chronometer accuracy question

[Canuck44]

I was at the range today and a guy had his chrono set up, I had a few pet loads he let me test for speed and the results were slower than I expected, they were as follows:

270 win 22" - 140 gr accubond with 51 grains of imr 4350 - 2636 fps, the hodgdon data shows about 2800 for a 24" barrel, I expected to be at about 2750 fps

270 win 22" - 90 gr sierra hp with 58.0 gr imr 4350 - 3091fps, interpolating the hodgdon data I expected about 3300fps

45/70 18" - 405 cast with 30 grains of imr 4198 - 1140 fps, hodgdon shows 1370 fps for a 24", I expected 1250-1300

45/70 18" - 300 gr hp with 53.7 gr imr 4198 - 2011 fps, hodgdon shows 2293 for a 24" barrel, I expected 2100-2150

45/70 18" - 350 gr fn - with 48 gr IMR 4198 - 1903 fps, interpolating from hodgdon data shows 2060, I expected a little better than 1903

I this I case of my expectations are higher than they shoudl be considering the shorter barrells? Hodgdon exagerating speeds to sell powder? Or a Chrony that needs adjustment?

[Pathfinder76]

Barrels, chambers, and throats are a law unto themselves.

The chrony could be off. Maybe. To check you could run 22LR ammo of known velocity over it. I verify with trajectory.

[elkhunter11]

A chronometer is a timepiece,you are referring to a chronograph that measures velocity.

The velocities listed in manuals are velocities obtained with one particular test barrel,one lot# of powder,one lot# of one specific primer,one lot# of one specific brass,and one lot# of one specific bullet.With the variances between lot#s of components and the variances in chamber and barrel dimensions,you can't realistically expect the velocities you obtain with your gun to match those printed in the manuals.You also can't expect that all loads listed in a specific manual will be safe in your gun,hence the disclaimer to begin with starting loads and work up while watching for pressure signs.

[Buckhead]

Going by the Nosler manual 51 grains of IMR4350 should give you about 2750 in a 24" barrel. .270 with 22" barrel I would expect about 100 fps less - say 2650 so you are pretty close.

IMO if you want maximum speed out of the ole .270 then you need to have at least a 24" barrel and 26" would be even better.

There are also a couple of other factors at play here;

1. Never expect to get the same velocities as indicated in a manual or on the net. The velocities given are taken under controlled conditions in a specific rifle or pressure barrel and may or may not be relevant to your rifle.

2. IMO chronies are good at reading relative velocities of different loads. I would not use one to determine if I am at maximum or not.

3. Your rifle might like a different (slower) powder better such as H4831 or RL22 if you are trying for a higher velocity.

With the 140 Accubond in my .270 I load strictly for accuracy only and I have found the most accurate powder in my rifle is IMR4831. I can only get about 2840 fps with this powder but I have no issue with that in a 22" barrel when the rifle will shoot MOA or a bit less on any given day. If I feel I need more speed I just move up a bit to my .300 WSM.

In your 45/70 I would say you are getting pretty good velocities considering your barrel length.

[Canuck44]

The slower speed explains why I was getting 12" more drop than expected out of the accubonds at 500yrds.

What would you guys reccomend for a crony? I am interested in buying one as it was fun to play with and would like to experiment further. Not to worried about how fast something is as long as its accurate but it seems like a fun toy and more info can't hurt when I am developing loads.

[222rem]

When powder manufacturers test speed of the bullets they do it in laboratory conditions in indoor range with specific temperature ,humidity,elevation above sea level etc.
If you can duplicate those condition including same bullet , primer and powder lots and take an average of number of shots they took your numbers would be closer to what they print in the book .Also velocity will very from rifle to rifle with same barrel length .
Just my thoughts .

[Pathfinder76]

Originally Posted by Buckhead

IMO chronies are good at reading relative velocities of different loads. I would not use one to determine if I am at maximum or not

What would you use then?

[sidetrack]

I just can't resist this one!
A chrono-whatever actually measures time. The time it takes for the bullet to pass between the two sensors. The little computer chip inside of it saves you the trouble of getting your calculator out and converting that time to a speed. After converting for you, it displays a speed, but the actual measurement is time.

If you've ever taken physics, you will know that speed and velocity are not the same thing, just like torque and horsepower are not the same thing. (Horsepower is torque x time.)
Speed is a measure of how fast something is going.
Velocity is speed and direction.

eg: 2700 fps is a speed.. .not a velocity
2700 fps NNW is a velocity, as north-north-west is a direction.
wheee! This is fun!

[elkhunter11]

A chrono-whatever actually measures time. The time it takes for the bullet to pass between the two sensors. The little computer chip inside of it saves you the trouble of getting your calculator out and converting that time to a speed. After converting for you, it displays a speed, but the actual measurement is time.

Yes it only directly measures time,but it displays only velocity that is calculated using the time and distance.Then again the speedometer in most vehicles does not directly measure the vehicles speed either.A sensor measures the rpm of a shaft or gear,or disc,then using tire circumference(and possibly the final drive ratio) the result is displayed as a speed on the speedometer.

[LongDraw]

No one mentioned temperature. You will see that variation in velocity from summer to winter.

[sidetrack]

Ho hum. And then there's that V word again. I haven't seen any chrono-thingies that told me which direction the bullet is going.

http://www.glenbrook.k12.il.us/GBSSC...Kin/U1L1d.html

[Dean2]

Chrony brand is inexpensive and relatively accurate. Inexpensive to replace the box if you accidently shoot it, and many are every year. For that reason the Beta versions are a good bet because the expensive read out gear sits on the bench out of harms way.

