Dramatic drop in velocity with exact same components - strange! Ever seen this??

5gauss

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I have a Browning X-Bolt in 28 Nosler I've been shooting for a couple of years with 143 gr Hammer Hunters. I recently got my hands on some ADG brass and N570 powder. I've been wanting to try some load workup with these new components. Went to the range a week ago to do pressure test and it went like this:

83 grains - 3465 fps with no pressures signs
84 grains - 3575 fps with no pressure signs
85 grains - 3600 fps with no pressure signs
86 grains - 3722 fps with slight ejector mark, no sticky bolt

Decided to focus around the 86 grain level (+/- .3 grains) to test accuracy.

Back at the range yesterday to try those loads and they shot .5 MOA BUT they all ran velocity around 3500 fps instead of around 3700.

I've been loading for 35 years and never seen anything like this. Components were exactly the same each time:

New ADG brass
N570 powder
CCI-250 primer
143 grain Hammer Hunter bullet

using Magnetospeed chrono

Anybody experienced anything like this?? It's driving me crazy trying to figure out why velocity would drop 200 fps with exact same components, seating depth, etc.
Could be digital scale issue. ALWAYS weigh an item of known weight on your scale to check calibration/accuracy before each loading session. Takes one second and eliminates one variable in the puzzle...
 

gigem82

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I don’t have any specific advice, but don’t assume the chrono is right. Be sure and double heck all of your connections, etc. may be worth a call to them they are super responsive. Just MHO.
 

Chas1

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before you go through a long list of eliminating all the variables I take your chrono and a friends and shoot through both for comparison or shoot a friends reloads of which your friend is confident on what velocities should be and shoot through your chrono and see.
 

Veteran

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I second the new brass vs. once fired aspect.

Other things I would check.

Get a borescope and look at your barrel. Shooting 3400 to 3500 velocities w copper bullets means you might need a good copper cleaning w Boretech CU2+ or Montana Extreme in addition to carbon cleaning sequentially as it layers up, carbon, copper, carbon, copper, etc.

Check your scales. N570 is very notorious as being a chunky monkey powder and does not always meter well for me. Anytime I throw N570, I weigh it on both a balance and an electronic scale until they both agree its exact.

Was there any difference in your neck tension or crimp possibly?
Did you lube necks or bullets w graphite or other dry lube one time and not the next when seating?

How do you measure seating?
With caliper or w adjustable seating die?

Though you state temperature was not a factor, did you cool the barrel between shots any differently?
Ammo can really be affected by sitting in a hot chamber vs. a cooled one.

Good luck on solving the mystery.
 

JKPhoto

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I think that the only things that would result in this level of change would be the fouling issue and the weight of the charge. One or the other. Just my $0.02
 

muleystalker

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While shooting the first session were you placing a round in a heated barrel and then taking your time to shoot, it’s real easy to cook a round with a hot barrel. While shooting F-Class where we are shooting 20 rounds at a time you can experience this if you camber a round in a hot barrel and then try to wait for a wind condition.
Are you using a beam scale, if so it’s real easy to bump one of the weights one notch and not realize it.
 

Paulstarr

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Butterbean has the right idea, do another batch and see what happens, sometimes chrono's fib the megnetospeed fits to the barrel and with most rifles has an effect on resonance.I use one and although I trust the velocities but in most cases it can have effect on the P.O.I.I take it the primers were for the same batch and the ambient temperature was around the same, if so,Mystery to solve.
 

ButterBean

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And before the OP has a chance to verify here we are

Fire 2.gif


Carbon Ring, Y'all forgot to tell him to check for a carbon ring, Break out the bore scope, Or wait, Maybe He didn't use an AMP annealer, I bet that's what it is, Yep I'm sure that's the problem

Really.gif
 

Csbc4fun

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And before the OP has a chance to verify here we are

View attachment 299879

Carbon Ring, Y'all forgot to tell him to check for a carbon ring, Break out the bore scope, Or wait, Maybe He didn't use an AMP annealer, I bet that's what it is, Yep I'm sure that's the problem

View attachment 299882
You Funny… Hand held drill and socket should do it! LOL…
 

nimrod_gn

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Station pressure almost identical. Air temperature and temperature of ammo almost identical. Case length, neck tension same. I even weigh the brass. Humidity very similar. Use digital scale that I calibrate before each use. All of this is still leading me to believe it must have been something up with the chrony.
 

Jer2170

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Pointman

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Could be digital scale issue. ALWAYS weigh an item of known weight on your scale to check calibration/accuracy before each loading session. Takes one second and eliminates one variable in the puzzle...
You can get scale calibration weights online from several sources. They can be used as a "sanity check" for the scale after calibration.
 

Doom2

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I was kidding about the humidity. Barometric pressure has almost no effect on chronograph readings, especially for the MS. For optical chronographs its on the order of 1/2 fps on a 55gr 223 round @15 ft from the muzzle for 1/2” mercury. For heavier bullets its less.
 
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