1/100th Grain Accuracy Scale?

[MCMXI]

Say I am loading 3.1 grains of powder and I want to know if my measure is throwing closer to 3.0 or 3.2, I just throw ten charges, weigh them all and divide by 10 and you’ll get “3.1x”.

I don't think this is a good method to determine the "accuracy" of the charges thrown. Consider the following example. Let's say you throw the following ten charges (in any order).

3.0, 3.0, 3.0, 3.0, 3.1, 3.1, 3.2, 3.2, 3.2, 3.2

Total weight is 31.0 so divide by 10 equals 3.10 but the standard deviation is 0.094 (close to 0.1) and only two of the charges thrown were the desired 3.1.

That said, there are so many other variables in reloading and shooting that completely negate +/- 0.1 grains of powder. If you're loading 4.0 grains then 0.1 grain = 2.5% of the total charge. If you're loading 25.0 grains then 0.1 grain = 0.4% of the total charge. If you're loading 75.0 grains then 0.1 grain = 0.1% of the total charge. So 0.1 grains has the biggest impact on small pistol loads which are intended for short barreled, sub 50 yard type firearms where 1/2" groups aren't the norm. For rifle loads such as .223 and up you're looking at less than 0.5% change in the total charge. If you think that +/- 0.1 grains makes a "significant, measurable" difference to your POI, let alone +/- 0.01 grains then you should have been in Beijing this week winning gold medals in every shooting event for the US.

 

[Matt304]

The effects may be minimal. But, if the goal is maximum accuracy, I think precise powder measuring is one step in the right direction. Any measurable gain which can improve your gun's accuracy can improve your group. The target is an addition of your group, and the gun's group. If you have a gun which groups 4", but you can hold to 0.25", then you will likely have a 4.25" grouping gun.

 

[MCMXI]

I think precise powder measuring is one step in the right direction. Any measurable gain which can improve your gun's accuracy can improve your group.

Matt,
I consider myself a careful reloader. I sort cases and bullets by weight, I trim cases to the correct length, I debur the flash hole and case neck inside and out, I weigh powder carefully and dump charges that are +/- 0.1 grains back in the hopper, I seat bullets 0.01 to 0.02 inches off the lands with competition dies, I use top quality primers, bullets and powder, I use a dial indicator to measure/adjust the headspace to within 0.001 inches, I neck size the cases only reducing the neck by 0.002 inches and follow the OCW method to "tune" the load and end up with ammunition superior to anything commercially available but I still realize that other variables have a far greater effect on accuracy. Some of those variables come from me and others from the environment. A 10mph crosswind can move a .308 168gr bullet more than 25 inches at 500 yards, how much difference would 0.1 grains of powder make at that distance with no wind? How much difference would 0.05 grains of powder make at that distance?

 

[MCMXI]

Out of interest, if you take the example of a .308 Win load consisting of a 168 grain SMK HPBT with IMR 4895 powder, the recommended range is 41.0 to 45.4 grains giving 2447 and 2758 ft/sec respectively. This is rather simplistic, but if you do a least squares fit, the representative equation is y=70.682x - 450.95 where y is velocity (the dependent variable) and x is grains of powder. Using this equation, an increase of 0.01 grains of powder to 41.01 grains results in a 0.7 ft/sec increase in velocity. 0.05 grains (45.05) gives a 3.5 ft/sec increase and 0.1 grains (45.1) gives a 7.0 ft/sec increase in velocity. When you consider that the bullet is already moving at 2447 ft/sec to start with, these increases seem insignificant. In fact, 7.0 ft/sec (from 0.1 grains increase) is about 4mph which is a fast walking pace!!

 

[jmorris]

I don't think this is a good method to determine the "accuracy" of the charges thrown. Consider the following example. Let's say you throw the following ten charges (in any order).

3.0, 3.0, 3.0, 3.0, 3.1, 3.1, 3.2, 3.2, 3.2, 3.2

Total weight is 31.0 so divide by 10 equals 3.10 but the standard deviation is 0.094 (close to 0.1) and only two of the charges thrown were the desired 3.1.

You are correct that it is an average. You are incorrect in you assumption of such a large standard deviation. If you throw 10 charges of 3.1 then weigh them all together you will then find what the average weight to the hundredth of a grain.

 

[MCMXI]

You are correct that it is an average. You are incorrect in you assumption of such a large standard deviation. If you throw 10 charges of 3.1 then weigh them all together you will then find what the average weight to the hundredth of a grain.

Sorry about that ... the standard deviation for the set is 0.09 and not 0.094 ... but that's still close to 0.1 grains. Personally, I wouldn't be comfortable summing the weights of 10 charges to find the average since as the data set above shows, the average on its own is virtually meaningless and can be misleading. The standard deviation tells the real story. The average of 20.0, 30.0 and 40.0 is 30.0 but the SD is 8.16 so again, the average doesn't tell the whole story.

Anyway, my apologies for going on about this. We're all free to do what we want and we all have different ideas about how things should or shouldn't be.

 

[Walkalong]

A 10mph crosswind can move a .308 168gr bullet more than 25 inches at 500 yards, how much difference would 0.1 grains of powder make at that distance with no wind? How much difference would 0.05 grains of powder make at that distance?

