Powder Lot Change Effects

[mehavey]

Having figured out where the Model-70 wants bullet standoff to be using the 175SMK last week, I also picked up another (several pounds) of IMR4831.

Just for grins, I ran a Lot# Change series today (weighing the cases this time)to see if velocity differences would be noticeable. (The lots were only several months apart)

Last Week @74DegrF:
Lot#255 (my old Lot of IMR4831) gave me 2,530fps last week using 53.3gr

This Week @73DegrF
Lot#255 gave me 2,527fps
Lot#265 (New Lot) produced 2,457fps (with unsually good StdDev)


Quite a difference for those who operate at/near MAX loads.
Be forewarned (again) to step things down before you step them back up.

[amamnn]

There are a lot of variables that will cause results to be different from one day to the next--powder is but one--that being said--powder was the subject and so----------------Assuming we are talking hunting loads here since the model 70 is a famous hunter--and since we are seeing a post that at least by inference implies that the new lot was not up to the old lot's standards--one is tempted to ask if the deer or elk was not harvested due to the 72 fps difference in velocity at the muzzle-----but--more to the point--maybe

The way to keep these kinds of surprises (in powder lots) to a minimum is to check the powder density (this is not the load density) in every new jug of powder and compare it to the previous jug. If you have an electronic scale this can be an easy process, especially if you have Richard Lee's dippers which are also metric.

One (1) gram weight is the same as one (1) cubic centimeter (cc) of volume of pure water. Unless your water has a lot of lead in it this fact can be used to your advantage when determining powder density. In my opinion this is something every serious shooter should do. It is easy and can save some time and/or grief down the road.

Powders today are a lot more consistent than in the past, BUT, as far as I know, the powder maker's association has not changed their minds about what was the allowable tolerance when I first learned about it, which length of time is nunyobiddness, and so a wide discrepancy can and does exist. Suffice it to say that some makers still have a pretty loose tolerance, which can cause you some distress if you get a jug that is on the verge of the light side tolerance, then buy one on the heavy side.

So then, you will fill a dipper as per Richard Lee's recommended method and by doing that you have the amount of your fav powder that fits in say, 2.2 cc, which is a medium size dipper. You will then weigh this on your digital scale in the metric mode and find that it weighs say, 1.8 grams. You then divide 1.8 by 2.2 and find that the density of your powder is .818181818 or rounded off to 82% of the density of water. You write this number on the jug. If the next jug is more than 5% different than the first jug--you might not want to MIX them and you may want to re-test your fav load that uses that powder.

If you are a REAL accuracy nut the 5% will be a 5% variation of the tested density not 5% of 100% which as mentioned, is the standard, .i.e. density of water. So--what we look for is a variation of 5% or more in the weight of our first jug--which was 82% The new jug would have to test only 77.9 % to be out of tolerance on the low side. If we were not so picky then a jug that tested at 77% would be on the edge of our tolerance. Remember, I did say a REAL accuracy nut.............like me?--you bet!

There is no powder maker who will guarantee exact lot to lot uniformity, so even the match shooter, who is watching this density situation like a hawk, knows he still has to test lots--which is why some of these folks buy 100 or more pounds at a time. There are some makers who seem to hold their tolerances within the 5% but still you never can tell when something will happen- witness the rash of powder recalls not long ago.......

Since more people have become aware of this situation, some of the powder makers (but not all, by any means) have advertised that they hold their lot to lot variation to closer tolerances. Do not believe it--check for yourself. No one checks every pound of powder to go out of the factory and thereby mistakes can and do go undetected--there has to be some trade off between exactness and afordability. But, these days, anyone who has moved the handloading operation into at least the 20th century has the means to do the process I have described.

In the end the thing that really counts is where the bullet lands on the target and what the impact (notice I did not use "impact" as a verb) of the terminal performance of the bullet was on that target whether to kill or score on a target, that is the object of this endeavor. How fast it gets there is but a secondary consideration--within limits.

[zplinker]

One solution to varying lots, is to take everything you have of that powder, and mix it...make one big lot. Work your loads up, and you're set for a good while.

[mehavey]

Interesting..... So I went downstairs and ran the comparison test. (No need to convert to specific gravity, just ratio the relative densities).

Old Lot#255: 15.18gr/cc
New Lot#265: 14.82gr/cc

Load Density Ratio: New Lot/Old Lot = 0.976
Therefore Predicted Velocity of New Lot (relative to Old Lot) = 0.976 x Velocity of Old Lot

Predicted Velocity Lot# 265 = 2,466fps
Actual.... Velocity Lot# 265 = 2,457fps

Interesting Indeed.

BUT... This would seem to apply were I throwing volumes, not weights. Since I was weighing charges, the New Lot#265 would occupy more case volume for the same weight -- nominally increasing pressures (ergo increasing velocity) all other things being equal.

But all things are not equal, so what I guess I'll do is run this simple range velocity test when changing Lots, esablish the relative burn ratios, and go with that.