Erratic Chronometer Readings

[Bvcg1]

Hey fellas please help me understand how I could possibly be getting such wide variations in bullet speed in these 17 shots:

3,152 fps
3,159
3,122
3,167
3,174
3,108
3,093
3,129
3,152
3,115
3,072
3,079
3,144
3,122
3,144
3,152
3,108

These are chronometer readings starting from last shot to first.
High: 3,174 fps, Low 3,072 fps

The results were 5.5 inch groups at 200 yds. I've had .75 MOA at 100 yds with this load, so I had felt pretty confident in it.



Caliber: .300 RUM
Bullet: Berger 185 gr VLD
Powder: 93.5 gr H-1000
Seating Depth:-.175 off lands (Magazine Fit)
Primer: Federal 215
Chrono: Competition Electronics Pro Chrono

Every load was precisely measured, then measured again, cases universally trimmed, etc. I do not go as far as to separate bullets by weight.

Could the jump to the lands cause bullet distortion, thus changes in pressure, resulting in bullet speed variations? I know Berger recommends backing off the lands incrementally until the best depth is found, but this is a hunting rifle and I feel a whole lot better with 2nd and 3rd shot cartridges that fit my magazine.

More importantly, could this possibly be a sign of barrel / chamber issues?

Also, on another note, I have been told that a 185 grain bullet may be a bit light for a .300 RUM... that a 200-220 grain bullet will be more stable and have better ballistics at long distances. Any thoughts there?

Thanks for your help!

 

[jmden]

Go with the 215 Berger and H1000 or Retumbo.

Did you have changing lighting conditions during the string? Typically the best conditions for accuracy and consistency of the chrono are consistently overcast (cloudy) conditions. Sunny conditions can result in higher readings. Changing lighting conditions can cause different reading coming from the chrono.

Using a micrometer seating die and a Davidson's seating depth checker to get the most consistent seating depths is something else to consider as well.

Is your bullet seating die seating plug for VLD style bullets? The alternative style has a much larger opening often causing VLD style bullets to make contact with the plug at their tip and not far down on the ogive, which is much preferable for consistent seating depths.

Is your chrono stable and not moving in the wind? Are you sure you are shooting through it at the same exact angle, etc. for every shot (shooting off a bench, or prone off bipod to increase consistency in this regard)? I put my chrono 10 yds out to keep any chance of muzzle blast from moving the chrono and causing a bad reading as the bullet is passing through.

 

[Bvcg1]

I have a Redding micrometer seating die, and the setting is 'glued' in place so it won't move, so everything is considtant as far as seating depth goes.

Regarding the chrono, is was sunny and the chrono was placed 12-15 feet from muzzle. No postion change or wind to speak of.

The thing is, in other times at the range, my friend shoots his 7mm STW through it and his readings are very consistent, while mine are not. I'm wondering if it could be a problem with the gun. Your thoughts?

 

[jmden]

I have a Redding micrometer seating die, and the setting is 'glued' in place so it won't move, so everything is considtant as far as seating depth goes.

Regarding the chrono, is was sunny and the chrono was placed 12-15 feet from muzzle. No postion change or wind to speak of.

The thing is, in other times at the range, my friend shoots his 7mm STW through it and his readings are very consistent, while mine are not. I'm wondering if it could be a problem with the gun. Your thoughts?

Weighing every charge carefully, of course? Try changing primers.

The micrometer seating plug (these are also designated as 'VLD' or 'standard', so make sure you have the VLD style or this can cause erratic seating depths) is designed to be used in conjunction with a seating depth checker, Davidsons being a good one. I seat the 300g Berger with a Forstner micrometer die, but I still get some inconsistencies that have to be measured with the Davidsons seating depth checker and then the micrometer seating plug turned a bit to adjust so they are all consistent based off the DAvidsons seating depth checker measurement. U seat them all 5 or 6 thou long, measure, then adjust each one down from there. The plug shouldn't be 'glued'--that's no better than a non-micrometer plug at that point and that's not what a micrometer plug is for.

I would suggest Shawn Carlock's: Reloading for Long Range Hunting

Defensive Edge Products

 

[MTBULLET]

first, replace battery, then try moving chrono to 30' from bbl.

 

[MMERSS]

With a 2+MOA group perhaps your chronograph is not all that erratic. In either case you should consider redeveloping a load. While at it, why not try some of the heavier bullets. The 300 RUM is a perfect candidate for the heaviest commercial bullets out there.

 

[Greyfox]

I have used that chronograph. Sunny conditions with the polished Bergers will give the variations that your seeing. As was stated in a prior post, wait for an an overcast day and set the chrono at 15' from the muzzle. Use a level to make sure the body of the chronograph is level with the barrel, and the bullet passes about 4-5" above the sensors.

