Chronograph speed vs load data

S

souther33

New member
recently I was loading some .223 ammo for a friend. I was using barnes 38gr varmint grenade HP FB seeded at 2.180 with brass trimmed to 1.745 (below avg) and 27.5gr of tac powder. That was the lowest charge given by barnes. barnes website stated that it would run 3681 FPS. I personally do not like very fast bullets thats why I chose the lowest charge. Before ever giving someone ammo that I load especially a round that I'm not very familiar with, like the 38r bullet. I test fire a round to make sure it fires. anyways I just recently got a chronograph and decided to test the velocity of the bullet to see how far off I or barnes was on FPS. I stood about 10 foot back and fired my first shot that read 3325 FPS. how could this be that far off. could it be the operator or the chronograph or barnes is just wrong. not wanting to fire all my friends bullets I stopped and began to think about it and here I am. asking some of y'all guys that went wrong. Being a 22 year old youngster I can safely admit I don't know everything.

by the way I will put some links at the bottom one for barnes load data for .223 and a blog about chronographs. What caught my eye was it saying to multiply by .75 and how many feet back you are then add to you FPS to get your real muzzle velocity. is there any truth to that.

thank everyone


http://precisionrifleblog.com/2012/...ctical-tips-to-increase-accuracy-reliability/

http://www.barnesbullets.com/files/2015/06/223RemingtonBRM5V9.pdf
 
You're missing an important piece of information: rifle barrel length. If the data was developed with a 26" barrel and you're using a 16" SR carbine, you'll never match book velocities. (Or if you are, please warn me so I can leave before you blow something up.)

Case in point: 55 gr FMJ .223 ammo from my M4gery runs 2800 fps, versus a factory rating of about 3000 fps from a 24" test barrel. It's a normal difference and to be expected.
 
First, run some .22 LR match ammo over your chrono. When I replaced mine with a different make a few years back, my numbers suddenly dropped 80 fps. The new one matched the advertised, and likely accurate, velocity for the rimfire match rounds. The typical chrono is pretty good, but it's not a calibrated lab instrument. Second, every rifle is different. Barrel length, chamber dimensions, etc. will give you a lot of variation. I say believe your numbers and go from there. Don't sweat it.
 
The bullet starts slowing down as soon as it exits the muzzle. To determine how much faster it would be at the muzzle vs. your 10 feet distance to your chrony, use one of the better trajectory calculators like JBM and you can see exactly the difference.

Barrel length is a major impact on velocity, but just some of the others include:

- different powder lot
- barrel roughness and diameter
- atmospheric conditions
- did you level your chrony? If it isn't absolutely level and perpendicular to your muzzle, the path between the sky screens is greater, thus lower velocity.
- and many other differences

It is normal to expect lower velocity than the test data listed achieved.
 
While playing with subsonic loads for my rifles, I fired some medium load 9mm and 40. thru the chronagraph and both are over max load speeds. I guess, I don't know?
 
What length barrel?
If 18", that's exactly what QuickLoad predicts for a 36gr VG/TAC/27.0/2.180"
(On the other hand, a 24" barrel produced 3,625fps)

(BTW: TAC is wasted on a short barrel/light bullet) ;)


.
 
Last edited:
+1 for jepp2's response and I'll add:

In testing the ammo company's guns are in a vise, yours was against your shoulder I assume. The rifle moving backwards does have an effect on velocity.

The ammo company's do their testing inside, in a controlled atmosphere and their light source is constant and does not change. You being outdoors means you have limited control of your light source and your powder temps could be way off from what the ammo company's was.
 
New one on me

In testing the ammo company's guns are in a vise, yours was against your shoulder I assume. The rifle moving backwards does have an effect on velocity.
Click to expand...

Please explain how a recoiling rifle affects velocity, or your source for that theory.
 
Newton's third law of motion gives us that the momentum imparted to the ejecta (bullet plus gas mass) of a gun will be equally and oppositely imparted to the gun and shooter. This is because the same force is applied equally and oppositely for the same amount of time. I've calculated this before and it works out typical .30 cal guns wind up going something on the order of 10 fps to the rear by the time the bullet exits. This subtracts from the velocity a universal receiver would achieve. It's just not a lot.

More of the velocity difference typically comes from the fact the SAAMI standard test barrel has a minimum dimension chamber, within half a thousandth of an inch, and a standard bore that, for all but five cartridges is 24" long to within a hundredth of an inch or better. A lot of manufactured guns have a different barrel length or a much less exact one, and more importantly, a looser chamber that doesn't confine the load as well and so can't get to the same peak pressure. Some guns have longer freebores than the SAAMI standard, and that works against velocity a little, too. So it all adds up.
 
Last edited:
And if ya think all that really sucks, you oughta check out the variation in EPA, gas mileage, and performance stats between advertised automobile specs, and your actual results with your new F 150.:p jd
 
You must log in or register to reply here.
Back
Top