My Chronograph reads and average of 2975 (SD around 30) and I am shooting Berger bullets BC .617 with 61 grains of IMR4831 in the 7RM. When I shoot at the range and adjust my ballistic chart to represent my true point of impact at 200, 400, and 600 yards, the velocity on the chart shows I should be shooting at 2730. I know that the actual POI is what is important and so I will use that info for my chart, but I was wondering if it is common to have a chrony fail or give bad readings? Am I missing any other factors here?
Chrony Problems
- Thread starter Rem 700
- Start date
RiverRider
Handloader
- Dec 9, 2008
- 1,454
- 107
The height of the scope above the bore is a pretty important part of the equation and if it's not accounted for you're going to see discrepancies.
Another thing that could be an unknown variable is the actual stability of your bullet in flight. Any wobble will decrease the effective BC and cause more rapid velocity loss than expected.
There's always the possibility of a poorly calibrated chronograph, too. If you have an opportunity to shoot your load over another instrument you might be able to eliminate or confirm the possibility.
Another thing that could be an unknown variable is the actual stability of your bullet in flight. Any wobble will decrease the effective BC and cause more rapid velocity loss than expected.
There's always the possibility of a poorly calibrated chronograph, too. If you have an opportunity to shoot your load over another instrument you might be able to eliminate or confirm the possibility.
- Thread starter
- #3
Yeah, I'm pretty sure I got my scope height right...at 2" (50mm objective lense). But I played around with that a little too on the ballistic program and it didn't make that much of a difference. Wobble might be happening. I never thought of that. The ballistics seem to represent the 2975 vel out to about 300 yards, but then the bullet starts dropping more like it had a 2730 muzzle velocity at I get out to longer ranges. What causes wobble?
Oldtrader3
Ammo Smith
- Nov 6, 2009
- 8,406
- 6
Most trajectories are calculated at 1.5 inch mounted height for the scope. The other factor is ballistic coefficient changes as the bullet flies. BC's are magic numbers that are an average over a set of velocities. Change flight conditions and/or velocity range and BC changes as well. Bullet instability or optimism by the Berger engineers both could change the results. I would suggest A:B'ing your chronograph against someone else's machine. I check my numbers by talking to people at the range while they are shooting their rifles cross their chrono, just as a baseline check.
G
Guest
Guest
RR,
Can you elaborate more on the Scope Height affecting the Chony readings. I didn't realize it could affect the outcome of the readings.
Thanks,
Don
Can you elaborate more on the Scope Height affecting the Chony readings. I didn't realize it could affect the outcome of the readings.
Thanks,
Don
- Oct 30, 2004
- 24,458
- 3,535
Any bullet will yield a different BC from different rifles. Too many variables, that is why you gotta shoot at long range.
Oldtrader3
Ammo Smith
- Nov 6, 2009
- 8,406
- 6
I assume that you are shooting a 168 grain match bullet (from the SD which you provided) and reviewing loading tables for 61.0 grains of IMR 4831, I would guess that the muzzle velocity is somewhat less than 2800 feet per second for this load. I think that your chrono is not reading right. Plus, the velocity is pretty close to your drop calculations. The only powder that will get you close to 2900 fps is Retumbo with that bullet in the 7mm RM.
- Thread starter
- #8
That's helpful to know. I was wondering what that load "should" be getting. I will compare with othe chronys at the range.
Concerning the question about scope height: The muzzle of the rifle sits about 1-2" below the scope's line of sight. So at the muzzle the bullet is 1-2" below the line of sight. The scope is adjusted to bring the crosshairs into the bullet's trajectory. How much you have to adjust the crosshairs to inline with the trajectory depends on how high the scope sits. For example, in my set up the difference between a scope height of 1" and 2" translates into about a 2 inch verticle POI change at 500 yards. So, it's not much, but it is important to get it right for long range. At 1000 yards it is just over a 5 inch change. I'm not an expert, so maybe somebody else can explain it better.
Concerning the question about scope height: The muzzle of the rifle sits about 1-2" below the scope's line of sight. So at the muzzle the bullet is 1-2" below the line of sight. The scope is adjusted to bring the crosshairs into the bullet's trajectory. How much you have to adjust the crosshairs to inline with the trajectory depends on how high the scope sits. For example, in my set up the difference between a scope height of 1" and 2" translates into about a 2 inch verticle POI change at 500 yards. So, it's not much, but it is important to get it right for long range. At 1000 yards it is just over a 5 inch change. I'm not an expert, so maybe somebody else can explain it better.
