do these velocities look right to you?

J

JDMLandscaping

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Managed to get out to the range and do some load development with my RPR Gen 3 in 6mm Creedmoor. It has a 24” 1:7.7 barrel. Shooting Hornady 105 HPBT match using Hornady 6mm CM brass and H4350. Started at 38.6 grains powder and increased by .2 grains up to 42.6 grains. Fired 3 round groups recording velocity on each and shooting for groups (as best as possible as it was pretty chilly at the range today). Got some funny results with the velocities and I’m not sure if it was my chrony or if it’s what’s to be expected with this load. I won’t show all my data as it’s a lot but hopefully you guys can get a good picture. This is my first time testing loading for a bench gun so any and all info would be great!

Chronograph: shooting Chrony “master chrony” model (older unit got used but in very good shape)
Rifle: RPR Gen 3 24” 1:7.7 twist
Cartridge: Hornady 6mm CM brass Hornady match 105 HPBT and H4350 powder COAL 2.795”

Data (as per chrony) averaged over all 3 rounds:
38.6 grains of powder gave 2715fps
39 2688fps
39.4 2729fps
39.8 2710fps
40.2 2746fps
40.8 2773fps
41.2 2820fps
41.8 2860fps
42.2 2834fps
42.6 2821fps

FPS is going up but I expected to be in or near 3000fps once I hit the 42 grains of powder and expected a larger spread in velocities, not 150 fps from slowest to fastest. I set the chrony up as per the directions exactly. Powder measured on a Lyman digital scale which I calibrated every 15 or so powder weighs.
 
I never did figure out the "why" part - would get days like that with my Beta version of the same machine. I always suspected had to do with the day's lighting - seemed to occur mostly when I did not put on those white "paraglider" covers - but never spent any amount of time to confirm that.
 
Direct sunlight reflecting off of the bullet will influence a chrony making the readings erratic. Best to test velocity on days with overhead cloud, or at least use the diffuser screens. Other than that, I have had some rare combinations of components that did exhibit erratic results and non-linear progression of velocity vs powder charge. But it only happened with an unusual mismatch of components to application. You seem to have made pretty "normal" choices. Perhaps ignition was erratic. You may want to try a hotter primer, Or check firing pin strength, protrusion, and free travel without any crud interfering.
 
Something happened at around the 41.8 load mark to change the equation. Yes those numbers seem odd, but I wouldn't necessarily suspect the chronograph.

Did you take a long break and let the gun cool down? Did you change the rate of fire? Did you clean the barrel. Were there any big changes in ambient temperature or wind that may have affected cooling?

When I do load development I never fire three rounds of the same load in sequence.
My load development goes as follows.

Step one, pressure test. Load one round of each load that I would want to explore up to where I expect the maximum to be, and then load two more. starting at the bottom fire one every 3 minutes. If i want to get really particular I bring a head gun and make sure the barrel has returned to ambient temperature. Repeat looking for signs of pressure up to max and then stop when excess pressure detected. Lets say this reveals 8 different charge weights that are safe.

Then I go back to the bench and load 3 or 5 rounds (typically five now) at each charge. Put a target with 8 circles. Fire 1 round from each charge at each target, then back to the beginning until I have all my groups fired. This helps to spread out any inconsistencies between temperature, barrel wear/fouling, shooter performance (first group is always rough) etc across all of the groups fired.

I've never seen velocity decrease like that with increasing powder charge unless some other variable was introduced to explain it. I've also never seen light affect the reading of the chronograph in such a small way. Normally changes in light cause registration failures, not readings that are 2% lower than expected.

Definitely something going on there.
 
Probably either the chrony or the powder measure are off. Do you have a balance beam that you can check the electronic scale against, or can you borrow another chronograph to compare velocities? Some electronic scales can go wonky. I don't know about the newest generation but the older lyman electronc scales didn't have a perfect reputation.
 
3 rounds is not much of a test of anything.

If the loads are mild for the rifle, powder and bullet, the velocities can be erratic, and with such a small sample, one very fast or slow round can skew the results, so you don't get the nice progression you are looking for. However, I use 3 rounds tests much like you did to get a hint as to what works. In this case, the answer may be "This power does not work." or "Loads are too mild to work."

Or, if the targets show a good progression towards accuracy, but the Chrony does not, the problem could be the Chrony.

I use a Chrony and get excellent results. But I only get good results on an overcast day (no direct sunlight - Chrony has to be looking up at clouds). And, muzzle is at least 15 feet from Chrony - preferably 20 feet.
 
