Scale Stabilization

I have some 200 grain sample bullets to test. My first test is to see how consistent is the weight from one bullet to the next. I’ve previously learned this is a critical factor for accuracy – to have all bullets weigh almost the same. (I’d like to see +/- 0.5 grains but know other shooters who demand +/- 0.1 grains.)

To get started in the testing, I went out to the reloading room – it’s chilly today with a storm moving in – snapped on the scale, an RCBS Rangemaster 750, and weighed all 104 bullets in random order.

After typing in their values on the computer, I had Excel calculate the stats.

Hmmm. Not as good as I was hoping. I’d like the Minimum and Maximum (i.e., the “Range”) to be a lot closer.

I decided to plot the values to see if anything else would jump out from the data.

There are several light bullets, yes, but notice also that, over time, the weight trends downward.

How can that be? I’m drawing the bullets at random from a plastic tub. How can I be getting lighter and lighter bullets?

You may have already guessed, it’s not the bullets, it’s the scale. Over time, the scale is giving me a lighter and lighter readings.

Looking at the plot you can see that, in the beginning, the scale was reading an average weight of about 201.3 grains per bullet. But by the end of the session, that average had gone down a full grain, to about 200.3.

Thinking back on my test process, I did (at least) two things wrong.

1. I did not turn the scale on and leave it alone for 15-20 minutes before starting – I figured it’s all solid state so there’s nothing to “warm up”.
2. And I didn’t periodically check the scale for a reading of 0.0 when empty.

That “warm up” period is apparent in the above plot. From the beginning until about bullet #75, that’s where the downward trending takes place. But after #75, if you eyeball average the data, it’s relatively flat. The scale has stabilized.

And how long did it take me to weigh 104 bullets and write down the numbers? About 20 minutes.

Bullet #75 was 3/4s of the way through so 3/4s of 20 minutes is 15. The scale needed 15 minutes to stabilize. (Funny. That’s what the instructions recommend. Gee, what’d’ya know!)

So, I did a second weighing of those same bullets again in a random order. This time, however, I gave the scale 30 minutes to warm-up. And during the weighing, I checked to be sure the scale returned to 0.0 before each new bullet – that doubled the amount of time required but, for the sake of discovery, I felt it was worth the time. Also, I noted any point when I had to re-zero the scale.

Here are the calculated stats.

Hmmm, that’s not very different from the first run. The range is now 3.0 instead of 3.1 grains but that’s only 0.1 grains different. And the Standard Deviation is a little better, 0.5 instead of 0.6 but, again, that doesn’t seem like much.

Let’s look at the new plot.

But the light bullets are still there. They aren’t an error in the measurement process. They are real. And now that the data is more trustworthy, I feel confident in making a judgement call.

With my goal of +/- 0.5 grains, from this batch I would keep those weighing from 200.5 to 201.5 (yes, I know that’s actually +/- 0.55 grains but it’s easier to see the rejects in the chart that way), there are nine (9) underweight, and five (5) that are too heavy. That’s 14 rejects out of 104 bullets or slightly less than 14%.

For general competition, I probably would just load and shoot 100% and not bother with weighing each individual bullet. My second pass through the bullets took almost an hour and included re-zeroing the scale six times but the “not quite zero” amount was never more than 0.1 grains. I think I could weigh bullets faster but, even so, it is a time-consuming process.

For the Tuesday evening fun shoot and practice 2700s, I would just load and shoot everything.

But for 2700s where the scores are going to the NRA and, in particular, for big events where some major prizes are at stake then, yes, it would be worth the effort. Indeed, making that ammo I would weigh each bullet and keep only those that are +/- 0.5 grains. And I would use all new brass. And after loading, I would check each round in a chamber gauge and then finally in the barrel of the gun that’s going to fire them.