ES, or SD?
Or the same cold?
Or prefouled cold?

 

[Firecat]

It seems like any time I post a question I often forget half the details. Thanks Mikecr for the questions. I would like to achieve adequate data to get some custom drop turrets from Kenton Industries. for this I would like to get average velocity, Extreme Spread, Standard Deviation, Hi and Low Velocity. If there is anything else you would think helpful for the purpose I stated I would appreciate knowing it.

 

[SBruce]

My opinion...........it takes at least ten shots of any given load to get enough data for a GOOD average, especially if you want to know SD's.

If it's for drop charts or turrets either one, it would probably benefit you to try and do your chronoing under similar conditions you intend on using that load. Example: if the .223 is going to be used in the winter, then chrony it during the winter. If you're loads are subject to being left in the truck overnight during the winter, then put them in the freezer before you chrony, average barometer reading, ect, ect.

Don't forget to measure exactly how far the crony is from the muzzle so you can let them know that too (TRUE muzzle velocity, not 15 or 20' away). Let them know the altitude where you did the shooting, but more importantly; let them know the AVERAGE altitude where you plan on using the turrets.

Barometer, Temperature and Altitude have a bigger effect than alot of folks realize. Especially with relatively low BC bullets and slow velocity calibers. The bullets you mention in the .22's are pretty low BC, and the .223 is relatively slow.

Good Luck!!

 

[SBruce]

One more tip:

Don't put the chrony too close to the muzzle!! No Less than 10 feet away, especially if using really slow burning powders.

I almost destroyed a chrony w/ a 25-06 shooting 85 grn. bullets and 4831 powder with the chrony sitting 10' away from the muzzle..........Muzzle Blast & Concussion. I didn't actually shoot the chrony, but the excessive blast pretty much tore it apart temporarily, bent the wire rods, tore the sky screens/diffusers, and even moved the tripod enough that it wasn't lined up with the bore anymore!!

 

[Mikecr]

A few statisticians hear might get all flip about it...
But I agree with 10shts Sbruce suggested if you're capturing cold barrel(1st shot) MVs -with your final load.

1st shots are usually lower in velocity. Way lower. So don't cheat, let the barrel completely cool between shots, as you'd only be cheating yourself.
It is important to establish 1st shot standard conditions as far as barrel fouling too.
Not to mention everything Sbruce pointed out.
I pre-foul totally clean barrels with WS2, and have higher confidence in my 1st shot, than any other. Not that the others are bad,, I just don't really care about them.

Oh yeah, another thing. Make sure you level your screens everytime. Believe it or not, any screen angle messes with readings. This stuff is that sensitive..
I put my crosshair on intended target, insert a laser boresighter, and verify the dot is centered the same in each screen. Use the diffusers also(even though screens will work without them). These reduce light variance from day to day. But shooting in the morning one session, to evening in another might cause velocity reading shifts because of sunlight angles.
Well that's all I can think of for now.

 

[Firecat]

Thanks a bunch guys. Advice taken. I am anxious to go give it a try. I've been wanting to get a chronograph for a while now. I guess its time to put it to good use.