Chronograph. Working or not? How can I truly tell?

[Lounger]

I’m wondering if I’m getting correct chronograph readings. My 300 Blackout readings were more than 250 fps slower than the data suggested. The data, which was from the Accurate Arms website, was for a Sierra 220 grain HPBT and used a 16 inch barrel. I used an 8 inch barrel and my 220 grain bullet was Berry’s spire tip. I know that a longer barrel will increase velocity, but this seemed to be quite extreme. Could it be that the copper plated lead bullet moves that much slower than the Sierra bullets? My info is below. I also used factory 5.56 rounds as a comparison.

300 Blackout

Aero Precision 8 inch barrel
Berry’s 220 grain Spire tip bullet
A1680 powder (10.9 grains)
LC (once fired brass) 5.56 brass cut and resized by Carolina Brass
CCI SRP #400
Suppressed with a Sig Sauer SRD762QD

Chronograph readings:
8 shots
Avg. = 828 fps
Spread = 64 fps
Stdev. = 21.9

This is roughly 250 fps slower than the data from Accurate Arms Load data. Even with a shorter barrel, should I be getting this much of a difference from the 1085 fps that the data says?

I used Federal 5.56 factory rounds as a comparison.

5.56

Odin .223 Wylde 16 inch barrel
Factory rounds
Federal, 5.56 55 grain fmj ammo (box states that muzzle velocity to be 3165 fps)
Unsuppressed

Chronograph readings:
9 shots
Avg. = 3031 fps
Spread = 137 fps
Stdev. = 41.8 fps

The chronograph was a Caldwell Precision (lower end chronograph @ about $100) placed at about 3 – 4 yards from muzzle.

My factory 5.56 rounds were about 130 fps slower than the box indicated, but that’s only about 4% slower. The 300 blackout loads were about 24% slower than the data suggested.

Any thoughts on my Blackout velocity? I assume that the only way to know is to use another chronograph, but I thought I’d inquire.

Thanks!

[20X11]

Yeah...I tried a caldwell and compared it to my existing chrono... It read consistently 300 fps slower. A good flush of 129.99.

[smustian]

25 fps per inch of barrel is a good ballpark from what I have seen. Given that, your 8" being 250 fps slower than a 16" barrel is reasonable.

[KTMRacer]

To me, it's another one of those wormholes that you can crawl off in, never to be seen again.
I go by the data my cheapo chrono produces. Is it right? Who knows, but it's what I use and with no issues in my loads or accuracy.
FWIW I've spent a lot of time in the archery world and competitions. I've seen a full gamut of chronos and given less than perfect lighting, temps, etc. they all vary brand to brand.

[steven11b]

Enter the data into a ballistic calculator and shoot at distance. If the shots are on then the velocity is correct. If you need to make elevation corrections then input those corrections into calculator and itll make the adjustments to the velocity and give you the correct velocity.

[Lounger]

Thanks guys for the speedy responses. I appreciate your help.

[dellet]

Without knowing the length of the Berry's bullet, I am guessing it is shorter and you seated it to the same length as the data. This also will lower the velocity.

Looking at the numbers you posted for the factory ammo, there are three possible issues. Crap ammo, crap chronograph, crap shooter.

I'll give the benefit of doubt to the ammo. That spread is too high for decent ammo.

As for shooter, it's important to have the chronograph far enough away from the muzzle to not pick up the muzzle blast. You might move it back one more step.

It's very important that the bullet take the same path over the windows with each shot. If the chronograph tilts or you shoot corner to corner it will screw with the numbers. Most people are reasonably aware of this. It will also lose Acurracy if the height over windows varies.

It might be that your next trip out gets a bit better just having some more experience setting up and using the chronograph. Use some of the same factory ammo if possible to compare.

[Lounger]

Thanks dellet.
I loaded the bullet to the manufacturer’s recommend oal (the one bit of data they did say). But I did shoot off hand and my chronograph might have been too close. I’ll use a bench next time.
Thanks for your reply.

[Roverhound]

My 9.5" pistol is consistently 200 fps slower than my 16" carbine with the same loads.

[hardcase]

If you have access to a power source, I would get one with infared light shields instead of depending on sunlight. I never hard dependable results on a cloudy day.

I do not have one but see positive feedback from the Magneto-speed that fits on your gun.