Chronographing

[TikkaT3Varmint]

Hi everyone

I'm new to reloading. I am loading 39gr Blitzkings on top of 27gr H4895. Using the RCBS digital scale (any possibility that this may be the problem?). I just bought a chrony F1 and tested my loads out. Problem is.... the highest velocity was 3880 and the lowest was around 3780. The others were anywhere in between. I started reloading to get more consistent ammo and I doubt that this is much better than factory ammo.

Could it be the scale is out? The chrony is out? Or is there something else I am not considering? Very frustrated.

Thanks in advance

Dalton

[ryutzy]

couple questions, Are you weighing each charge? Is all the brass trimmed to exact same length? When you lube your cases make sure all lube is completely cleaned out of your case necks. Are all your bullets seated to the exact same length? (Ogive to case head) Primer pockets and flash holes cleaned and deburred?

I have also noticed with my chronograph that for best accuracy the chrono needs to be in direct sunlight. Cloudy days seem to give weird or no readings at all.

[Jim White]

- Changing light conditions can affect chronographs.
- How high or low (from shot-to-shot) the bullet travels over and left/right of the sensors.
- Muzzle distance from test-to-test.
- Muzzle blast, if any.

These are just a few things "that may" cause some issues. Of course, ammo can cause it too, neck tension, brass length, seating depth etc... (I believe ryutzy mentioned a few).

[DoubleUp]

+1 on everything already said. It would be good if you could check your chrono against another by shooting through both of them. I try to set up the exact same parameters each time. measuring 10 ft from the front sensor to the forward edge of my shooting bench. That puts 11 ft from the barrel to the sensor. Muzzle blast in a 204 is a lot greater than you might think and can cause erratic readings. Also be sure the chrono is level each time and try to set up as JW said to fire directly over center of the sensors and not have the back sensor skewed from the front.

Consistency is king in the reloading, shooting, and chrono set-up. Get a pattern and routine to do it the same way each time.

[TikkaT3Varmint]

Thank you for your replies. I do weigh each charge. Cases all trimmed to exact. Clean cases after sizing so there is no lube. Bullets are all seated the same depth.

And I also measured 12 ft from muzzle to the chrony so that shouldn't be an issue.

I guess I will have to measure my chrony against someone elses. And maybe my scale as well, as it weighs and dispenses the powder for me so maybe is not entirely accurate?

Thanks again

Dalton

[Rick in Oregon]

Inconsistent neck tension could be the culprit here......

[ryutzy]

Based on your reply TikkaT3varmint I think I agree with Rick here. Neck tension is likely the issue.

[TikkaT3Varmint]

Not sure what you mean by this... Please explain

[Joe O]

Could be bullet tension,however,if you are using a die with expander ball ,the neck tention should not be a factor,in a no turn chamber.Try sorting a box of bullets by weight,then sort a group of the same weight by OAL (base to Ogive).Now when you seat each batch they will be the same distance off the lands (your choice).The set-up info for the F1 crony is another must.Keep in mind,you can still have little groups when the SD and extreme spread is double numbers.

[TikkaT3Varmint]

Thank you.

I just realized one thing that was mentioned..... I do not deburr the flash hole. I simply clean the primer pocket. I have not been able to find a flash hole deburring tool for the 204. Even looking on the rcbs website i can not find it. Would this make a big enough difference to throw my velocities off that much?

[Glen]

What did your groups look like? Little bugholes with 100fps variation isn't all that bad. Patterns with 100fps variation isn't all that good.

[TikkaT3Varmint]

Groups are good. Just can't help but think they could be better if the velocity wasn't fluctuating.

[ryutzy]

TikkaT3Varmint

Flash hole deburring probably is not your culprit, however it could help. If you are trying to shoot the best groups possible then you do want to deburr the flash hole. Brad at CRT has the best flash hole tool that I have ever used. I highly recommend his products as do many others on this forum. http://www.customreloadingtools.com/crt_013.htm is his website and this is the link to the specific tool.

[Wrangler John]

There are just too many variables to point to any one factor as the cause. First variable is that each shot is a separate chemical reaction - the chances of each reaction running to the same terminal point is astronomical. Variations in primers plays a part, as does case dimensions, the metallurgy of each piece of brass, powder charge variation, type of powder, barrel condition during the run, and so many obscure factors that all add up in differing combinations making a determination nearly impossible. Then we come to exterior factors, the chronograph itself may have a 0.5% accuracy range (the manufacturer claims 0.5% under ideal conditions, but my experience with open channel sonar flow meters and optical turbidity meters suggest that there is a greater fluctuation due to environmental variables), temperature, wind, air density, light angle, opacity, ogive glint, all may serve to decrease actual measurement accuracy upwards of 10%. Many more factors including muzzle blast, which can really wreak havoc with results - even muzzle blast from nearby shooters.

Then we come to the chronograph itself. The Chrony F1 has a screen spacing of about one foot. Generally, longer distance between screens produces greater accuracy due to more accurate clock cycle measurement. The Chrony also places its sensitive electronics down range where they are exposed to blast effects, which, if it doesn't effect accuracy directly, the circuitry may have been adjusted to provide less sensitivity to compensate for shock wave distortion.

I use two chronographs: an Oehler Model 33 and a new production Oehler 35P. The Model 35P is actually 2 chronographs in a single housing and uses three screens. There is a start screen, and two stop screens, one for the primary channel, and one for the secondary channel or "proof" chronograph. Screen spacing is two feet per channel or 4 feet total. The proof channel provides a means of determining the validity of result for a single shot. If the two agree within a small velocity, then the result is accurate, if the second or proof velocity is way out of range, then the result can be discarded as erroneous. For example, here are some results for a load:
Shot 1 = 3541 - 3528
Shot 2 = 3549 - 3535
Shot 3 = 3555 - 3542
Shot 4 = 3573 - 3559
Shot 5 = 3592 - 3578
Shot 6 = 3600 - 3587
Shot 7 = 0000 - 2700 deleted
High = 3587
Low = 3528
ES = 0059
Avg = 3554
SD = 23
So here we find shot 7 produced an error and was deleted from the tally. The remaining shots were either 13 or 14 fps between the primary and proof channels thus valid. The ES was 59 fps. This is about the only way to be certain the data itself is valid. The older Model 33 chronograph did not have this function, yet the results for the same cartridge as shot with the 35P are within a few fps. Thing is that the Model 36P costs $590 for the kit, which is why I didn't purchase one until this year when they became available again. How much does this all matter? All I know is that some of the most accurate loads I have shot have Extreme Spreads of 100 fps or more. Do not do what we are all prone to - chasing after things that don't matter. If your groups are good, then don't expect more from that load. In obtaining lower extreme spread I suggest trying a new powder or two, that has been the single largest factor for me. H 4895 may not produce the lowest ES in your rifle compared to say Benchmark or Reloader 10X (examples only). Then try adjusting seating depth. Remember that the .204 Ruger is fairly long throated, so if the bullet leaves the case before the shank enters the bore, pressure will bleed off around the bullet until it enters the rifling and blocks the bore. That's the function of freebore - a pressure safety valve. Seating the bullet in or out changes the relationship a minuscule amount and may decrease extreme spread.