We have a winner!

 

[MCMXI]

If you throw 10 charges of 3.1 then weigh them all together you will then find what the average weight to the hundredth of a grain.

From http://en.wikipedia.org/wiki/False_precision

"False precision occurs when numerical data are presented in a manner that implies better precision than is actually the case; since precision is a limit to accuracy, this often leads to overconfidence in the accuracy as well.

In science and engineering, convention dictates that unless a margin of error is explicitly stated, the number of significant figures used in the presentation of data should be limited by the precision of those data. For example, if one instrument can read to tenths of a unit of measurement, calculations related to data obtained from that instrument can only be confidently stated to the tenths place, regardless of what the raw calculation returns or even if other data used in the calculation can be obtained more precisely. Even outside these disciplines, there is a tendency to assume that all the non-zero digits of a number are meaningful; thus, providing excessive figures may lead the viewer to expect better precision than actually exists."

I hope someone finds this useful.

 

[jwr747]

get on the internet and find info on the Army Marksmanship folks.they take ammo loading to the point of being fanatical. there may even be some dvd's out there that take you thru their process. jwr

 

[Matt304]

I wasn't ever actually trying to measure 1/100ths of a grain, guys. I simply wanted a scale more accurate than +/- 0.1 grain. That equates to a possible range of 0.2 grains. 1/100th was the next logical step in my mind for a scale.

If plus 0.1 grain gives + 7FPS, and under 0.1 grain gives - 7FPS, thats a spread of 14FPS with a standard scale.

OK, now let's apply that to a bullet trajectory.

.277 diameter, 150 grain, 0.5 BC, 2800FPS vs 2814FPS.

At 400 yards, that is a 0.4" trajectory variance.

At 600 yards, that is a 1.0" trajectory variance.

At 1000 yards, that is a 4.1" trajectory variance.

I don't know about others, but throwing 0.4MOA out the window at 1000 yards doesn't sound very good to me.

 

[MCMXI]

I simply wanted a scale more accurate than +/- 0.1 grain. That equates to a possible range of 0.2 grains.

Matt, a scale that measures to the nearest 0.1 grains has an "error" range of 0.099 grains since for example, a weight reported as 30.0 grains could be anywhere between 29.950 and 30.049 grains. I haven't seen your calculations but that error range alone would reduce your estimated 4.1" trajectory variance to 2.05". In addition, we can assume as many hits above the intended POI as below so the error is now +/- 1.025" or +/- 0.1MOA @ 1000 yards. The WORLD RECORD for benchrest at 1000 yards is 1.403", so for someone going for the benchrest world record using the kind of specialized equipment that is required, a scale capable of measuring to 0.01 grains might be a worthwhile investment. It would be interesting to ask Tom Sarver what kind of scale he uses.

"Tom was shooting a big 30 wildcat he calls the “300 Hulk”, a shortened 338 Lapua Magnum necked down to 30 caliber, pushing 240gr Sierra MatchKings (0.711 BC) at close to 3000 fps. Tom is using about 85 grains of H1000. He anneals the Lapua brass after every firing, and the brass that set the record was on its 58th firing!"

Personally, if I can shoot 0.5 MOA (5.25" groups) @ 1000 yards with what I've got then I'm a very, very happy camper!!

 

[Matt304]

1858,

Ah, I see now. I didn't realize that was how the scale tolerance worked out.

I'm not aiming for any world records myself, but I just thought to myself about spending the money once on a good digital scale, and not needing another down the road sometime. So something with a very good accuracy potential sounded appealing to me.

The guy who set the 1000 yard record must have been using a very long barrel. Either that or it's that the 300RUM for which data I am looking at is simply inefficient. Hodgdon claims only 2,863FPS from a 220 grain over 89 grains of H1000!

But who cares about that; a 1.4" 1000 yard group is something to talk about.

 

[DILLONHELP]

A part of the equation that so far has been ignored is the VOLUME variation from one pice of brass to the next. If you measure the volume of twenty pieces of brass from the same manufacturer and lot, you will find the internal volume varies by much more than .1 or .2 grains. Consistancy in the powder volume to pressure vessel (case)volume is what gives the least variation in velocity. A consistant powder charge weight alone is insufficient.

 

[MCMXI]

A part of the equation that so far has been ignored is the VOLUME variation from one pice of brass to the next.

Not really ... that's why many here (me included) sort cases by weight after trimming and neck sizing. Once again, I sort them using a scale that reports the weight to 0.1 grains (max. weight is 1500 grains) which is good enough for me.

I'm not aiming for any world records myself, but I just thought to myself about spending the money once on a good digital scale, and not needing another down the road sometime.

Matt, I can appreciate that thought process ... and if my RCBS ChargeMaster Combo scale suddenly grows another decimal place I won't be sending it back!!

We have a Mettler balance where I work that is accurate to 0.0001 grams (0.1mg) which is 0.0015 grains but at a cost of over $8500 I won't be taking it home for the weekend. I could do some reloading at work though ... hmmmm.

Mettler XP504 balance (photo)

Mettler XP504 Overview