I am not a real in depth reloader, just like to find loads that produce good accuracy and enough zippy zoom to whomp whatever is on the other end, not a bench rest target master. So, there are things the detail men know/do always that I do not, or barley dimly grasp they may be really, really important. Note - I taught myself to reload, never took a class, just what I gleaned on the internet and a few old books.
Now, having said THAt spiel, here is what I find interesting. In the .223/5.56mm cartridge, it has been posted in at least one if not more articles that the caliber tends to lose 25 FPS for every inch shorter on the barrel, within the limits of traditional lengths. In other words, a load that produces 3,200 FPS in a 24 inch test barrel SHOULD, in theory, be limping along about 3,000FPS in a 16 inch tube. I have seen this with published data compared t my own results, using a ProChrono in good repair.
Now we move to this one - the Hornaday 55gr FMJ/BT bullet, (old standby and a good cheap slug), is listed as zipping along at 3,182 on top of 25.5 grains of Accurate Arms 2520, out of a 24 inch test barrel. This is the published data from Western powders or 5.56mm. So, I raised the starting load to 26 even, because I can, and ran it across the ProChrono today. Based solely on my extremely limited examples from myself, I believe I can reasonably expect velocity in the 2,900 FPS area, a decent step.
Average velocity for the ten testers was a plodding 2,722. Temp about 70 degrees, elevation approx 1400 feet, humidity mid 30s, no wind to speak of. Yes, ten ain't much, but I don't have a bunch to waste, like to start small and work up to something nice.
So, oh gurus of the electrons, thoughts?

TL/DR - ran loads through chrono, velocities with published load data approx 200 FPS too slow from estimate, what gives.

A few possibilities (and there could be multiple contributing to your slower speed):

1.) Case capacity can play a huge role in velocity; for instance Federal brass may be thinner (thus having greater internal capacity) than say Winchester.

This means that it will take more powder in Federal brass to achieve the same velocity/pressure as you'd get with a lower charge weight in Federal Winchester.

*I'm using those brands to illustrate a point; not making any specific statements about Federal or Win brass.

If the data was created using a specific brand of brass, it may not be representative of what you can expect from all makers in that caliber.

2.) Not all batches of powder are at the same burn rate; some lots of the same powder are faster/slower than others.

3.) Sometimes bullet bearing surface lenghts can vary between lots; this can mean some lots are faster/slower than others.

4.) Those velocities in the loading manuals are generated off what are effectively bolt-guns; if you're shooting this in a semi-auto, it's almost always going to be slower.

I actually never considered case volume - I recycle everything, so cases for me are marked on the reload bag label as "mixed". :)
You're right about bolt vs auto, didn't think about that one. Thanks.

How confident are you with your scale's measurements? Do you use check weights to verify the scale is reading at or near the targeted charge weight before using the scale?

Do you have a second scale to cross check with? I worked with 2 scales for years until I optimized my one scale and now use check weights to make sure its spot on before doing a load ladder.

It likely is a combination of factors stacking up to the lower velocity, but I'd start with the scale.

How wide was the velocity spread on the 10 rounds?

I lose 100 fps going from a 24" barrel to a 20".

I lose 200 fps going from 20" to 16".

A 20" barrel is very efficient in .223. A 16" and shorter not so much.

I do not yet have a lot of experience measuring and reading rifle data, but I almost always get lower velocities than published data. Enough so that I called a couple of times to discuss the variances. (BTW, they are right, you are wrong, just ask them ).

They must be engineers.  

Yea, I am usually chronographing less than book.   Powder makers seem to be the most optimistic than others.  Hornady is just as guilty.

I guess if I called them to talk they would tell me I have a slow barrel.  (Therefore I presume they do their testing with tight bore, tight chamber barrels).

The following was posted by  WES from MSTN many years ago on here.
For reference that's about as close to realistic and follows ~ relatively close to what I see with velocity drop as barrels get shorter.

I haven't had my chrono that long but so far.
Western powders TAC 5.56 data is the closest thing i've seen to realistic velocity when you figure in barrel length reductions (like MSTN shows below) for the 77 gr loads i've used.  I have no idea if the rest of their data is close or not.......

