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LocoGringo

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Feb 9, 2013
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I tested the rounds I made for my son's rifle and it started out very consistent. My procedure was to shoot 5-shot groups and get chronograph speeds as I shot the groups. The chronograph info was the most important and the real purpose I was at the range.

The load is Hornady brass, Barnes TTSX 120 grain 7mm bullets, Federal 210M primers and BL-C(2) powder for 7mm-08. For precision testing, I shot 5-shot groups ranging from 43.2 grains of powder to 43.6 grains in .1 grain increments. Every 5-shot group was 3/4" except for 2 "flyers". Even with the "flyers" the groups were respectable. I'm going for a low recoil hunting load and this fits perfectly. The rifle is a bit heavy and is equipped with a 7" suppressor. I settled on 43.3 grains of BL-C(2) with the bullet seated about .050" off the lands. I use Dillon Precision case lube (alcohol with lanolin) to lubricate the cases for resizing, so a little lube gets on the inside of the case neck when I spray.

I was getting very consistent muzzle velocities...until I didn't. For the first 5-shot group, 4 of the shots were within 15 FPS of each other except 1 which was WAAAY different...67 fps less.

After all 20 rounds were fired, the average was pretty solid but there were a significant number that hit really high and really low. The average for all 20 was 2865 fps with a high of 2940 and a low of 2778. There were about 3 or 4 that were in the 2900s and a few sub-2800s. The temperature was about 50-55 degrees outside with high humidity, so I'm wondering if the powder is temp sensitive or if case lube could have caused some variation or because the powder is a low density load since it's close to the minimum.

Groups were a little larger than I would have liked, but the largest was 1.13" which isn't bad. I wasn't concentrating as much on getting a tight group as making sure I didn't hit my chronograph. So I'm wondering what might cause such a wide dispersion for muzzle velocities? It's definitely good enough for hunting purposes in East Tennessee, but I'm looking to learn also.

Another question is how many rounds do you chronograph to develop a reliable ballistics chart?

@Walkalong @Varminterror
 
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I'd try some with out the suppressor to rule that out, poa will likely change and the barrel node to. Could be neck tension, your shooting a lot of copper so maybe your barrel is fowled. Seems some powders aren't the best with mono bullets, there's probably a reason the load data is typically using different powder then the same weight jacketed bullet.
 
The load was developed with the suppressor, not independently of the suppressor. The rifle was also thoroughly cleaned beforehand...it was a fresh bore and the rifle itself is new enough that there is no buildup from being fired. It has less than 400 rounds total through the bore.

It's a full custom rifle: Defiance Rebel action, Krieger barrel, Timney trigger and HS Precision stock. The scope is a 3-15x42 Tract Toric scope.
 
What is book load range? I've found ball powders usually like to be on the higher side of the load range to get consistency.
If I get random velocity swings on the bottom. I add powder.
If I'm already at or near max. I start looking at my seating depth.
 
Just me- when working up a load,
I'm going up in the increments looking
for accuracy first and foremost. Velocity
doesn't figure in at all. And I'll only
work on changing one variable at the
time. Unless you're just looking for
the highest velocity, I'd put the
chronograph aside until I achieved
the accuracy I was after. Just me though
 
Is the brass all trimmed to the same length? If you crimped after seating the bullet case length will effect velocity.
 
Bfh_auto said:
What is book load range? I've found ball powders usually like to be on the higher side of the load range to get consistency.
If I get random velocity swings on the bottom. I add powder.
If I'm already at or near max. I start looking at my seating depth.
Click to expand...
Book load range was 43.1-47.8c.
Skgreen said:
Chrono Battery on the way out?
Click to expand...
A possibility. The 9v battery is old. I might change that for next time. Thanks, I hadn't thought of that.

Ranger99 said:
Just me- when working up a load,
I'm going up in the increments looking
for accuracy first and foremost. Velocity
doesn't figure in at all. And I'll only
work on changing one variable at the
time. Unless you're just looking for
the highest velocity, I'd put the
chronograph aside until I achieved
the accuracy I was after. Just me though
Click to expand...
The .1 grain progression of loads was my second pass. Originally, I had used .6 grains to find an accuracy node. The .1 grain was to try to find out how wide that node was. I had already settled on 43.3 which is .2 grains above minimum and all of these rounds were the load I had settled on. I just wanted to confirm precision and zero while collecting chronograph data. Accuracy and precision were "good enough" for deer hunting here in East Tennessee. It's a solid enough of a load (even the low velocity fluke) out to 400 yards.

