Muzzle velocity estimate without Chronograph


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SLCscottie
August 12, 2010, 04:19 AM
I don't have access to a chronograph at this time and I don't want to spend the money. And I just need a rough rule of thumb or experience with this matter.

So what should I expect the muzzle velocity difference to be when using a
Rock River AR15 with a 16 inch barrel 1 in 9 rate twist instead of a
Remington 700 26, 1 in 12 twist (Hornady Data used from their reloading book)?
My guess is a loss of 150 fps.

I reloaded the 223 Rem using Hornady Data:
55 gr #2266 SP w/c
B.C. 0.235 C.O.L.: 2.200
Varget 25.5 gr
Velocity per book 3100 FPS

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Norrick
August 12, 2010, 06:17 AM
i would get a box of ammo with the velocity data printed on it, and hopefully it tells you the barrel length. you could then switch ammo and see how much lower or higher the groups appear than the reference ammo (dont adjust scope after the sight in for the ammo with printed data)

It wont tell you much other than 'its faster' or 'its slower' but its something... (faster would be higher and slower would be lower because it would have more time in freefall).. but then again with different ammo the point of impact can change anyway, so maybe not so accurate...

JDGray
August 12, 2010, 06:22 AM
A chrono is the only way to tell. I've seen some vastly different velocities from reloading manuals, and we seldom use what they use in componet combo or equipment. Sierras manual was very close to actual velocitiy, on the last load work up I did, but your results may differ:) 168gr GMM ammo is rated at 2600fps from a 24" barrel according to the box, and it clocks exactly that through my 20" barreled Savage 10FP.

SLCscottie
August 12, 2010, 07:22 AM
I agree that a Chrono is the best way to go.

I've been doing some searching after posting this question.

I estimate roughly 40 fps every two inches after reading the article posted at this sight.


http://www.loadammo.com/Topics/October05.htm

SLCscottie
August 12, 2010, 07:32 AM
so instead of 3100 fps for a 26 inch barrel I may be getting 2900 fps for a 16 inch barrel. I don't even want to start with the twist rate.

(10 inch difference / 2 inches) time 40 FPS = 200 fps change.

So yea a Chrono is the way to go. Just don't have access to one.

I will have to go play with this at the range.

Grumulkin
August 12, 2010, 08:49 AM
On huntingnut.com, you can download free ballistics software. Shoot your gun at 100 and 200 yards and measure the bullet drop from 100 to 200 yards. Plug in the ballistics data like ballistic coefficient into the program until you get a similar drop with a certain velocity and that's the velocity.

This is obviously not as good as a chronograph but it should give you a ball park velocity estimate.

R.W.Dale
August 12, 2010, 12:08 PM
^^^^

It Might get you in the parking lot or on the same side of town as the ball park but I doubt it'll get you in to see the game.

Velocity calculated by BC is a problem as bullet mfgrs like to pad their numbers, bc varies by velocity and atmospherics also play a part.


Every barrel is different too some guns have fast bores and some slow and the difference can be great, which throws FPS per inch rules of thumb out the window since you don't know what you're starting with.

In short till you buy a chrony ($100) you will continue to live in the dark ages with regards to your muzzle velocities.

JimKirk
August 12, 2010, 05:43 PM
Here is some data I have gathered...

From Hornady # 7 Manual... Using 26" barrel:
55 grain SP / 22.7 grains Reloader 10X Published Velocity... 3100 FPS
Same load, same bullet out of 16" barreled DPMS 1/9, Chronographed ...2860 FPS
So -10" of barrel = -240 FPS ... Theoretically out of "my" barrel, but ...

Now we know that to be sure of exact lost, we would have to cut the same barrel down and measure after each cut. Most folks are not going to do that. That velocity loss would only apply to that barrel also. The next barrel may or probably will be different.

Jimmy K

don
August 13, 2010, 01:11 AM
Norrick, I think you may have that bassackwards. According to Richard Lee's manual, the faster bullet would shoot lower due to the fact that it is in the barrel less time hence less barrel rise during recoil. The slower bullet is in the barrel for a longer period of time thus allowing the barrel to rise higher before the bullet exits the barrel. I have found this to be true through mny own experience.

SLCscottie
August 18, 2010, 04:40 AM
While at the gun store, I did see different velocities for about the same load while thumbing through the different reloading books comparing Varget for 55 gr.

SLCscottie
August 18, 2010, 04:48 AM
JimK,
-240 FPS

That helps.

Krochus,

You got me laughing.

atblis
August 18, 2010, 03:11 PM
ballistic pendulum
http://hyperphysics.phy-astr.gsu.edu/hbase/balpen.html

ranger335v
August 18, 2010, 05:59 PM
"Norrick, I think you may have that bassackwards."

Correct. I've demostrated it numerous times to folks who swore otherwise.

Norrick
August 19, 2010, 07:09 AM
I guess its one of those things a physics lab cant really help you with. sorry for the bad advice, Ive never had to use it since I have a chrony.

Hondo 60
August 19, 2010, 10:01 PM
Without a chronograph I can't see that any numbers would be more accurate than just pulling it outta yer butt.

Really, a chrono is the only way.

Case in point: I reloaded some .357 mag - 180 gr MBC Stryker over 8.0 gr AA5.
Man it almost snapped my wrist from recoil. I thought they were flying. When I chronoed them, they were only 1000 fps or so. About 200 fps less that the MUCH milder feeling Winchester factory ammo.