Most i've looked at are way way off.....

Barrel material twist etc all factor in............ stainless and 1/8 twist shoot slower than chrome lined......... fn barrels so far shoot faster than anything i've tried.

MK 262 VELOCITY DATA
__________________________________

7.5" 2053 FPS ... 10.5" 2363 FPS
DIFFERENCE 310 FPS
DIFFERENCE/INCH 103 FPS

10.5" 2363 FPS ... 14.5" 2576 FPS
DIFFERENCE 213 FPS
DIFFERENCE/INCH 53 FPS

14.5" 2576 FPS ... 16" 2669 FPS
DIFFERENCE 93 FPS
DIFFERENCE/INCH 62 FPS

16" 2669 FPS ... 18" 2769 FPS
DIFFERENCE 100 FPS
DIFFERENCE/INCH 50 FPS

18" 2769 FPS ... 20" 2818 FPS
DIFFERENCE 49 FPS
DIFFERENCE/INCH 25 FPS

M855 VELOCITY DATA
__________________________

7.5" 2244 FPS ... 10.5" 2639 FPS
DIFFERENCE 395 FPS
DIFFERENCE/INCH 132 FPS

10.5" 2639 FPS ... 14.5" 2861 FPS
DIFFERENCE 222 FPS
DIFFERENCE/INCH 56 FPS

14.5" 2861 FPS ... 16" 2938 FPS
DIFFERENCE 77 FPS
DIFFERENCE/INCH 51 FPS

16" 2938 FPS ... 18" 3046 FPS
DIFFERENCE 108 FPS
DIFFERENCE/INCH 54 FPS

18" 3046 FPS ... 20" 3061 FPS
DIFFERENCE 15 FPS
DIFFERENCE/INCH 7.5 FPS

XM193 VELOCITY DATA
_________________________________

7.5" 2364 FPS ... 10.5" 2755 FPS
DIFFERENCE 391 FPS
DIFFERENCE/INCH 130 FPS PER INCH

10.5" 2755 FPS ... 14.5" 2984 FPS
DIFFERENCE 229 FPS
DIFFERENCE/INCH 57 FPS PER INCH

14.5" 2984 FPS ... 16" 3075 FPS
DIFFERENCE 91 FPS
DIFFERENCE/INCH 61 FPS

16" 3075 FPS ... 18" 3245 FPS
DIFFERENCE 170 FPS
DIFFERENCE/INCH 85 FPS

18" 3245 ... 20" 3254 FPS
DIFFERENCE 9 FPS
DIFFERENCE/INCH 4.5 FPS

One of the first things I bought when I started reloading was an RCBS check weight set - I always use it.

I tried again with a load with the same powder that should develop 2900...2730. The rifle is consistent with every OTHER load. I am beginning to think this powder is bad somehow.

Unless you left that canister lid open for a year, that powder isn't bad.

Alot of factors to consider.

- primer, bullet, case , crimp , seating depth different than published data and of course barrel length  along with atmospheric conditions can lead to differences

- My question , how do the powder companies measure the velocity? At muzzle, 10 feet away or some other method such as calculations instead of a chrono..

I believe i read somewhere that if you place a chrono 10 feet away add 10% for muzzle velocity... I can't remember where i read it or if i heard it or dreamed it.

Even the path the bullet passes through a chrono can give different results as can battery life.

Imo, published velocity is entertainment. Use chrono as a single piece of the puzzle.

From my Labradar on a 223 shot string:

V5 is 5 yds from the muzzle.
Avg 18.2 FPS drop in velocity or 0.65% variance from muzzle velocity.

Shot ID - V0 - V5
1. 2797 - 2778
2. 2811 - 2793
3. 2798 - 2780
4. 2774 - 2756
5. 2743 - 2725
6. 2807 - 2789
7. 2794 - 2775
8. 2794 - 2776
9. 2724 - 2706
10. 2752 - 2734

I've had old powder produce lower velocities, but it was probably 30+ years old and turning red. Powder can absorb water over time and weight increases. Did some side by side testing with new powder of the same type to see the velocity shift lower. It wasn't as much as I expected. Definitely not 200fps.