First time using chronograph

declan1980

I bought a chronograph on eBay, and today was the first chance I got to use it.
I was testing my usual hornady 55gr v-max hunting rounds, and I was quite surprised by the results.
I fired ten shots taking care to try and keep the same amount of time between shots.
The lowest reading was 3032fps and the highest was 3186fps. That gave an extreme spread of 154fps. My average velocity was 3116fps, which was pretty close to the 3100fps I had calculated using jbm.
I was surprised by the extreme spread with these rounds considering that they would be considered premium quality ammo, but since they are the most accurate rounds I've used in my gun I'm still happy enough to keep using them.

tomcat220t

What speed are they advertised at ?What lenght is your barrel ?

declan1980

They're advertised at 3240fps. I'm shooting them out of a 24" 1:8 twist tikka barrel

declan1980

I forgot to mention that the sun was shining when I started, but it started to get a bit overcast for the last four shots, this was when the velocities started to drop. Would this have had the effect of showing lower velocities, or am I just trying to find excuses?

Cass

The weather can effect readings. Well not so much weather as sun. The sunshades should be used in strong sunlight and in overcast conditions no shades. The best weather i find is a cloudy day thats not too dark.

Either way you should always try and shoot in the same conditions.

After saying all that i was testing a box of lapua ammo once, and a shot went 2.5" low. When i checked the chrono the speed was down by over 200fps on the other rounds. So you can get a good spread even with good quality factory ammo.

The only thing a chroo is good for is when this happens you can be happy knowing its not a fault with your rifle, scope or technique.

poulo6.5

declan1980 wrote: »

I bought a chronograph on eBay, and today was the first chance I got to use it.
I was testing my usual hornady 55gr v-max hunting rounds, and I was quite surprised by the results.
I fired ten shots taking care to try and keep the same amount of time between shots.
The lowest reading was 3032fps and the highest was 3186fps. That gave an extreme spread of 154fps. My average velocity was 3116fps, which was pretty close to the 3100fps I had calculated using jbm.
I was surprised by the extreme spread with these rounds considering that they would be considered premium quality ammo, but since they are the most accurate rounds I've used in my gun I'm still happy enough to keep using them.

i chronoed a couple of different rounds for my .243 and the same thing was showing for me.
i tested 75gr hornady custom that was considerably slower than advertised but the 95gr superformance was over 100fps faster than advertised.

while testing the high/low shots would equate to high/low speeds. thats the joy of factory ammo.

clivej

So here's the stupid question of the week!!

Is the difference in the velocities of the factory rounds that there is a difference in the amount of powder going into the cases????
And is the difference only just a couple of granules of powder?
Or is there more involved like cases or the primers?

Kramer

clivej wrote: »

Is the difference in the velocities of the factory rounds that there is a difference in the amount of powder going into the cases????

Could be powder or neck wall thickness/neck tension or case capacity or bullet seating depth or bullet uniformity or primer seating or flash hole inconsistency etc. Lots of possible reasons TBH.