This is an odd one to me. I loaded up 90 rounds of 556,
once-fired LC brass of two recent years, Hornady 2267 bullets (55gr
FMJ/BT with cannelure), CCI 41 primers, and 25.0 grains of BL-C(2). All
cases sized with RCBS dies, and trimmed to the same length, 1.750".
Cartridge overall length was 2.200 as per Hornady 9th ed., each round
was lightly crimped with a Lee Factory Crimp Die, and each round dropped
into an L.E. Wilson Case Gauge.
Three mags were loaded with 30 rounds each. Starting with a clean rifle,
all 90 rounds were fired over 10 minutes or so with no additional
cleaninig, no "mag dumps", just a continuous, moderate firing rate, over
a chronograph. The goal was to put some rounds through this relatively
new rifle, see how it performed after having the barrel lapped, feed
ramps polished, some other minor stuff. The groups were fine, although I
used close targets (25 yd) because I'm not concerned about accuracy
yet.
Part 1 of 2



Tarkid

Part 2 of 2



Ok, to the question: what do you make of the three groups?

Statistic	Mag1	Mag2	Mag3
Hi Velocity Lo Velocity Ave Velocity   Spread Std Deviation	3189 2807 3087 382 75	2789 2569 2652 220 45	2726 2594 2660 132 40

What the heck would cause a drop of 400 fps average velocity, 13%, after
the first magazine of 30 rounds? And the 2nd and 3rd magazines were
very close to each other? I've been scratching my head over this for a
week now. Anybody out there have a sane explanation?
For reference, this rifle has an 18" heavy stainless steel barrel. It
was quite hot when finished, but not alarmingly so. Any ideas?
Tarkid

 
 
 
 
 
 



PS-for some reason this board thinks my 1500 character original post was >2000 characters. I just don't understand the new math...

I'm going with incorrect powder levels.

Only thing imo that covers the issue.

Did you weigh every charge?

Txl

I'm going with bad chrono data.

Did you fire any experimental control rounds before during and after the sessions?

Did the lighting conditions change during the session?  Did the skyscreens move due to muzzle blast?

Powder was dispensed with an RCBS Uniflow Powder Measure. Don't know if you've used BL-C(2), but it's extremely fine and measures beautifully. I could maybe buy this explanation if we were talking about three or five rounds, but not 30!



The chronograph being the problem could make sense. What you don't see in the status I included in the post are the number of values in each string. After throwing out obviously bad rounds (187 fps, is one example), there were 30 measurements in the first string, 17 and 19 in the 2nd and 3rd, respectively. Another issue is that I had the chrono way too close due to range constraints. It's possible that affected it. Sheesh, you've made me realize I might have damaged the darn thing. After the 556, I shot a series of 9mm cast bullet loads made with a new mould and it seemed ok, but I'll have to get it out and look it over.



Thanks for your thoughts, much appreciated.



Tarkid

You might be having chronograph issues.  
If you could borrow a chronograph from a friend, I'd shoot another string and "proof" one against the other.  
There should be no more than a 4-5% spread between them.

A lot of conditions could be present that might induce chronograph errors.  Ground glare, muzzle blast, mis-aligned sensors, etc.
I don't know if these conditions were present, just some alternate points to consider.

I chrono should be at least 15 feet away...depending on manufacturer.

Did you load/make 30 rounds and load them into a magazine

or

Did you load/make 90 rounds and load them into the magazines?

If you loaded/made 90 rounds and loaded them into a magazine not caring what order the round to mag was then it looks like the chronograph was the culprit.

I'd say chronograph issue or some slightly over charged rounds. Could be inconsistent crimp as well.

Isn't blc2 temp sensitive?
That one mag could have set in the sun and warmed up?

Given the placement of the chronograph close to the muzzle, the decrease in velocity over time, and the change in SD there are a few options, but no meaningful conclusions can be made without doing another test under more controlled conditions.

1. The chronograph may have moved and angled a bit so that the bullet crosses the sensors at an angle.  It's really measuring speed by measuring the time (by measuring the voltage that accumulates in a capacitor between the time the first and second sensor is tripped) it take the bullet to cross the fixed distance between the sensors.  The problem is the distance changes if the unit is angled up or down or side to side.  The distance between the sensors is really the hypotenuse of a right triangle, and the more the unit is angled relative to the path of the bullet, the larger the "short" side of the triangle gets, and the more the hypotenuse will grow relative to the baseline length between the sensors.    

The two largest and most common mistakes I see with shooter using chronographs are a) failing to ensure the chronograph is both level and precisely aligned with the path of the bullet from muzzle to target, and b) failing to ensure the chronograph is mounted securely so that its alignment doesn't change.  Most shooter set them up haphazardly, then blame the chronograph when they get bad data.

2.  The velocity is consistent after your first 30 rounds, the extreme spread is narrowing as reflected by both the extreme spread numbers and the improving SD.  More importantly, your velocity numbers in the last two sets are about what I would expect from 25 grains of BLC-2 in an 18" barrel with a 55 gr FMJ-BT.

