velocity issues

[HunterGreen]

What you folks think ?

When chrono my new loads I noticed every two or three shots would be very different in velocity.

3025,3015,3033,2870,2902,3044, ????

Neck sizing 7mm mag, neck case outside diameter .306 after running them thru nk sd. Never had a problem, unless my scale is off

 

[live2huntmt]

Does your POI change when the chrono reads the extreme velocities? If the POI does not change with the chrono... I would lean towards chrono error.

 

[HunterGreen]

Good question,

Roughly an inch,. Do you think my neck is too tight ?? .306-.307 for winchester brass 7mm remington magnum..

 

[joseph floerke]

i dont even use a chrono anymore for long range applications i just go out to 1000yds and walk the bullet in until i have established my MV.

 

[Broz]

I would say there are some "flukes" in that string. My next guess is you are using a "Shooting Chrony" or some other less expensive unit with screens. Also if it is and it is positioned less than 18 ft. from the muzzle you need to move it out.

I have had these same issues and it caused me to loose trust in chronographs. But once I bought a Oehler that all changed. Now I mainly use a Magneto Speed that I tested to be equally accurate with the Oehler.

It is nice to leave the shop with a worked up load and accurate data entered in my ballistic app and not have to fudge numbers to get it to match up. Accurate data input is a great thing.

Jeff

 

[HunterGreen]

I bought pro chrony, I have it standing 6 feet in front of muzzle.. I think I should be 10 feet or more.. just saying

1 ol timer said to check neck on case for complete roundness. He said even .0025 off will allow powder gasses to escape before pushing bullet out.. I also notice @500 yds I went from 48 out of 50 rd hitting 8 inch gong to 17 out of 50rds.

Using imr4831 63.4gr nosler 140 ballastic tip coal 3.330 factory barrel, jewell trigger .2 ounce, A5 medalist Bell&Carson stock

 

[Broz]

I bought pro chrony, I have it standing 6 feet in front of muzzle.. I think I should be 10 feet or more.. just saying

First your chronograph is not known to be one of the best for accuracy in readings. But I can tell you first hand if you move it out to 18' or more you will get better readings. The muzzle blast is shaking the unit among other things.

Jeff

 

[HunterGreen]

Yeah I think maybe 12 feet minimum than.

Question, how does those neck concentricity Gages work ???

I am I within the normal neck tightness of .306-.307 ??

 

[Canadian Bushman]

Neck tension is more dependent on the id measurement since we dont know the thickness of the neck or roundness. Measuring the od is an estimated guess.

I dont think neck tension or concentricity would cause those variations by themselves. Could you tell us a little about how and with what you weigh your powder?

 

[HunterGreen]

Mr. Bushman I learned a few things already, thanks for telling me about measuring inside not outside diameter on neck. Also you mention measuring case neck thickness, umm
Whats the purpose of that ?? Just asking..

I would be delighted to give all info,
Weighting them with rcbs 1010 beam scale

 

[MontanaRifleman]

I have a ProChrono, and IMO it's one of the better cheap chrony's. It usually gives fairly accurate info if light conditions are good but fluctuating light will cause fluctuating readouts. It's good enough to get you in the ball park if you use it properly and know its limitations. It's better than my Shooting Chrony.

Listen to Bros... get it out to 18' I have usually done OK @ 15', but 18' would be better. 10' is way too close.

I am using a Magneto Speed now. Was with Bros when we tested it against the Oehler 35. It was right there with the Oehler.

 

[Broz]

Yeah I think maybe 12 feet minimum than. QUOTE]

Why do you think I stated 18 ft or more? Because 12 ft will work?? I am just trying to help you from my personal first hand experience.

Jeff

 

[Canadian Bushman]

Mr. Bushman I learned a few things already, thanks for telling me about measuring inside not outside diameter on neck. Also you mention measuring case neck thickness, umm
Whats the purpose of that ?? Just asking..

I would be delighted to give all info,
Weighting them with rcbs 1010 beam scale

Well if your case thickness has large variations in the neck and you size all the neck O.D.'s to .307, now those variations are transferred to the I.D. and result in variations in neck tension.

Now we dont know what your neck thickness is (unless i missed it). If its .012" and you sized your neck to .307 that infers the I.D. is .283. Thats a .001" press fit. Thats good. If its .014 that puts you at .279 (.005) which is pretty tight.
This could cause trouble! Likely you would have felt it seating bullets. Like i said ealier though this is only an estimated guess.

I personally want to think its a powder or primer problem because 100-200 fps is a large jump, and the variations in vertical impact are significant. Is there any way your scale could be acting up, or a possibility your powder or primers got damp?

Im not trying to be a know it all here, just brainstorming. Im sure we'd all like to get you back to whooping up on that 500yd target.

 

[HunterGreen]

Well if your case thickness has large variations in the neck and you size all the neck O.D.'s to .307, now those variations are transferred to the I.D. and result in variations in neck tension.

Now we dont know what your neck thickness is (unless i missed it). If its .012" and you sized your neck to .307 that infers the I.D. is .283. Thats a .001" press fit. Thats good. If its .014 that puts you at .279 (.005) which is pretty tight.
This could cause trouble! Likely you would have felt it seating bullets. Like i said ealier though this is only an estimated guess.

I personally want to think its a powder or primer problem because 100-200 fps is a large jump, and the variations in vertical impact are significant. Is there any way your scale could be acting up, or a possibility your powder or primers got damp?

Im not trying to be a know it all here, just brainstorming. Im sure we'd all like to get you back to whooping up on that 500yd target.

Umm well, this Saturday I will bring extra table and start @18' , secondly I just bought 2 hrs ago a Hornady electronic scale seems its pretty accurate more faster and accurate than rcbs 1010 as of now.

I sincerely appreciate all replies, keep em coming. I do have 6 2 pounds dehumidifiers bags in the safe along with electronic dehumidifier, cci magnum primers been using lately. I did find two boxes of fed large mag primers, so I'll try those later. These casing are on there 3rd shot