AJC1 said:
We're you .005 or .050 off the lands as the manufacturer recommends. Blc2 is very temperature sensitive, and ball powders are not a great choice for low density loadings.
Click to expand...
I'm sure you are right. It's .050" as recommended. I was trying to remember all of the numbers from memory since I'm at work and don't have my notes in front of me. Thank you for helping with the clarification.

Blue68f100 said:
Were you allowing the barrel to cool between shots? Did you have a round in the chamber cooking while waiting between shots?
Click to expand...
Not completely. When I was getting enough mirage to interfere with seeing the target clearly, I stopped and waited. It is a heavy contour for hunting...the heaviest "hunting" contour Krieger offers. The suppressor got much hotter than the barrel did. The barrel was never too hot to touch, but it did throw some mirage. I have a cover on the suppressor.

I did not have a round cooking in the chamber. I left the bolt open until I was ready to fire to aid in cooling.


Thanks for all of the replies. There's a lot to consider here.
 
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LocoGringo said:
Yes, brass was all trimmed to length and very consistent. No crimp was used.
Click to expand...
I use charts like this to try and resolve what powder to focus on. Using this and the load data provided by Barnes you see RL-15 is on both charts which is great. The part that I focus on is that the best accuracy is found at the start load for rl15 but still gives 80% case fill. I prefer 90-105 but this is just an example of how I do this. Big game appears to be the big winner on both pages. The good news is it's readily available. I don't prefer ball but that's just me. I won't buy leverevolution for 30-30 and I'll stick to ar-comp and 4064.
upload_2023-1-12_18-3-38.png
 
AJC1 said:
I use charts like this to try and resolve what powder to focus on. Using this and the load data provided by Barnes you see RL-15 is on both charts which is great. The part that I focus on is that the best accuracy is found at the start load for rl15 but still gives 80% case fill. I prefer 90-105 but this is just an example of how I do this. Big game appears to be the big winner on both pages. The good news is it's readily available. I don't prefer ball but that's just me. I won't buy leverevolution for 30-30 and I'll stick to ar-comp and 4064.
View attachment 1126915
Click to expand...
My thinking for choosing BL-C(2) was that it looked like it filled the case more than the alternatives I was looking at in my Hornady load manual among the powders I have on hand and it will be for a limited number of rounds until my son can handle a little more recoil and I can move up to a 140 grain bullet. This is a custom rifle (the serial number is his initials and birthdate) I spent 13 years in planning, buying and having built that I hope he will appreciate one day. I don't want him to hate shooting it. I have lots of Varget and H4895, 4 pounds of H414, 2 pounds of BL-C(2), 1 pound of IMR 4850 and 4064, and a small amount of RL15. I eliminated the powders I have less than 2 pounds of because 1 pound isn't enough to get through testing AND production of enough rounds for inventory and shooting without searching for/buying more powder.

Basically, I went with what I have that will best fill the case AND give adequate performance for deer hunting in East Tennessee. I have NO doubt there are better powders out there to accomplish what I'm trying, but this was my thinking with what I have on hand. Also, don't forget, I'm going with MINIMUM rather than MAX because I'm trying to minimize recoil.

What I'm trying to figure out in this post are possible causes for such wide velocity swings.
 
LocoGringo said:
A possibility. The 9v battery is old. I might change that for next time. Thanks, I hadn't thought of that.
Click to expand...

I use my chronograph a lot, and it's not uncommon to have errors because of poor light, side light, wind, whether or not I use the sun screens, a low battery... and sometimes it just doesn't see a particular bullet well. It's a little bit of voodoo, sometimes, to get it to work right.

If you are concerned about the variations, I would load 10 rounds up of your final load and run them across the screens... 5 rounds is barely enough to establish a data set for a load, although I understand you would be reluctant to do that with your entire ladder. When I find a possible final load for a firearm, I load 20 of them up as carefully as I can and run them all... checking for velocity and accuracy. That will tell me if the load is reasonable in a number of areas. If you are able to call your shots (or have a spotter) you can plot your string and the velocity data... an errant reading might show up as a flier, etc, proving the point that it actually was different, not necessarily a chrono anomaly.