As to whether a Chrono measures speed or velocity, it is incorrect to assert they measure speed because they don't display the direction of flight. While the direction in north, south is not relevant to the measurement, the bullet must travel as precisely square to the unit and from the front to the back sensor, so the measurement is time of flight over a specific, directional distance. Quote from your link Sidetrack "Velocity is a vector quantity which refers to "the rate at which an object changes its position". Velocity is a vector quantity. As such, velocity is direction aware. ". Which is precisely what this type of Chrono measures. (Way too much fun. Shall we next debate how many angels can dance on the ogive of a Berger 168 grain 30 caliber VLD.)

For testing max loads you can buy the gear to make your own gun into a pressure gun.
http://shootingsoftware.com/index.htm

Relatively inexpensive, and works very well. Only way to know for sure what the pressure of your loads is.

[Dick284]

I've had the ability to on one occasion put 3 chronies nose to tail and shoot through them. They all read different, and not by 25fps either, but by up to 125fps difference.

Conclusion was, dont get all bent on the numbers.
Verify them by checking down range trajectory.

So far my chrony seems to pan out pretty close when I crunch the trajectory to the instrumental velocity.

Other things that make chronies do weird things are:

Not set up fully open. or bent to actually slightly over open.

Varied lighting conditions, bright sun and shadows crossing a chrony does some mighty weird things to velocities.

Not shooting through the same very specified window of bullet travel, aimlessly poking shots through the screens will give varied readings.

A Chronograph is just but one tool available to the reloader and shooter, it is neither definative nor totally necessary for the most part.

Thngs that are necessary and definative are Practice and range time.

[sidetrack]

Dean2

"it is incorrect to assert they measure speed because they don't display the direction of flight".

Ha ha ha ha ha ha!
Maybe read the definition of "speed" again. It's not that hard!
Ha ha ha ha ha

[PoppaW]

The chrony is just part of your kit. If yours reads 100 or 50 or whatever high or low it doesn't matter as you have to shoot and prove your trajectory anyway. Its like your reloading scale that has 100gr of powder in it and you put it in another scale and it reads 102gr. Is it a bad scale? Not if its consistent every time. Just like the chrony. If yours reads 25fps low at 3000fps then who cares. I just use it to compare loads with different powders to see which is best in my gun for speed. So if you want a faster gun get a faster reading chrony

[Buckhead]

Originally Posted by Canuck44

The slower speed explains why I was getting 12" more drop than expected out of the accubonds at 500yrds.

What would you guys reccomend for a crony? I am interested in buying one as it was fun to play with and would like to experiment further. Not to worried about how fast something is as long as its accurate but it seems like a fun toy and more info can't hurt when I am developing loads.

I would recommend the Chrony F1 to start. Not much money ($100 or so), works as good as any and can be upgraded if you wish.

[Buckhead]

Originally Posted by chuck

What would you use then?

To decide if I am a the maximum charge for a particular powder?

Pressure signs - loose primer pockets, gas leakage around primers, how flat the primer is, how my rifle is changing in recoil and sound of report as I work up, ejector marks,etc.

Once I am satisfied with the load and am getting the accuracy that I want then I will Chrony it just to see what the velocity might be.

Then I will do some shooting at increasing ranges 100, 200, 300 and 500 so I can to check the amount of bullet drop.

Checking the bullet drop against some ballistics tables or software, then I wouild know if my load is really running faster or slower than my chrony indicates.

If I'm satisfied with what I am seeing with both accuracy and bullet drop then I am good to go with using that load for that rifle. Otherwise it is back to the drawing board with a different powder. If after I've tried half a dozen powders that should work well with a particular bullet and things are still not working then it is time to try other types of bullets. Some rifles just prefer one bullet over another.

[Grizzly Adams]

That was my first big surprise, when I got a chronograph. The loads I took from my Speer manual, were consistently slower than the manual claimed, from the same barrel length. I was trying to be conservative and was using the lower powder charges as starters, but after I figured out these guys were maybe doing a bit of a sales job, I upped them to the middle ranges, without any problems. Just for fun, I chronographed a bunch of 22 Lr and the velocities there came close to what was advertised. I've got one of those Chronys and sometime poor lighting can affect the reading.
Grizz

[elkhunter11]

Checking the bullet drop against some ballistics tables or software, then I wouild know if my load is really running faster or slower than my chrony indicates.

Then again those tables or calculations are based on the ballistic co-efficients provided by the bullet manufacturers,which may or may not be accurate.Barnes for one,recently issued new BC numbers for many of their bullets.In most cases,they are reasonably close,but don't expect them to be too precise.

[Buckhead]

Originally Posted by elkhunter11

Then again those tables or calculations are based on the ballistic co-efficients provided by the bullet manufacturers,which may or may not be accurate.Barnes for one,recently issued new BC numbers for many of their bullets.In most cases,they are reasonably close,but don't expect them to be too precise.

You are correct. I would not expect there to be an exact match - but they do give me a reference that I am in the ballpark (so to speak).

[twofifty]

Working up loads, I place the chrono at the start of the bullet's path to the target.
Then take notes of the measured velocities for the rounds fired at each target: gets two things done at once.

Big bonus is that this makes it much less likely to drill your chrono as you have a consistent point of aim, and the bullet's path (i.e. distance travelled) between the sensors is the same for each shot.

[Canuck44]

Can I use the same chrony for my bow? Do they read down that slow?

[elkhunter11]

Can I use the same chrony for my bow? Do they read down that slow?
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Mine does.