6.5 PRC Powder Charge/Velocity Ladder Test. No Flat Spot

[gregmark24]

My buddy and I have been working together on load development for our rifles; his is a 6.5 PRC. In load testing I found a velocity flat spot with my 243, also with good POI, but he has not.

Twice now with once-fired Lapua, his powder charged different loads have tracked Gordons Reloading Tool almost exactly. No nodes. The first ladder was 0.010 off the lands with a IMR7828SSC. The second, ladder was with H4831SC, and 0.070 off the lands. He's using Fed 215M primers, producing ~50 ft/sec higher velocities than non-magnum primers (which GRT predicted), and 147 gr ELDM's. At test range of 100 yds, he produced the lowest vertical dispersions with loads 2, 5, and 7 (see chart below); and corresponding velocity SD's of 12.0, 8.9, and 4.6.

Questions:
With 0.6 gr between loads, why were there no discernible velocity flat spots, see graph - average velocities linear with powder charge increments?
He's got a carbon barrel--does the carbon damp out the harmonics/resonance of the barrel? We'd appreciate any advice you can lend.

Velocity Curve vs H4831SC w/ GM215M primers

Vertical dispersion - ignore Load 1 and load 10

 

[WeiserBucks]

Go shoot at long distance and decide from there.

 

[Justice1327]

How does GRT determine a change in velocity by switching primers? Is there an option to select various types of primers?

 

[Opa-lopa]

Velocity "flat spots" are a figment of small sample size; ignore them. Plot the data, compare w/ velocity vs charge weight from load manual. Is your data above or below the manual? If you're below, you probably either have a shorter barrel or worn throat. If you're above, it's either due to a longer barrel than the manual or your primer-powder interaction is driving the bus. Velocity doesn't come free, approach max load with caution. Load what shoots smallest groups consistently.

 

[jeb405]

In the past year I feel like load development has become as controversial as barrel break in. I most often find what you did, no velocity flat spot. Some look for smallest group, some best ES and for Satterlee method similar vertical dispersion (positive compensation). If it were me and loads 8-9 were safe from max I'd split the difference and if good groups then Bob's Your Uncle. Some would do a bullet depth seating, to me if groups do what you want no need. Easy to shoot out a barrel overthinking load development.

 

[badbull]

I'm no expert but my thoughts are that your flat nodes will show better with a .2 - .3 gr change in powder charge. Use the info gained above and perform another ladder to define the results.

 

[Culpeper]

Work up a load until you first start seeing minor pressure signs. Then back off .5 grains and find the best seating.

 

[jeb405]

I was going to comment as badbull did about size of increments. I am not familiar with 6.5 PRC but I know the increments I use are much larger for my RUM than my 308, I think of it in % of total load not in grains. Maybe that is not correct.

 

[BrentM]

It looks like you've not reached the higher nodes from what I can see. Generally there is a larger step at .5 grain increments in fps until you hit the upper node and max charge. I may have missed if you tested to max or not but that's my opinion. I can get the speeds you show in a 18" barrel

 

[lancetkenyon]

In my opinion, you don't need a velocity flat spot. That is super hard to tell with a single shot in each charge weight anyway.

You want a vertical dispersion flat spot. And testing should be done at 400-600+. Look for 3-4 in a row that have very little vertical. That is where your node is.

 

[P7M13]

I think how you are evaluating your data is messing you up. Who cares what GRT predicts?
You appear to have a node between loads 4 & 5. I'd suggest doing a workup between 50-52 with .3gr increments. Look only at the empirical results, and do not do a linear regression.

 

[DallasLR]

Charge weight, seating depth, and barrel tuning by ladder test with 3-5 shot groups cannot do anything but fool you with randomness. If you do a 5 shot ladder test and think you found a node. Repeat the test again. And you have a good chance of the node moving. Repeat the test 10 times and you have a 99% chance the node will move. This is a frustrating fact. But those three things do not change group size. The barrel of a gun moves before the bullet leaves. So weight of gun is the biggest factor in precision. Next are the components you use and how consistent they are. If your components were theoretically 100% consistent then given any Charge weight, seating depth, and barrel tuning for that given component set you would have the same precision as any other combination of the same components. Think about it. Why would there be a precision node for say powder charge? Or even a velocity node? It is counter intuitive and if you asked anyone who had never been infected with the idea what happens when you add powder they would tell you the right answer; the bullet goes faster. That's the only thing that changes and a ladder test of 3-5 will never statistically tell you otherwise.

 

[BrentM]

In my opinion, you don't need a velocity flat spot. That is super hard to tell with a single shot in each charge weight anyway.

You want a vertical dispersion flat spot. And testing should be done at 400-600+. Look for 3-4 in a row that have very little vertical. That is where your node is.

He's doing velocity ladder test, satterlee method. Different than a traditional ladder that tweaks a load for specific range like a traditional vertical dispersion ladder.

 

[BrentM]

My buddy and I have been working together on load development for our rifles; his is a 6.5 PRC. In load testing I found a velocity flat spot with my 243, also with good POI, but he has not.

Twice now with once-fired Lapua, his powder charged different loads have tracked Gordons Reloading Tool almost exactly. No nodes. The first ladder was 0.010 off the lands with a IMR7828SSC. The second, ladder was with H4831SC, and 0.070 off the lands. He's using Fed 215M primers, producing ~50 ft/sec higher velocities than non-magnum primers (which GRT predicted), and 147 gr ELDM's. At test range of 100 yds, he produced the lowest vertical dispersions with loads 2, 5, and 7 (see chart below); and corresponding velocity SD's of 12.0, 8.9, and 4.6.

Questions:
With 0.6 gr between loads, why were there no discernible velocity flat spots, see graph - average velocities linear with powder charge increments?
He's got a carbon barrel--does the carbon damp out the harmonics/resonance of the barrel? We'd appreciate any advice you can lend.

Greg, here is what I suggest. Take the book max and throw out the lower half of the charge weights. Start in the middle and work up in .5 grain increments to 1-2 grains over max. Be prepared to pull bullets. Shoot your velocity ladder until you get the first signs of pressure. That charge is now your max charge. Drop 1.5 grains and load in .2 grain increments 2 shots each. Shoot and record the velocity in the 2 shot increments and look for the node. Likely you'll see a spot in there about .5 grain wide with lower sd and flat node.