Velocity swing in 7mm wsm with IMR 7828ssc

[Hiwayman92]

Just got my build done, and I'm working on building a load for the 175 ABLR. H1000 seems less accurate thus far, but more consistent velocity. I'm trying to wrap my head around why h1000 has a 50 fps velocity range, and 7828 is giving upwards of 100 fps, but is still grouping 1/2 minute at 100 yards without attempting any fine tuning with seating depth yet. I've deburred the flash holes, and properly prepped the brass. What can I do to get more consistent velocity? This is my first day with a new Caldwell chronograph, and by the end the old 9v battery was dying.

 

[elkaholic]

Just got my build done, and I'm working on building a load for the 175 ABLR. H1000 seems less accurate thus far, but more consistent velocity. I'm trying to wrap my head around why h1000 has a 50 fps velocity range, and 7828 is giving upwards of 100 fps, but is still grouping 1/2 minute at 100 yards without attempting any fine tuning with seating depth yet. I've deburred the flash holes, and properly prepped the brass. What can I do to get more consistent velocity? This is my first day with a new Caldwell chronograph, and by the end the old 9v battery was dying.

That is a lot of spread! I would be very suspicious of the Chrono readings. If you can get some RL26, it should work well with that combination.......Rich

 

[MagnumManiac]

Just got my build done, and I'm working on building a load for the 175 ABLR. H1000 seems less accurate thus far, but more consistent velocity. I'm trying to wrap my head around why h1000 has a 50 fps velocity range, and 7828 is giving upwards of 100 fps, but is still grouping 1/2 minute atb 100 yards without attempting any fine tuning with seating depth yet. I've deburred the flash holes, and properly prepped the brass. What can I do to get more consistent velocity? This is my first day with a new Caldwell chronograph, and by the end the old 9v battery was dying.

This is more common than you think, velocity variation has little to do with group size, it's barrel harmonics that dictate how tight a group is at 100yrds. Small ES/SD numbers do not influence group size under 500yrds. They influence vertical at longer ranges because bullets hit higher or lower on target, that's all.
Let me educate you, and others on this.
A barrel oscillates in little circles as a bullet travels down it, when it's spraying bullets all over the target, the bullets are exiting the barrel at different points in the oscillation. When groups are tight and consistent, the bullets are exiting the barrel at the same point in the oscillation. It is commonly believed that consistent barrel time, and therefore, velocity, must be the same. I do not agree with this. Before I owned a chronograph, I had several loads that shot exremely tight groups to 300yrds in 3 different cartridges, never thought anything was not quite right with them until I shot them at 800yrds and beyond. They nearly all had extreme vertical at thise distances, except one, that I still use today. When I got my chronograph, I checked all of those loads, and the ones that had vertial had quite wide velocity variance, one was above 150fps, but it still shot tiny little groups. How could barrel time differ so much, but groups be so small?
I still do not know the answer to this, but, there you go.

Cheers.
lightbulb

 

[LoneTraveler]

Are you using weighed powder loads? Are you using a proven lot of primers? incosistant primers, Too hot primers or cold primers for a load can make you sit and scratch your head. Are you still on the starting load. May have not reached the accuracy node in the rifle yet. Just some thoughts to check, Good Luck.

 

[Hiwayman92]

I'm using federal match 215 primers from the same box. I am hand trickling and weighing each charge individually. All I can think is either trying 210 primers or paying much closer attention to the scale since the powder is super short cut, and when I'm used to one piece of powder trickling in, it's trickling 3-4 to make the scale move. Is this a manual scale issue?

 

[Varmint Hunter]

I'm using 7828ssc with Fed 215M primers under the 160 Accubond bullet in my 7WSM and get single digit SD.

I wonder if the variance that you are experiencing is due to inconsistent neck tension.

The very first thing I'd verify is the accuracy of the chronograph readings. Shoot the same loads over a different chronograph.

I put a Chrony chronograph directly behind my Oehler 35P and the difference in the readings was considerable. I sent the Oehler back to the company for testing and they found it to be reading accurately. That left the readings from the Chrony chronograph very suspect.