Thanks for all the input so far. As for my chrony set up I did have the sun shades installed on it and there was no direct sunlight. It wasn’t an overcast day but it also wasn’t clear but because there was no direct sunlight I don’t think that was enough of an issue to throw velocities off this much. My powder scale i purchased not even a year ago new and like I said I made sure it was calibrated with the weight they include to do so every 15 powder throws or so. The instructions for the chrony said to set it up at least 10 feet from the muzzle for centre fire rifles and so I paced off 10 feet and put tripod down in front of me so it was around 11-12 feet from muzzle. I was trying to find my velocity nodes so this was rather disappointed as basically I shot $70 worth of reloads for nothing. I’ll have to ask around and see if I can borrow a better chrony or buy one on Black Friday
 
I've shot 39.1grs H4350 with the 105gr BTHP, did 2846 fps from my 24" barrel. Alpha brass which has less capacity than Hornady brass

Factory 103gr ELDx ammo does 3150 fps
 
JDMLandscaping said:
Thanks for all the input so far. As for my chrony set up I did have the sun shades installed on it and there was no direct sunlight. It wasn’t an overcast day but it also wasn’t clear but because there was no direct sunlight I don’t think that was enough of an issue to throw velocities off this much. My powder scale i purchased not even a year ago new and like I said I made sure it was calibrated with the weight they include to do so every 15 powder throws or so. The instructions for the chrony said to set it up at least 10 feet from the muzzle for centre fire rifles and so I paced off 10 feet and put tripod down in front of me so it was around 11-12 feet from muzzle. I was trying to find my velocity nodes so this was rather disappointed as basically I shot $70 worth of reloads for nothing. I’ll have to ask around and see if I can borrow a better chrony or buy one on Black Friday
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I'm not convinced the chrony is trouble.
 
How much does the Time a cold cartridge spends in a warm chamber relate to the ambient temperature to any extent, and affect burn rate ?
I always try to be consistent with my loading/firing time
 
I first heard of a Velocity to Charge Weight notion in Pierre Van Der Walt's book "African Dangerous Game Cartridges". He quotes from Johan Loubser, who is a ballistician, formerly with South Africa's Somchem, more recently with Western Powder in Miles City, Montana. His concept is based on evidence of a constant increase in velocity, per given unit of powder increase, until a "tipping point", beyond which the hand loader has exceeded limits. He discusses Average Velocity per Charge, and Dynamic Velocity per Charge. But even that information shows no evidence of a decrease in velocity after an increase in powder charge. The discussion is about the predicted versus the actual amount of increase in velocity for a given incremental increase in powder. I have not read anybody refuting his concept.
 
boxhitch said:
How much does the Time a cold cartridge spends in a warm chamber relate to the ambient temperature to any extent, and affect burn rate ?
I always try to be consistent with my loading/firing time
Click to expand...

It can certainly have a sizeable effect. A few seconds would be negligible but minutes different from round to round will be noticeable. How much depends on a lot of variables.
 
JDMLandscaping said:
So what do you think is the trouble?
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Hard to say without having observed everything. Scale drift could cause your charges to be lighter than you think. Temperature changes or a an inconsistent interval between groups. Just guesses.

Were the bigger loads compressed?

3 rd groups are a relatively small sample size.

You posted the average velocities for each group correct? How tight were the velocity spreads? Was it a single slow round dragging down the average ornwas it consistent?
 
The only problem I had with my old Chrony was getting a consistent light source, as is mentioned throughout this thread.

A couple of decades back I finally had enough of it and made up shadow box, with overhead 12 volt lights, over the sensors. This worked really well for me. One thing, the lights from the bulbs needed to be filtered as they were just to bright. It was easier and cheaper to install filters, than it was to include a rheostat into the circuit.

The great thing about the above system is that results were very consistent shot to shot with the same loads. There was still an average 25fps spread but that's normal over 10 rounds. The same is true with my magnetospeed, as well as few times I've shot over a labradar.

Velocity increases are not consistent with powder volume other than you should be seeing increases, however slight with each increase in the charge. UNTIL YOU REACH THE SATURATION POINT.



I don't believe your 6mm Creedmore would be capable of reaching the saturation point of your rifle's bore. That doesn't mean you can't reach critical pressures in the case.


A now deceased friend of mine showed me his Chrony shadowbox, after seeing and using mine. His approach was different, but had the same effect on the sensors, likely even more consistent then mine.

He set up two smaller 12 volt bulbs at each end. The thing is, they were on the bottom of the box. The interior of the box had been painted eggshell white. His box was much more sophisticated than mine. Typical of Brian. It worked very well. He had set up the bottom of the box with a bracket for his Chrony. This was necessary because of his light set up. My shadow box just allowed the Chrony to fit inside.

The real critical thing about a shadow box is the height of the light source over the sensors. We both finally settled on the same height as the diffuser panels that are normally set up on rods.

Both of us painted fluorescent strips on each side of the openings to the box. These were scope reticle matching marks, which allowed enough room over the Chrony to save it's life.

I learned a long time ago that measuring velocities and shooting for groups at the same time was a recipe for disaster.
 
bearhunter said:
The only problem I had with my old Chrony was getting a consistent light source, as is mentioned throughout this thread.

I learned a long time ago that measuring velocities and shooting for groups at the same time was a recipe for disaster.
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Maybe with an old school chrony. With a Magnetospeed or a LabRadar, it is not only possible, it is almost mandatory in my mind. Why screw around wasting ammo doing one or the other when you can do both at once. Ammo is expensive, range time is expensive, transport to and from the range is expensive. Time is expensive if it is wasted in travelling to/from the range and waiting for days or weeks between outings after making adjustments. One can learn a lot by having the info immediately while doing the test/load development. While I personally don't reload at the range, I know of a few others that do so, or have a means to modify loaded cartidges, by pulling bullets, adjusting loads, and reseating on the spot.
 
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