If you belong to a club, ask around. I'll bet someone has a chrono. They may not want you to shoot over it, but I'd bet they'd be willing to shoot your firearm.

lefteyedom
August 21, 2010, 03:01 AM
Bite the bullet and spend a $100 and get a chrony. It is not like the 1970 when they were cost prohibitive. You will save money on the ammo not wasted.

JASmith
January 23, 2011, 08:38 PM
I just posted an online velocity estimator that might help: http://shootersnotes.com/calculator/velocity-estimator/

The estimator is effectively a modest extension of the Powley calculator. It allows you to use your powder even though it might not be one of the IMR powders the Powley calculator was built to handle.

You will also need to enter a reference load that uses the same powder but with a charge weight equal to or greater than the one you use in your load. Same for the bullet, it will need to be the same weight or heavier than the one you use and more likely even better if its the same brand. Barrel length for reference load is needed along with your barrel length.

Let me know how it goes!

Thanks!

kelbro
January 23, 2011, 09:29 PM
Everyone has different takes on this but my question for the OP is, "Why?" What does the difference in velocity mean? Odds are that the accuracy will not be the same from two different barrels so what difference does the velocity make?

1SOW
January 23, 2011, 10:47 PM
Odds are that the accuracy will not be the same from two different barrels so what difference does the velocity make?

I'm just learning here, but would it give Approximate effective max range and sight/scope changes to get on the paper using published ballistics data?

okespe04
January 23, 2011, 11:12 PM
http://www.walmart.com/ip/F1-Chronograph/4382668


Best price I have found and you can't beat 97 cent shipping. Mine arrived a few days ago, I have not taken it out yet but my Daisy lever action BB gun averages 238 fps!

Edit: I can also flick a penny at 39 fps!

SLCscottie
January 23, 2011, 11:59 PM
Thanks for the information guys. I'm glad the system alerts my email.

The reason I want to know the velocity change is that I bought the Nikon scope with BDC (bullet drop compensation) for AR15's and wanted to avoid going with a new longer barrel (yea I know I'm cheap. same with the chronograph). So I wanted to compare what change in bullet drop with where it needs to be for the Reticule on the scope.

JASmith
January 24, 2011, 12:29 AM
Those are excellent reasons to know how the velocity for your rifle is different from the published data.

By now you have probably looked at the predicted trajectory for your best estimate of the velocity, so you have some idea of the changes of impact.

A related question: What is the largest change in impact where you don't care? By this, consider that most of us are hard pressed to shoot better than 1 minute of angle. This means that we can't tell within about five inches where our bullets hit at 500 yards. What is the change in velocity needed to make this difference?

Answering that question will tell you how important a chronograph is.

(Of course, some of us are so addicted to better understanding that we'll get one even if we 'prove' we don't need it!)

kelbro
January 24, 2011, 08:00 AM
The reason I want to know the velocity change is that I bought the Nikon scope with BDC (bullet drop compensation) for AR15's and wanted to avoid going with a new longer barrel (yea I know I'm cheap. same with the chronograph). So I wanted to compare what change in bullet drop with where it needs to be for the Reticule on the scope.

Gotcha. That makes sense. I have a Leupold with the Varmint Hunter reticle and I just set up some targets at variable ranges to find out where the subtensions actually struck. Moved the targets in and out and when it all lined up, I used the rangefinder that I will use hunting to determine the distances. The kill zone on coyotes and prairie dogs get pretty small at 300yds +. For my application the velocity was not relevant, just the drop.

Hansli
January 24, 2011, 09:45 PM
The only answer to the original question is without a chronometer you can't know how fast the bullet is going. Even with one you'll need to clock several to find a mean. Would you time a race by counting Mississippis or tell time by the sun?

rsrocket1
January 24, 2011, 10:12 PM
Reloader 10x is a much faster burning powder than Varget. Over 95% of 10x is burnt by the time the bullet is 12" down the muzzle. Only 84% of the Varget is burnt by 12".

Being that Varget is still burning in the barrel even at 26", you'll have a bigger difference in muzzle velocity. @ 16", the Varget is still generating 12,000 psi. At 26" it's down to 8,000 psi so you do lose a lot of oomph out of that last 10".
I would estimate about 350 fps loss. But that's all just number crunching.

...get the chrony.

Better yet, calibrate the scope with real live firing at the range. Nothing beats real shooting! Besides, its a lot more fun.

If you can't place the shots with an adjusted scope whether its the bullets, your particular gun, or your aiming, does it matter?

twofifty
January 24, 2011, 10:15 PM
According to an 1860s account of developments in naval artillery during the Age of Sail (specifically the American Revolutionary War & Napoleonic Wars), a reliable method was devised more than 200 years ago to measure cannon ball velocities.

In the 1700s, French artilleryists designed a mechanical contraption to measure projectile speed. Spies working for the English eventually spilt the beans to the outside world.

Here's the gizmo:

Imagine a large but lightweight cylinder, say 6' diameter and 40' long. This cylinder rotates at a steady speed within a support cradle, by means of a geared flywheel. The no. of revolutions per second is known and constant.

Each end of this cylinder is fitted with a stout paper disk, divided and marked into 360 degrees. The disks are indexed to each other so their degree readings correspond.

How the gizmo works:

Now fire a cannon ball through the steadily rotating cylinder. Note the degree at which the ball pierces each disk at it travels through the rotating cylinder.

We now have enough data to compute distance travelled in a given time = velocity.

Like with modern day chronys, it was important to figure out how far back the muzzle needed to be. As now, it was also important to aim so the ball exactly transits the cylinder's longitudinal dimension, i.e. straight through.

Pretty ingenious.

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