Guy Miner
Master Loader
- Apr 6, 2006
- 17,874
- 6,472
Rem 700 - I'd check that chronograph against a known velocity load. All sorts of things can induce errors into chronograph reading. Light/angle/intensity. You may, or may not, be getting a correct reading. I tend to think that you are getting a reasonably close reading, given your powder charge.
Checking Berger's manual, the 168 gr VLD can hit 2900 - 3000 fps with several different slow burning powders. I've seen well over 2900 fps using that bullet in the past myself. BTW - it is an excellent choice for long-range accuracy...
Berger lists 59.4 grains of IMR 4831 at 2887 fps as a max load. Given the track record of the various 4831's in long-range target shooting, I can understand why you chose it as a potential powder.
Consider though, RL-22, which I've used with excellent results, including tight SD numbers.
If you want more velocity, Berger shows IMR 7828 SSC, Norma MRP, Viht N560, RE-25 and Ramshot Magnum all producing just over 3,000 fps. Several other powders including RE-22 will get you into the 2900's - where you already are with IMR 4831.
For more consistent SD numbers, consider going to a bushing type neck-sizing die. This eliminates the expander ball, and can produce very consistent bullet seating - which I've seen reduce the SD numbers down to single digits.
So - what about the drop?
The best thing I can tell you is that long-range shooting has a lot of variables, and the very best way to check them is at the range. If that's how much elevation your rifle/scope/ammo combination needs to be on target at 500 or 600 yards... Well then, that's how much it needs.
Sometimes I can make a good, educated guess at how much elevation I'll have to dial in for a given load, using the various charts and ballistic programs available. Sometimes though, I come across a combination that just doesn't click with the models. In that case, I go with what I really find, at the range.
Hope some of this helps. Wore out a few barrels figuring this stuff out the slow & expensive way.
Regards, Guy
Checking Berger's manual, the 168 gr VLD can hit 2900 - 3000 fps with several different slow burning powders. I've seen well over 2900 fps using that bullet in the past myself. BTW - it is an excellent choice for long-range accuracy...
Berger lists 59.4 grains of IMR 4831 at 2887 fps as a max load. Given the track record of the various 4831's in long-range target shooting, I can understand why you chose it as a potential powder.
Consider though, RL-22, which I've used with excellent results, including tight SD numbers.
If you want more velocity, Berger shows IMR 7828 SSC, Norma MRP, Viht N560, RE-25 and Ramshot Magnum all producing just over 3,000 fps. Several other powders including RE-22 will get you into the 2900's - where you already are with IMR 4831.
For more consistent SD numbers, consider going to a bushing type neck-sizing die. This eliminates the expander ball, and can produce very consistent bullet seating - which I've seen reduce the SD numbers down to single digits.
So - what about the drop?
The best thing I can tell you is that long-range shooting has a lot of variables, and the very best way to check them is at the range. If that's how much elevation your rifle/scope/ammo combination needs to be on target at 500 or 600 yards... Well then, that's how much it needs.
Sometimes I can make a good, educated guess at how much elevation I'll have to dial in for a given load, using the various charts and ballistic programs available. Sometimes though, I come across a combination that just doesn't click with the models. In that case, I go with what I really find, at the range.
Hope some of this helps. Wore out a few barrels figuring this stuff out the slow & expensive way.
Regards, Guy
RiverRider
Handloader
- Dec 9, 2008
- 1,454
- 107
DON":1bq7462j said:
I never said that Don. What I meant was that the scope height has to be known for trajectory calculations. I guess I was not real clear on that.
RiverRider
Handloader
- Dec 9, 2008
- 1,454
- 107
Rem 700":2xyb1hfb said:
It's just a product of some degree of instability. The absolute best BC is achieved when the bullet is perfectly stabilized. Any instability creates drag and accelerates velocity decay.
Another reason to love the 17hmr , I use it for verifying my chrony every time I set up. The 2550fps with the 17 gr V-max is ussually bang on give or take maybe 10 either way. This way I know my chrono'd load is very close to actual velocity again give or take a few fps. I always stay at least 15, from the muzzle as closer seems to get more erratic.
No experience personally with the bergers, but as a side note Sierra list the 168 gr MK with IMR4831 with max load of 62.5 grs and 2900 fps. Just a bit of a cross reference which is something I seem to have a habit of doing no matter cal or bullet.
No experience personally with the bergers, but as a side note Sierra list the 168 gr MK with IMR4831 with max load of 62.5 grs and 2900 fps. Just a bit of a cross reference which is something I seem to have a habit of doing no matter cal or bullet.
G
Guest
Guest
RR,
Sorry for the error on my part. I thought I was missing out on some good information on Chronograph's. This thread is very interesting too.
Thanks for the reply though,
Don
Sorry for the error on my part. I thought I was missing out on some good information on Chronograph's. This thread is very interesting too.