3.  You started with a "relatively new rifle" so some of the decrease might be due to the barrel breaking in.    There's also the issue of heat.  90 rounds in 10 minutes isn't exactly a low rate of fire and the barrel will indeed get quite hot.  Hot barrels expand very slightly, and as it expands the bore enlarges very slightly as well, perhaps enough to account for some of the velocity loss.

4.  BLC-2 is temperature sensitive, but in the .223 and .308  that works out to something on the order of about 1 to 2 fps per degree F, so you'd have to show at least a 200 degree F change in temp to get the velocity differences you are seeing, and given the effects of the round heat soaking in a hot chamber, I'd expect the later rounds to be faster, the opposite of what you observed.

 
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My advice is to ensure your chronograph is properly aligned and mounted securely, so that you are shooting directly across the sensors. In other words, when you look down the rifle at the target the chronograph should be level, the Vs from the supports for the sensors should be aligned/parallel to each other, and you should be shooting directly over the middle of the each sensor.  That's actually far more important than muzzle to chronograph distance.  If you're using a powder with little smoke, and a small round with less muzzle blast, you can get by with a closer chronograph distance, provided the chronograph still doesn't wobble or change alignment when you shoot.    

Once you've gotten things properly set up, repeat the test firing 1 shot out of a clean bore, ignoring that cold bore data point, and then firing one round per minute, recording 10 round groups.  Shoot three groups and then compare the data for all 10 groups.  

You'll have an increase in temp over the whole 30 rounds, but it should be much less and the first 10 shots should reflect more or less cool barrel performance, while the last two groups will show a more representative warm barrel performance.  It will only take 31 minutes and you'l have better data than 90 rounds in 10 minutes, and you'll be abusing the barrel a lot less.

If you see a consistent decrease in velocity again, then it's probably an artifact of the barrel warming up, although the change in velocity should be a lot less than you saw before.

If you don't see a significant difference between the first, second and third groups, then your initial results were probably an artifact of breaking in the barrel.

I would lean toward this angle.  How close were you?

Your geometric arguments are flawed.  I'll explain in a minute - brb.

The geometry of the sensor is fixed when the sensors are screwed to the base.  This fixes the sensor separation distance between the bases.  It also fixes the pointing angle of the sensors.   The latter (pointing angle) should be 90 degrees upwards but is subject to error, if for no other reason than normal manufacturing tolerances.  If the separation distance is correct (usually 24") but the fields of view are not perfectly parallel, there will be a calibration error in speed measurement.  

The problem with non-parallel sensors is the effective distance between them is now a function of the distance above the sensor.  This is bad and leads to variations in measurements, even if the bullets were moving at exactly the same speed.

The skyscreens should be in the field of view of the sensors.  However, they can be grossly moved from shot-to-shot by the muzzle blast, the supersonic shock wave and even the wind.  Movement of these is a likely cause of missed readings.

None of the above requires the sensors to be perfectly horizontal.  None requires the bullet to traverse the screens exactly every time ("aligned").  Angular errors are minor unless the misalignment is huge (10 degrees is easily seen visually on a 24" chrono base and even so, it causes minimal error, e.g., <0.5% error.  As long as the misalignment is constant, its small offset error will also be constant.  

What is required is that the bullet's passage be through the field of view of the sensors and there must be enough contrast between the bullet and the background to trip the timing circuit.  I am uncertain if the shock wave causes enough distortion (diffraction) of the illumination to trigger the sensors.

My final thought.  If you get inconsistencies like those, replace the battery.  Some chromos "eat" batteries and batteries that have been stored a long time lose capacity.  Batteries sitting in the hot sun at the range also lose capacity.  Take fresh batteries and bring lots of spares (cheap compared to time at the range and cost of shooting).  

Well, that is what I have found in my work chronographing and in study of the devices.  

One thing I found was, as my chronograph got older, it gave more and more inconsistent readings.  Eventually, I replaced it and since then have not had the problem.

This is why a lot of folks, including me, sharpie paint the bullets I am testing over the chrono.
In bright sunshine, even with the shade covers on, it can be hard to pick up the bullet.
I try to give the machine every advantage I can to be accurate.

As far as shock wave, yes it will mess up the chrono and give you bad readings. Sometimes very crazy readings like 5,000+ fps that is an obvious error. Its the errors that are close to what you are expecting that is a problem and you will never know unless you shoot multiple strings.
I have found that a minimum of 12' is needed for .223, and I usually step off heel to toe 15 steps(since my foot is almost a foot long)

<removed> not suitable for a technical forum. dryflash3  

Thanks guys for all the thought you put into your replies.



FYI, I always mount the chronograph atop a Gitzo metal tripod, with a 3-axis adjustable head that has a bubble level on top, so I know it's level. That darn tripod is steel and HEAVY, so I'm confident it's not wobbling around. It had a new battery that day. It will be at least a month before I can retest since the lower is out for some engraving and I just mailed the scope back to the manufacturer to correct an issue. It's going to be a long month!



The chrono in question is a Competition Electronics ProChrono Digital. I'll let you all know how it goes when I get the rifle back together and retry with the chronograph at a proper distance.



Thanks again for taking the time to reply with your thoughts.



Tarkid