I would also tend to agree with AJC... BL-C(2) would not be my first pick for an accuracy powder, I would likely try to find an IMR powder. BL-C(2) is a ball powder, typically requiring a Magnum primer (because of temperature sensitivity; ) I've not found ball powders to be the best for accuracy purposes... but that's just my personal experience. Proof is in the pudding, however... if you are getting the results you want with BL-C(2)... then that's the right choice. :)
 
LocoGringo said:
My thinking for choosing BL-C(2) was that it looked like it filled the case more than the alternatives I was looking at in my Hornady load manual among the powders I have on hand and it will be for a limited number of rounds until my son can handle a little more recoil and I can move up to a 140 grain bullet. This is a custom rifle (the serial number is his initials and birthdate) I spent 13 years in planning, buying and having built that I hope he will appreciate one day. I don't want him to hate shooting it. I have lots of Varget and H4895, 4 pounds of H414, 2 pounds of BL-C(2), 1 pound of IMR 4850 and 4064, and a small amount of RL15. I eliminated the powders I have less than 2 pounds of because 1 pound isn't enough to get through testing AND production of enough rounds for inventory and shooting without searching for/buying more powder.

Basically, I went with what I have that will best fill the case AND give adequate performance for deer hunting in East Tennessee. I have NO doubt there are better powders out there to accomplish what I'm trying, but this was my thinking with what I have on hand. Also, don't forget, I'm going with MINIMUM rather than MAX because I'm trying to minimize recoil.

What I'm trying to figure out in this post are possible causes for such wide velocity swings.
Click to expand...
I apologize I locked on 140 instead of 120. I was trying to explain my process for accomplishing what your goal is. In the senerio I provided the big game is reported to be the most accurate at the start charge. I fully appreciate your goal, but I missed the mark. I will redo my numbers if that helps with the 120.
40.6 grains of that imr 4895 gives 89 percent case fill would be temperature stable and should be decently accurate.
 
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Charlie98 said:
I use my chronograph a lot, and it's not uncommon to have errors because of poor light, side light, wind, whether or not I use the sun screens, a low battery... and sometimes it just doesn't see a particular bullet well. It's a little bit of voodoo, sometimes, to get it to work right.

If you are concerned about the variations, I would load 10 rounds up of your final load and run them across the screens... 5 rounds is barely enough to establish a data set for a load, although I understand you would be reluctant to do that with your entire ladder. When I find a possible final load for a firearm, I load 20 of them up as carefully as I can and run them all... checking for velocity and accuracy. That will tell me if the load is reasonable in a number of areas. If you are able to call your shots (or have a spotter) you can plot your string and the velocity data... an errant reading might show up as a flier, etc, proving the point that it actually was different, not necessarily a chrono anomaly.:)
Click to expand...
I tested the chronograph with my .22LR rifle beforehand to make sure lighting was sufficient to pick up the larger, faster bullet of the 7mm-08. I figured if the chronograph saw the "high velocity" minimag .22, it would see the 7mm-08. Weather conditions were cloudy, so the sunscreens probably weren't even necessary. I shot a total of 22 rounds of the final load over the chronograph (2 didn't get picked up by the chronograph) and the results were described in my post above.
 
Did not see it mentioned but I have had to black marker bullets before to get reliable chrono readings.
 
Is this the first time using this chrono and how has it performed in the past?
Whose chrono are you using and how far away from the muzzle/suppressor was it set up? I switched to a labradar but muzzle blast can affect a shoot through, although I’m not sure how the suppressor would affect it.
When I used a shoot through I always used the screens to get a more even lighting on the sensors. Cloudy days aren’t necessarily ideal, what you want is a very consistent light on the screens. A completely cloudless sky, with screens, and the sensors being in the shade worked best for me. An even-lit overcast sky works really well too. Light and dark clouds do not help. The best way to get consistent results is get the light kit and build a big box to shield everything.

While it could be the chrono, it could also be the load. Consistent single digit SDs take work. As to the question, assuming a normal distribution, I’ve heard 50-100 is a reasonable sample size for a reasonably confident result. There’s no way I’m going to use up that much resource so my puny 3-5 shot samples will have to do until I think I’ve found a node where I’ll load up 10 and see what those do and use that’s my minimum sample size.
 
LocoGringo said:
how many rounds do you chronograph to develop a reliable ballistics chart?
Click to expand...

Statistical confidence depends upon the sample set. A highly variable set requires more samples to find a statistically valid average than does a more consistent population. In general, a 10 shot string will be plenty for most single or low double digit SD sets - one quick litmus test is whether the Average is roughly centered in the Range/Extreme Spread and the ES is at least 4, but preferably closer to 6x the Standard Deviation.