 

[375fan]

Lot of good answers there. What is your twist rate?
I am in agreement that you need heavier bullet for the 300rum, try 215 or 230 hybrids, 208 amax, using H1000.

 

[notajeep]

My experience with the F1 Chrony. I was getting readings as low as 1300 FPS with my known prairie dod loads that should have been reading 2590 FPS. I had to adjust the height of the chrony so the bullet was passing somewhere between 4-6" above the eyes of the crony. My F1 worked great when I learned how to use it. In your case I really don't feel this is the problem but it easy to check.

 

[Bvcg1]

Lot of good answers there. What is your twist rate?
I am in agreement that you need heavier bullet for the 300rum, try 215 or 230 hybrids, 208 amax, using H1000.

Twist rate is 1 in 10

 

[MontanaRifleman]

I find chronographing on overcast days to give me the best results. That is why I will soon be ordering an acoustical chrony which is completely independent of light conditions. I have seen 50-60 fps jumps from overcast to sunny conditions. A couple of years ago, I got a second chrony and set it right behind the first. Readings were anywhere from 5 to 70 fps different on a sunny day.

A saw a guy at the range a few days ago with some battery operated LED lights taped to the top of his sky screens. He said they worked very well for him. More consistant light.

Changing the battery is another good suggestion.

That said, you may indeed have a large ES with your load for whatever reason, but chances are that some of the low and high reading are in error. Hard to say.

I agree that heavier bullets are best to get the most out of a RUM, but I have shot 180 E-Tips out of mine with numerous sub .5 MOA groups. This is out of a Sendero.

Your accuracy and ES could be due to any number of things.

How accurate is your scale? I had a 10-10 start acting up on me once.

Have you damaged your crown?

Action screws and bedding?

Scope and mount screws?

Scope?

Fouled bore?

Throat erosion? How many rounds down your 300 RUM? They can go quick if you shoot them hot.

It could be any of these or something else or a combo of things.

 

[Greyfox]

I find chronographing on overcast days to give me the best results. That is why I will soon be ordering an acoustical chrony which is completely independent of light conditions. I have seen 50-60 fps jumps from overcast to sunny conditions. A couple of years ago, I got a second chrony and set it right behind the first. Readings were anywhere from 5 to 70 fps different on a sunny day.

A saw a guy at the range a few days ago with some battery operated LED lights taped to the top of his sky screens. He said they worked very well for him. More consistant light.

Changing the battery is another good suggestion.

That said, you may indeed have a large ES with your load for whatever reason, but chances are that some of the low and high reading are in error. Hard to say.

I agree that heavier bullets are best to get the most out of a RUM, but I have shot 180 E-Tips out of mine with numerous sub .5 MOA groups. This is out of a Sendero.

Your accuracy and ES could be due to any number of things.

How accurate is your scale? I had a 10-10 start acting up on me once.

Have you damaged your crown?

Action screws and bedding?

Scope and mount screws?

Scope?

Fouled bore?

Throat erosion? How many rounds down your 300 RUM? They can go quick if you shoot them hot.

It could be any of these or something else or a combo of things.

I bought an acoustic Superchrono for much the same reasons. I have now used it about four or five times and I'm not convinced that it's the answer that it's cracked up to be. The specs are .5-1% error and that's about what I can see best case if everything is lined up perfectly. At 3000 FPS that 15-30 FPS error. It's better I think for getting a true velocity average regardless of light conditions, but not for establishing ES. For my serious ballistic work, I think I'm going to stick with my Ohler on overcast days. IMO

 

[lloydsmale]

heres what id try first IF YOUR NOT ALLREADY SHOWING PRESSURE SIGNS. Bump it up another grain of powder to raise the pressure a tad to see if that helps that 1000 burn a bit better. If you see some improvement try another 1/2 a grain at a time till either it cures it or you see pressure signs. Trying another primer might work too. Maybe a cci mag rifle but i have to doubt it as ive had better luck lighting of real slow powders with the primers your using. Only differnce is i use the match version (which might be worth a try too if you can find some). Another thing id try too is to shoot a different rifle, one you know has nice tight spreads to see if it is your chonograph giving bad readings.

 

[MontanaRifleman]

I bought an acoustic Superchrono for much the same reasons. I have now used it about four or five times and I'm not convinced that it's the answer that it's cracked up to be. The specs are .5-1% error and that's about what I can see best case if everything is lined up perfectly. At 3000 FPS that 15-30 FPS error. It's better I think for getting a true velocity average regardless of light conditions, but not for establishing ES. For my serious ballistic work, I think I'm going to stick with my Ohler on overcast days. IMO

Have you shot the acoustic chrony against the Oehler and if so, what were the results? My smith has an Oehler and I was going to have him compare when I got the acoustic. Really nice not having to worry about light, and having a large shooting window and not having to assemble anything to set it up.