100 fps velocity swings

[blaster113]

I was working up practice loads with BBI .40 165s, and had 2 of 5 shots read about 100 FPS faster than they should have. Other 3 were right where expected. Powder was Solo 1000 from a new jug, Rem 1X cases, SR primers. Powder charge was 5.1 grains and the drops were pretty consistent. Chrono used was my trusty Pro Chrono which has been pretty much spot on; my loads have always chronoed within 1-2 PF points at majors. I ran the chrono in the shade at about 2pm. Never had issues before with this method. Is it the powder, chrono, or some weirdness from shooting in the shade? 3 more shots with factory ammo and N320 loads were right on the money.

[Steve RA]

Possibly the chrono, at times they can be a little touchy!

[HI5-O]

The lighting angle causes most of my problems. Sometimes I use tape to block out some of the light coming at an angle. I use a PACT timer/chrono.

What was the distance?

Edited November 19, 2012 by HI5-O

[Hi-Power Jack]

Jonathan, using a chrono is a little like accuracy checking - if you fire five shots

and take the best three - you'll have a 1" group, and two "flyers".

Try taking a 20 shot chrono group and average it - I look for the lowest number

of the 20 shots - and make sure that lowest number is at PF. And you should

find some velocities below and above your average - you won't find 17 of the

20 shots to be within a few fps of one another - you'll find a range.

Not too meaningful, IMHO, to fire 3 - 5 shots over a chrono.

[lcs]

I was working up practice loads with BBI .40 165s, and had 2 of 5 shots read about 100 FPS faster than they should have. Other 3 were right where expected. Powder was Solo 1000 from a new jug, Rem 1X cases, SR primers. Powder charge was 5.1 grains and the drops were pretty consistent. Chrono used was my trusty Pro Chrono which has been pretty much spot on; my loads have always chronoed within 1-2 PF points at majors. I ran the chrono in the shade at about 2pm. Never had issues before with this method. Is it the powder, chrono, or some weirdness from shooting in the shade? 3 more shots with factory ammo and N320 loads were right on the money.

Always try for noon when the sun is directly overhead. The angle of the sun's light is causing the erratic readings. The chrono should not be in the shade and make sure you are far enough back from the chrono.

[Classic_jon]

This is probably not it but, something you may want to double-check just to eliminate it:

A long time ago I had some similar issues that took me a while to track down. Most of it was due to the cheap bullets I was using not being consistently sized and the second part was a loose seating Die that allowed the OAL to vary and therefore have a bad crimp.

I would say to grab a handful of the bullets and gauge them and then check your Dies to make sure they are tight and set up correctly. It cant hurt to check and should only take 5 minutes or so.

[CZinSC]

Try weighing a sample of bullets. I recently got a box of 500 125 FMJ's from BBI and the weights ranged from 123.5 to 126.4. Not sure if that will throw off the velocity by 100 fps, but it certainly doesn't help.

FWIW, I weighed a sample of Montana Golds I have with the same scale right after I weighed the BBI's, and they were all within .4 grains.

[blaster113]

Wow, lots of responses quick. I had a feeling it had to do with the chrono setup. I only fired 5 shots to get a ball park figure to see if I needed to go up, down, etc. on the powder charge. Distance to chrono was about 10 ft. Dies on the press are tight so that's not it; oal and crimp are consistent. I'll make some adjustments on the chrono setup and try again. 5.1 of Solo with my last jug got me 172 pf with a moly 170 so charge weight should be good

[blaster113]

And I weighed a bunch of bullets, most were 163-164 with 1 or two actually 165.

[HI5-O]

next time you chrono, let me know and we can run mines in front or behind yours.

[jlamphere]

From a statisitcs standpoint, your problem is the sample size is to small. In order for a sample to be normal, for practical purposes, the sample size needs to be 20 or more. A normal sample in statistics is very important because the rules that govern how the statistics are calculated assume the sample is normal. Not meaning to be picky and all of the other suggestions are valid, just thought I would a my two cents.

[Chronodude]

With 165's you're running a little over 1000fps for basic major. Speed of sound/blast is about 1200fps. A blast shield in front of the chrono may be the answer. Use a target/piece of cardboard with a 4x4 hole that lines up with a plane about 4" above your sensors.

2nd guess...light angle.

Chronoman

[blaster113]

With 165's you're running a little over 1000fps for basic major. Speed of sound/blast is about 1200fps. A blast shield in front of the chrono may be the answer. Use a target/piece of cardboard with a 4x4 hole that lines up with a plane about 4" above your sensors.

2nd guess...light angle.

Chronoman

thanks, will try that. I usually chrono in the shade so that the light doesnt fall directly on the sensors.

the damn chrono is truely voodoo! that looks like DonT's lovely chrono box in the avatar. I was shooting my match ammo with N320 and jhps throughout the week (10 rounds at a time) prior to our state match in that chrono box with IR screens. Nice feeling very consistent low SD/ES 170-171 pf. load. Of course come match day and its a bit cooler and rainy/overcast. 167 pf. If I loaded lighter I probably would've went minor. that rule about loading to 170+ pf for matches is no joke.