Help with large discrepancy in chrono readings

[cb4128]

Hey guys, had a pretty bizarre experience with my 264 win mag recently. In June I went to the gun range and set up my chrono and shot 5 different rifles through it to test loads and get drop data nailed down. 4 of my guns I was happy with and were around what I expected for speeds.

My 264 win mag wasn't as accurate as I remembered it being and I was shocked to see the average across 5 shots was 2,904 fps - this is with factory Nosler Custom 130gr accubonds - box listed speed is 3,100...I've never chrono' this gun so nothing to compare it to.

Fast forward to last week and I cleaned the barrel with Boretech Eliminator and had it showing no copper and no other fouling when I was done - it was really dirty and took probably 60 patches to get it clean. Took it to the range again, same setup, shot multiple rifles through the chrono and it was showing an average of 3,200 fps over 7 shots! It shot much better and the other 2 rifles I shot were within reasonable expectations of fps as well.

So, my question is - could an extremely fouled barrel really explain a 300 fps velocity change? Temperature was within 2 degrees the 2 different range trips and same box of ammo both times.

Will be interested to hear if anyone else has experienced something similar.

[bboswell]

What was your ES & SD each day?

[SabreKiller]

What was your ES & SD each day?

^^^^I have the same question.

[Gunnyart]

What type chronograph also.

[cb4128]

It's a pro chronograph the one with the metal rods and plastic white covers...

First day - 2904, 2911, 2930, 2943, 2904 (2,906.33)

Second day - 3182, 3253, 3159 (different ammo 3079, 3079; different ammo 3267, 3259, 3292)

[SabreKiller]

Is the ammunition from the same lot?

How far away did you have the chrono set up? Just curious

[cb4128]

Same box of ammo! Same distance both times - about 6-8' away

[Outbreaker]

What was the cloud cover like? Bright day vs cloudy day can jack with them at times,

[stxhunter]

Any way for you to try it with a magnetospeed?

[cb4128]

sure if someone wants to donate it for me to borrow? haha

[Pushbutton2]

What was your ES & SD each day?

Can you expound for me please?

[stxhunter]

sure if someone wants to donate it for me to borrow? haha

If you were in the Houston area or I was still up in DFW I'd lend you mine. Maybe someone in the area has one you could borrow.

[cattlelackranch]

Can you expound for me please?

Es= extreme spread (difference between highest and lowest velocity)

SD= standard deviation (eliminates the high and low reading and gives you the avg of the difference as a whole, kinda) basically it tells you the probability of your velocity +/- your SD. Larger sample size and smaller SD is good.

[cattlelackranch]

Help with large discrepancy in chrono readings

Was your ammo the same temp each time?
(Leave it in a hot vehicle or out in the sun)

Did you let a round sit in a warm chamber longer?
Fresh battery in chronograph?

I have a pro chrony and have shot over shooting chronies all had varying readings. I felt they were close but off slightly.

I have a lab radar you can shoot behind if you want to come to Rockwall.

[tgridley]

To answer your question yes a very dirty barrel will not only effect your accuracy but your velocity to.