Range Report Velocity Decline During Range Session?

Grump

Gunny Sergeant
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Minuteman
Oct 23, 2008
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Temps varied a whopping 4 degrees F this morning, it was the same batch of ammo, and they just kept going slower and slower.

First six:
2575.2 avg
ES 22.8
SD 7.8
100-yard zero spot on for elevation, .4 mil left from where I started.

200 Yard, 3 shots:
2554.1 avg
ES 21.7
SD 12.0

300 yard, 4 shots:
2541 avg
ES 27.5
SD 11.6

400 yard, 3 shots:
2540.9 avg
ES 23.7
missed SD, can't remember if chrono uses sample or population, don't want to bother

500 yard, 3 shots:
2515.5
ES 27.9
SD 15.0

600 yard, forgot to start chrono. @#$#&%^$&!

M1A, new med-hvy bbl, cold dirty bore.

Last year during load development, my 4064 loads did this through several sessions (e.g., 2580 one day, 2555 or so two months later at the same temp and altitude, down to 2530 maybe 2 months later when things were about 10 degrees warmer).

Anyone see this?

Causes?
 

shadow4

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Jul 6, 2005
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Aiken SC area
Re: Velocity Decline During Range Session?

How much do you trust your chrony?

Also consider that as the light conditions change so might the readings (unless your using an infrared model). One other thing to think about is that as you added elevation (as target distance increased), the bullets were not flying across at the same height through the chrony as previously. I've noticed on mine if I shoot through the very top of it, it reads a slower velocity.......YMMV

I've seen my velocity change through out the day with no temp increase or decrease. I chalk it up to chrony accuracy and shooting across it inconsistently. My next model will be an infrared model to try to eliminate these things. Plus I won't have to rely on good light conditions to get descent readings.
 

Sure_Shot

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Minuteman
Oct 27, 2001
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Schuyler Co. NY
Re: Velocity Decline During Range Session?


Not sure what your brand of Chronograph is but lighting has quite an effect on consistent readings.

Obviously some brands more than others.

Sky screens help but don't always keep it consistent.

It's just one possible explanation - if sunlight across the sensors changed much during the shooting session.
 

Grump

Gunny Sergeant
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Oct 23, 2008
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Re: Velocity Decline During Range Session?

Older PACT with the shot timer. The one without the download data ability.

I liked my previous dedicated Chrono better, but it must have bounced out of the back of the truck.

All shots were high through the sensor "window", just below the diffuser screens. Maybe a 2 inch difference, from a starting point of 12, so I doubt much effect from that. It's worth investigating, though.

Lighting--spotty overcast as T-storms gathered behind me, in shade of the clouds all the time. Some brightness variation between the 1st and 3rd strings, but back to the same as #1 for the rest.

I understand random variation and instrument tolerances, but this was a bit much. Anyone ever heard of a barrel shooting slower as it heats, instead of faster?

Maybe I should try Hornady's SuperFormance 178s next weekend when it gets back above 100???
 

BryanLitz

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Commercial Supporter
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Re: Velocity Decline During Range Session?

It's good to be wary of the chronograph and the effects of light and the angle that you're shooting through it. Good ideas and are quite possible causes of erroneous measurements.

However, it's not unheard of for a new barrel to actually shoot slower (or faster) as it breaks in. how new is the barrel? Typically I don't take a barrel seriously with regards to load development, accuracy, velocity, etc until it has over 100 rounds thru it. There's just too much changing early on.

-Bryan
 

Grump

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Oct 23, 2008
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Re: Velocity Decline During Range Session?

I think I'm at about 300 rounds now.

Cleaned it once or twice, always down to two patches of no copper trace (even after letting the patch sit for 20+ minutes--they "bloom" IME) with Sweets and Butch's.

Don't even know how long the throat is. Just load everything to 2.80 to 2.83. I don't believe in ammo that's not mag length.
 

Alan Griffith

Sergeant
Full Member
Minuteman
Re: Velocity Decline During Range Session?

Since you're breaking in a somewhat new barrel, it may be you're wearing out your throat. My 6.5x47 Lapua has 820 rounds down it. I measured the vel about 10 days back, about 760 rounds and the velocity had dropped off 60-65 fps from when I finished my load development. Basically, you'll need to measure your throat wear and if that's the culprit, then you'll need to up your powder charge about .2 gr and maybe even move your seating depth out you find a sweet spot and want to keep it there.

Alan