Thanks for the reply though,
Don
Antelope_Sniper
Handloader
- Jan 25, 2008
- 2,618
- 0
Remmy, you are getting a velocity 45 fps different then predicted, with a SD OVER 30.
The difference in your two expected values is close to your SD.
What was your sample size?
3 shots, 10 shots, 30 shots?
What all factors did you allow for?
Barametric pressue?
Temperature?
Altitude?
Were you using a G1 BC with a bullet that more closely resembles a G7??
You have way to many variables, and too high an SD to conclude your chronograph is wrong. In addition, most shooters attempt to come to these conclusion with too small a sample size, and without repeating the experiment. 45 fps is just normal variance between day to day shooting at the range.
The difference in your two expected values is close to your SD.
What was your sample size?
3 shots, 10 shots, 30 shots?
What all factors did you allow for?
Barametric pressue?
Temperature?
Altitude?
Were you using a G1 BC with a bullet that more closely resembles a G7??
You have way to many variables, and too high an SD to conclude your chronograph is wrong. In addition, most shooters attempt to come to these conclusion with too small a sample size, and without repeating the experiment. 45 fps is just normal variance between day to day shooting at the range.
Oldtrader3
Ammo Smith
- Nov 6, 2009
- 8,406
- 6
You have to go by your TPI (true point of impact) at longer range. That defines your ballistic coefficient and is the only way to truly define BC, all other methods are theoretical and empirical.
- Thread starter
- #16
Using G1 - 6.17; shot 4 shot groups (sub MOA at 475); Included the temperature, pressure, and altitude in the online program I use: http://www.biggameinfo.com/BalCalc.aspx
Yeah, I knew that it would be difficult to convince myself that the chrony is wrong, but just a little baffled at how my ballistics seem to match what my chrony says out to about 300-350 yards and then it drops off much faster than the vel says it should.
But you all have made a good point: Shoot at the range and use that statistic. So that's what I will do, but I just like to learn as much as I can and don't like it when something happens that I don't understand. But that's life, right?
Thanks everybody.
Yeah, I knew that it would be difficult to convince myself that the chrony is wrong, but just a little baffled at how my ballistics seem to match what my chrony says out to about 300-350 yards and then it drops off much faster than the vel says it should.
But you all have made a good point: Shoot at the range and use that statistic. So that's what I will do, but I just like to learn as much as I can and don't like it when something happens that I don't understand. But that's life, right?
Thanks everybody.
RiverRider
Handloader
- Dec 9, 2008
- 1,454
- 107
I can sympathize! When I finally took the plunge to buy my first chronograph, I thought all the answers would reveal themselves...and for every answer I got, ten new questions came up. It just never ends, but it sure is interesting!
Antelope_Sniper
Handloader
- Jan 25, 2008
- 2,618
- 0
Remmy, I owe you a bit of an appology, I misread your original post.
You are saying there is a difference of 245fp, not 45 fps.
The 45fps difference would fail a double tail T test at the 95% confidence interval, but the 245 FPS difference would pass this test, meaning that yes, there is a statistically significant difference.
Your chronographed velocity is within 10 FPS of what is predicted by Quickload, so I don't think the issue is with your chronograph.
The VLD bullets do not fit the G1 BC very well. Since Berger published their G7 BC, I'd run it again on that. In addition, so ballistics program will very from on to the next.
Next, I'd consider the wind. Did you have any head wind? Head wind will cause your bullets to dive. In addition, a left had wind will also cause your bullets to dive. If I recall correctly a 20mph left hand cross wind will drop me an addition 6 inches low and left at 600 yards in the STW.
In any event, this is an important step in your long range education. Now that you are starting to see just how many variable can effect your shot past 500, that real learning can begin.
You are saying there is a difference of 245fp, not 45 fps.
The 45fps difference would fail a double tail T test at the 95% confidence interval, but the 245 FPS difference would pass this test, meaning that yes, there is a statistically significant difference.
Your chronographed velocity is within 10 FPS of what is predicted by Quickload, so I don't think the issue is with your chronograph.
The VLD bullets do not fit the G1 BC very well. Since Berger published their G7 BC, I'd run it again on that. In addition, so ballistics program will very from on to the next.
Next, I'd consider the wind. Did you have any head wind? Head wind will cause your bullets to dive. In addition, a left had wind will also cause your bullets to dive. If I recall correctly a 20mph left hand cross wind will drop me an addition 6 inches low and left at 600 yards in the STW.
In any event, this is an important step in your long range education. Now that you are starting to see just how many variable can effect your shot past 500, that real learning can begin.
- Thread starter
- #19