Also, as has been noted, monometals won’t shoot worth a hoot from close to the lands, 50thou or longer, not 5thou.

Considering your challenges, if you exclude the possibility of fault with the Chronograph, leaving fault with the ammo, then ES’s over 100 are almost always primary ignition issues, and not terribly uncommon for ball powders.

Neck tension, primer power, and fill ratio would be my first stones to be turned, in this case.

Temperature sensitivity at 50-55F isn’t going to cause that kind of inconsistent ignition, not alone, at least. 50-55 just isn’t “cold.” That wouldn’t be the rabbit I would chase, not first, at least. BL-c(2) does like high fill ratio, and lower temps will exacerbate an already bad fill problem, but I’m not really chasing temperature as the issue here, personally.

I’d swap to a magnum primer if you haven’t already, and if you are, then I’d confirm neck tension. Given 3 thou neck tension, I’d start stacking in a little more powder. If you get to a recoil level you don’t like, then I’d add rifle weight and a muzzle brake, and then I’d simply change powders to something a little less finicky.
 
lordpaxman said:
Is this the first time using this chrono and how has it performed in the past?
Whose chrono are you using and how far away from the muzzle/suppressor was it set up? I switched to a labradar but muzzle blast can affect a shoot through, although I’m not sure how the suppressor would affect it.
When I used a shoot through I always used the screens to get a more even lighting on the sensors. Cloudy days aren’t necessarily ideal, what you want is a very consistent light on the screens. A completely cloudless sky, with screens, and the sensors being in the shade worked best for me. An even-lit overcast sky works really well too. Light and dark clouds do not help. The best way to get consistent results is get the light kit and build a big box to shield everything.

While it could be the chrono, it could also be the load. Consistent single digit SDs take work. As to the question, assuming a normal distribution, I’ve heard 50-100 is a reasonable sample size for a reasonably confident result. There’s no way I’m going to use up that much resource so my puny 3-5 shot samples will have to do until I think I’ve found a node where I’ll load up 10 and see what those do and use that’s my minimum sample size.
Click to expand...
This chrono has been solid and I've used it for 10+ years. It's not one of the more expensive ones, but it has given consistent results with both of my .308s, my 3 6.8 SPC loads and even my 9mm STI Marauder. It's a Pact Model 1. I usually set it up about 3-5 yards away from the muzzle. This time it was approximately 3 yards away. I've seen it where the muzzle blast shakes the screens and it still gives good readings. There was no shake from muzzle blast during this test although it did fail to pick up 2 shots.

These clouds were an even color that cast a consistent light. It was actually good lighting conditions.

I've just not seen such a wide dispersion in muzzle velocities like this before. I haven't actually tested a lot of different loads although I've been loading for a while, so in that regard, I guess I'm still a bit new to "chrono'ing". Come to think of it, this is the first ball powder load I've chrono'd. I always test for precision first and if I find a good precise load (in this case, .75 MOA), I chrono it to make a ballistics chart.

I think Varminterror is on the right track although I have no idea how to measure neck tension. I'm probably going to have to do a little more testing with a fresh battery and see what happens.
 
#1 save that H414 for when your son graduates to the 140s. My wife’s 7-08 loves it with all the 140s I’ve tried in hers; beat out RL17, RL19, IMR 4350 every time I’ve switched bullets.

#2 let your barrel cool til it’s like warm to the touch, not “not too hot to touch.” I don’t know if this will fix your SD, but it should improve accuracy. They sell battery powered fans to blow air through your barrel that significantly reduces the time it takes to cool.
 
LocoGringo said:
I have no idea how to measure neck tension.
Click to expand...

Neck tension = [(2 x neck wall thickness) + bullet diameter] - measured neck diameter with bullet seated

It’s just the difference of how much the brass is squeezing the bullet, vs. if they were stacked together without any pressure or interference. That difference should be something 2thou to 4thou.
 
Varminterror said:
Neck tension = [(2 x neck wall thickness) + bullet diameter] - measured neck diameter with bullet seated

It’s just the difference of how much the brass is squeezing the bullet, vs. if they were stacked together without any pressure or interference. That difference should be something 2thou to 4thou.
Click to expand...
Thank you...I knew there was a reason I tagged you.

Since there can be a lot of variation (I assume) in the thickness of the neck wall brass, do you do this for every round, or a small sample, or what?
 
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