Just a thought.

 

[Hiwayman92]

Neck tension could have something to do with it. This was my first time using my new redding dies, and I tried to set them up just like the instructions say, but maybe I'm in error? Almost all brass from this round were on their second firing, however a few were on their first. I didn't pay attention to which was which. Could that make a difference? They were initially full length sized, and then neck sized after they were fired. Should I give up on ssc or keep working with it?

 

[Varmint Hunter]

Personally, I found that 7828ssc gave the best accuracy, low SD and good velocity in my rifle so I wouldn't give up on it too quickly.

It is important to use brass that is as consistent as possible. That means starting with new fresh brass (no range pick-ups), prepping them to a reasonable degree, and to keep them sorted by the number of firings. Brass work hardens and mixing brass from different or unknown number of firings will lead to problems that may be hard to diagnose. Factory ammo that is from different lots can give you just as much trouble when the brass is used for reloading IMO.

You can save yourself a lot of time and money by purchasing a few hundred pieces of quality brass from a single lot (if possible) and working from there. Keep strict records of the number of times reloaded and when brass was trimmed.

Annealing is another issue but it is best handled by experienced reloaders. Brass that is properly sized and not loaded too hot can be used many times without annealing.

 

[Hiwayman92]

I think I figured out my problem. As I slowly hand trickle powder into the tray on my scale, it's not coming up accurately. Today I was careful to measure each charge, then tick the scale to settle the needle again. Some times it was right on, some times up to .3 grains heavy. I loaded h1000 today due to the lower Es I experienced yesterday, and got an Es of 43 with 65.5 grains, 20 with 66 grains, 12 with 66.5 grains, and 9 with 67 grains. Another interesting thing I found, is speed was only an average of 2 fps slower with 66 grains than 67. 66 was actually 11 fps faster than 66.5 grains. Weird... Also, this is my first long throated rifle. All the load data I've found shows saami lengths. I see no pressure signs yet, am I correct in assuming that when I'm seating out to 3.105 instead of 2.860 i will have less pressure and velocity with the same charge? If so I need to keep working up, since I'm only getting 2843 fps with a 180 Berger at 7,200 altitude.

 

[elkaholic]

I think I figured out my problem. As I slowly hand trickle powder into the tray on my scale, it's not coming up accurately. Today I was careful to measure each charge, then tick the scale to settle the needle again. Some times it was right on, some times up to .3 grains heavy. I loaded h1000 today due to the lower Es I experienced yesterday, and got an Es of 43 with 65.5 grains, 20 with 66 grains, 12 with 66.5 grains, and 9 with 67 grains. Another interesting thing I found, is speed was only an average of 2 fps slower with 66 grains than 67. 66 was actually 11 fps faster than 66.5 grains. Weird... Also, this is my first long throated rifle. All the load data I've found shows saami lengths. I see no pressure signs yet, am I correct in assuming that when I'm seating out to 3.105 instead of 2.860 i will have less pressure and velocity with the same charge? If so I need to keep working up, since I'm only getting 2843 fps with a 180 Berger at 7,200 altitude.

I think you may have a combination of things there. The 100+ spread you experienced would take a full 2 grains or more of powder to do that (all else equal). Sometimes if you have reached, or rather exceeded, max pressure, your velocity can actually decrease, but that would normally show up as excessive pressure on your case. Several other things have already been mentioned that could also contribute, and there are more. Just leaving a cartridge in a hot chamber for a while before shooting can cause considerable pressure (velocity) increase! This is especially true of powders with high temp sensitivity. It does appear that you have made some progress with the powder charge, but I am still a little suspicious of the chrono being erratic. That 100+ spread is rather telling to me.
As one poster accurately described earlier though, es does not always have a lot to do with accuracy at 100 yards. I have seen single digit es that would not group and 50' or more spread that would almost shoot one hole. Keep working on your loading including the power charge, neck tension, seating depth, different primer, etc. and see if your es stays consistent. If you continue getting readings as high as you did earlier, I would try another chrono. As far as your coal being much longer; if you are referring to the same charge in a rifle that has been throated, yes, the pressure will be lower. Normally though, seating closer to the lands will raise pressure a little, and some times jamming bullets can raise it considerably.
I am not suggesting this is your problem, but once you get things figured out, I would still highly recommend that you try some RL26.......Rich

