How Accurate Does a Powder Measure Have to Be?

S

Swifty Morgan

New member
I'm getting ready to crank out some 10mm after a long layoff. How accurate should a powder measure be before I quit trying to improve it? I can't remember.

Here are the last 5 weights I got today: 12.6. 12.5, 12.4, 12.5, 12.4.
 
Well, if you are getting no more than a .2 gr. variance out of five throws, I'd say you are ready to proceed. FWIW, the only recipe I hand measure with a beam scale if for my target .223; I do my best to stay dead on at 22.7 gr. with H322, and I get consistent results. Not saying I know, but I would have to wonder how much difference .2 gr. would make in a handgun round. I guess, as soon as this plague lifts, you'll get to find out. ;)
 
Thanks for the help.

It turned out leaving the powder measure in a humid workshop for several years was not a good move. Spent a fair amount of time cleaning it up yesterday.
 
To be real technical, what you are looking at is "how PRECISE" your measure is.

Accuracy is like a firearm that shoots a group evenly around the bullseye
Precise is like a firearm that shoots a tight group size.

Accurate and Precise is a tight group right on the bullseye.

So long as your scale is reasonably accurate
(A dime weighs 35 grains. .. 35.000589 grains)

We want PRECISION... each throw should be about the same.

Since you are working up a load from small to big carefully... and determining that your load is safe... so long as every powder throw is the same and you always use the same method of measuring, you'll be fine.

If you do something like clean the corrosion from a powder measure... I would worry that you've changed the accuracy and would check to be sure it's still throwing an accurate load, too.

The danger lies in a precise powder measure that consistently throws heavy loads and one is consistently shooting +P loads without knowing it.

That's where weighing a dime comes in. It's a poor man's way to check accuracy when you don't own a test weight.

Does anyone have a different test object that isn't a dime? Maybe something about half that weight?
 
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I don't think reloaders need a powder measure that throws exact and repeatable charge weights until ammo with a one tenth grain spread shoots no worse than 2/10ths MOA extreme spread at 100 yards.

Exact charge weights is about last on the list of the 6 or 7 requirements for best accuracy. Until then, a half percent spread in charge weights of the right powder can shoot under 3/10ths MOA at 100 when the other stuff is good.
 
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I wasn't shooting for any weight in particular. I was trying to get the measure working consistently. The actual charge I needed today was 10.0. I thought there was no point in chasing 10.0 until I had some idea whether the measure was working correctly.

I got it within +/- 0.1 grain, and I guess that will have to do.
 
It's Accurate No. 7 in a Hornady Lock-n-Load progressive. Seems to be working! I filled 77 old Starline cases.

This measure was made before they put baffles in them. I don't know if they're helpful or not.
 
Swifty Morgan said:
I wasn't shooting for any weight in particular. I was trying to get the measure working consistently. The actual charge I needed today was 10.0. I thought there was no point in chasing 10.0 until I had some idea whether the measure was working correctly.
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Maybe I'm off base but, IMHO, if you don't know what charge weight you're looking for, you can't assess the performance of the measure.

Tolerances can be expressed three ways: You always start with the preferred measurement, whether that's a dimension, a weight, or a volume. From there you can have the tolerance expressed as plus-or-minus (the most usual), plus-only, or minus-only.

If your goal is 12.5 grains and you get weights ranging from 12.4 to 12.6, that's a tolerance of plus-or-minus 0.1 grains. Not too bad.

But if your goal is 12.4 grains and you consistently get loads from 12.4 to 12.6, the error is always on the high side, by as much as .2 grains. If you're sneaking up on a maximum load, consistently throwing more than what you want is not a good thing.

Refer to stinkeypete's post above on the distinction between "accuracy" and "precision." If you have a recipe for a maximum load of 12.5 grains and the measure consistently throws 12.6 -- well, that's very precise (repeatable), but it's also consistently over the maximum load, so that's not very helpful.
 
I was trying to get consistency, and I used the word "accuracy" to describe it. If it's consistent at 12 grains, it should be consistent at 10, so I worked to get consistency before trying to get the exact weight I wanted.
 
In general they are as accurate as the operator and powder used. The question should be "can you get it accurate enough for your purposes ?" Grab a scale and find out, you are the only one that has the answer
 
With a powder such as Accurate #7, you should be able to keep the charges easily within the +/- 0.1 grain variation.

As Bart B has already mentioned, don't worry about it a whole lot.
 
The problem, as I see it, is the zero weight of the balance. My RCBS digital will zero at -121 grains (weight of the pan) but occasionally drift to -122. This is at a time when I am trying to weigh 140.0gr of H4350. At at a zero of -121 I get 40.0gr If the zero increases to 122 between weighed charges what would the real charge weight be? I'm thinking it is actually 39.0gr and I should trickle another 1.0gr. But if we're talking abut +/- 1.0 grain spread, no big deal?
 
Doesn't matter.
As long as the screen reads "Zero" when the powder goes in, you're getting an absolute weight differential.

(BTW, my RCBS Chargemaster's negative reading may drift a tenth when I pull the pan off, but never a whole grain.)
 
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