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Old May 27, 2009, 08:58 AM   #1
Qtiphky
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Velocity Differences?

I don't have a chrony so I was wondering if there was a way to calculate the velocity of a round?

That said, here is the data.

From the Hodgdon website with a 24" BBL and Win SR Primer: 55 gr SPR SP, 23 Grains of H335 is the starting load and is listed at 3018 fps.

From my Sierra manual with a 20" BBL and Rem 7 1/2 Primer: 55 Grain SPT, 23.6 Grains of H335 is the starting load and is listed at 2700 fps.

I loaded 55 Grain SPR SP, 23.6 Grains of H335 with CCI Benchrest Primers (that's all I have been able to get since November). I am shooting them out of a 16" BBL in my Bushmaster and this is the most accurate round so far, but I am wondering why such a discrepancy in the listed velocities.

Do I really drop another 300 fps because of the 4 inch shorter barrel? Are these rounds really only running around 2300 fps?
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Old May 27, 2009, 09:15 AM   #2
brickeyee
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Quote:
I don't have a chrony so I was wondering if there was a way to calculate the velocity of a round?
Even with QuickLoad (internal ballistics software) the best you can reliably do is make an educated guess.
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Old May 27, 2009, 09:28 AM   #3
freakshow10mm
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The primer is different, the OAL might be different, the altitude that the data was testing might differ from each other and your location, etc. There are variations, it isn't the gospel.

For a educated guess, take the ratio of actual versus max powder charge and multiply that by the max velocity listed.

23.6/25.3 X 3203= 2988. From a 24 inch barrel you should get about 2988 fps. Figure anywhere between 20-50fps per inch of barrel difference. Even that isn't accurate as I've had rifles that only lost 10fps per inch with some powders. Best way to know is buy a chronograph for $100 or so and test the velocity.
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Old May 27, 2009, 08:51 PM   #4
SL1
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The easiest and cheapest way to find out is to make friends with a guy with a chronograph.

The next cheapest way to do it is to zero at 100 yards and then shoot at a much longer range (300 to 500 yards or more) and measure the bullet drop at the longer range. From that, a ballistics table will give you a ROUGH estimate of your muzzle velocity. Just find the muzzle velocity that matches your long-range drop for YOUR bullet's ballistic coefficient.

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Old May 27, 2009, 10:27 PM   #5
Shoney
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Combinations of factors can cause great variances in velocity/pressure. Here are a few factors:

Primer: amount of striking force on primer, strength, presence of powdered metal (Aluminum & others), brisance - a measure of the rapidity with which an explosive develops its maximum pressure
Barrel: length; tightness of bore; height of the lands; distance of bullet to lands; temperature of barrel;
Bullet: bearing surface of bullet, alloy of bullet; shape of bullet;
Brass: new/used elasticity; manufacturer, volume;
Powder: new, aged, old, batch powder was from;
Weather: ambient air temp., barometric pressure, humidity
Elevation: above sea level
Other: I am sure I have not listed all.

Now, mix and match them. Would you care to guestimate the number of possible combinations?

Even if you have a chronograph, there are no guarantees that it is or will remain accurate. That’s the fun & frustration:barf: of reloading.
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Old May 28, 2009, 04:42 AM   #6
kiwi56
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Velocity Difference

There are just too many variables involved. I use Load From a DisK and it is pretty darn close but still not exact.
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Old May 29, 2009, 07:09 PM   #7
James R. Burke
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I have never had the manual match the crony. It is just a estimate and thats all. I know its hard getting all the stuff you need when starting out but a crony is a good piece of equipment to have, and you can get a pretty nice one for a fair price.
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Old May 29, 2009, 07:40 PM   #8
arkie2
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Short answer: get a chrony.

I know that wasn't what you wanted to hear but it's the only sure way to know the answer to your question. Plus it's fun.
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Old May 30, 2009, 12:49 AM   #9
rg1
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Like already said, without a chrono it's just an estimate, but with Hornady 55 fmj's and H335 in a 20" barreled AR using H335 powder I get approx. 2850 fps with 23.2 grains. I use RP 7 1/2 primers and get nearly the same velocity using RP or WCC cases. A rough estimate of velocity differences from 16" to
20" is around 100fps but will vary from rifle to rifle and ammo used.

Last edited by rg1; May 30, 2009 at 12:56 AM.
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Old May 30, 2009, 08:04 AM   #10
Qtiphky
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Getting a Chrony!

That's pretty much what I had decided before I threw it out there. But you never know what answers lerk and if I could've gotten the answer without spending the money that would have been great.

That said, now I need to study the other thread about which one to buy.
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Old May 30, 2009, 09:30 AM   #11
Gbro
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Quote:
"Buy A Crony"
That's pretty much what I had decided before I threw it out there. But you never know what answers lerk and if I could've gotten the answer without spending the money that would have been great.
You got a very good alternative with SL1's post

Quote:
The next cheapest way to do it is to zero at 100 yards and then shoot at a much longer range (300 to 500 yards or more) and measure the bullet drop at the longer range. From that, a ballistics table will give you a ROUGH estimate of your muzzle velocity. Just find the muzzle velocity that matches your long-range drop for YOUR bullet's ballistic coefficient.
You will be double checking the same information used to calculate a ballistic coefficient.
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Old May 30, 2009, 10:49 AM   #12
B.L.E.
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Quote:
freakshow10mm said "For a educated guess, take the ratio of actual versus max powder charge and multiply that by the max velocity listed."
I think that for a better educated guess, take the ratio of the actual verses max powder charge and multiply by the maximum kinetic energy listed. Then uses the kinetic energy formula backwards to solve for velocity.
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