 

[Jordan Smith]

This is more common than you think, velocity variation has little to do with group size, it's barrel harmonics that dictate how tight a group is at 100yrds. Small ES/SD numbers do not influence group size under 500yrds. They influence vertical at longer ranges because bullets hit higher or lower on target, that's all.
Let me educate you, and others on this.
A barrel oscillates in little circles as a bullet travels down it, when it's spraying bullets all over the target, the bullets are exiting the barrel at different points in the oscillation. When groups are tight and consistent, the bullets are exiting the barrel at the same point in the oscillation. It is commonly believed that consistent barrel time, and therefore, velocity, must be the same. I do not agree with this. Before I owned a chronograph, I had several loads that shot exremely tight groups to 300yrds in 3 different cartridges, never thought anything was not quite right with them until I shot them at 800yrds and beyond. They nearly all had extreme vertical at thise distances, except one, that I still use today. When I got my chronograph, I checked all of those loads, and the ones that had vertial had quite wide velocity variance, one was above 150fps, but it still shot tiny little groups. How could barrel time differ so much, but groups be so small?
I still do not know the answer to this, but, there you go.

Cheers.
lightbulb

The oscillation is actually more of an elliptical shape rather than little circles, with a horizontal component- more like a figure 8. There are two factors in this sine wave oscillation that lead to accuracy, and minimal vertical spread at distance- bullets exiting the muzzle at a node (small groups at short range), and bullets exiting the muzzle as the barrel exits a lower node or enters an upper node (minimizing vertical dispersion at distance).

As the barrel oscillates, it reaches as point as it's rising where it briefly stops before it starts coming back down again, which is known as a node. When the bullets are timed so that they exit the muzzle while the barrel is in one of those nodes, the muzzle's motion is minimal, so bullets exit the same way, or very close to the same way, every time. Whether a slower bullet exits the end of the node, or a faster bullet exits at the beginning of the node, they are leaving the muzzle with the barrel in about the same position, and will produce small groups even with large ES muzzle velocities.

Minimizing vertical spread is a similar concept, but depends on getting the bullets to exit the muzzle when the barrel is entering an upper node, or exiting a lower node. If you can picture the barrel rising before briefly stopping and coming back down, a bullet traveling down the barrel faster will leave the muzzle sooner while the barrel is pointing lower in its trajectory. A slower bullet will leave the barrel later, when the barrel is pointing higher as it reaches the top of its vertical motion and enters the upper node. So to recap, the faster bullet exits the muzzle when the barrel is pointing lower, and the slower bullet exits when the barrel is pointing higher. This means that the slower bullet is aimed a bit higher than the faster bullet, which neutralizes the difference in trajectory between the faster and slower bullets (the slower bullet is aimed a bit higher, so hits the same POI at long distance as the faster bullet). The same is true as the barrel exits the lower node. This is the theory behind the Audette ladder test. Obviously the lower your SD and ES numbers are, and the closer to the node that the bullets exit the muzzle, the more accurate your load will be at all ranges.

Hope that helps clarify the question raised at the end of your post.

 

[Jordan Smith]

I would focus on ensuring consistent neck tension, and making sure your chrono is giving reliable readings (which I'm skeptical of).

 

[Hiwayman92]

What can I do to ensure consistent neck tension? I'm using redding neck dies, a vld deburring tool, deburring the flash hole, and using match grade primers. What am i missing? Also, the chronograph must be working fiNE now, all of my h1000 loads were within 20 fps. It was either a chronograph issue, new and once fired brass issue, bad powder measurement, or a combination of all. I loaded some more 7828ssc rounds tonight, I will report back when I get a chance to shoot them. I need my oal Guage to show up so I can start messing with length... none of the h1000 loads are shooting much better than 1" so far.

 

[Jordan Smith]

Anneal your brass every few firings. I just noticed your post about your powder scale. That's also an obvious source of velocity variation.