IMI 5.56 Cartridge Case Weight Distribution

awshoot · Apr 19, 2020

Now I wouldn't normally do this for 5.56 because I don't have a precision 5.56 rifle and what I have I use for plinking. But I've got time on my hands and so after cleaning up some once fired IMI brass, I weighed each piece against a reference cartridge on a digital scale. I cut the primer pocket crimp with a cutter -- I don't have a swager so some of the variation could have come from slight differences in how well I reamed out the primer pocket crimp. I trimmed the shells and then deburred and chamfered. I randomly selected a shell and put that on my digital scale and tared that. I used that as a reference for all other shells and luckily, I just happened to choose the most numerous weight. My scale weighs to the hundreth of a grain but only in 2/100th increments. For my sorting, I used the weight displayed to the tenths and dropped the hundreths amount.

The results (weight difference in grains | count):

ageingstudent · Apr 19, 2020

awshoot said:
Now I wouldn't normally do this for 5.56 because I don't have a precision 5.56 rifle and what I have I use for plinking. But I've got time on my hands and so after cleaning up some once fired IMI brass, I weighed each piece against a reference cartridge on a digital scale. I cut the primer pocket crimp with a cutter -- I don't have a swager so some of the variation could have come from slight differences in how well I reamed out the primer pocket crimp. I trimmed the shells and then deburred and chamfered. I randomly selected a shell and put that on my digital scale and tared that. I used that as a reference for all other shells and luckily, I just happened to choose the most numerous weight. My scale weighs to the hundreth of a grain but only in 2/100th increments. For my sorting, I used the weight displayed to the tenths and dropped the hundreths amount.

The results (weight difference in grains | count):

View attachment 686269 View attachment 686270

So what do you think from your testing? My experience with IMI 5.56 brass is good. I have no complaints. I think it's at least as good as LC. If you didn't pick a random case and just did an average of all prepped cases I'm going to bet your spike would be closer to the middle of all cases.

awshoot · Apr 19, 2020

I too like my IMI brass and have no issues with it though as a caveat I would say I load to plinker spec (I drop powder from a powder measure which testing for that powder, gives -0.1 to + 0.3 grains of charge) and shoot it from an el cheapo rifle I got for fun. So this was mostly just a curiosity thing.

As for the spike, I don't think picking a different case would have changed the spike in its relative position -- for example if I chose one of the cases that currently measure at -1.0 grains and called it zero, I'd have a spike at +1.0 grains.

My thoughts are several actually. First, It seems odd to me that I did not have any case that was either -0.1 or +0.1 grains. That result is just weird and I wonder if there is some quirk to my scale that caused that. Secondly, the ones that did measure "zero" in the majority displayed as "0.00" -- which seems odd too. Once I got away from zero, the results in any particular tranche seemed fairly distributed, for example, I'd see negative 0.60, 0.62, 0.64, 0.66, and 0.68 quite a bit. So in the end, I'm questioning my scale.

The other thing I wondered was about the manufacturing process -- presuming it was not my primer pocket trimming that caused variation (a bad presumption I think) -- what would have to be true for most of the cases to weigh X, and for most of the ones that varied from that weight, to weigh less than X rather than be somewhat evenly distributed between more and less? It almost seems like IMI is aiming for a maximum weight of the case and will accept variation below that to some degree, but not above it. I wish I knew more how cases were manufactured and the machines used to make them.

Anyway, that's what I've been thinking about. I did take the zero cases and segregated those, then put the cases from -0.1 to -1.0 grain cases in a separate bag (and put the heavies and the light ones in separate bags too). At some point, I'll load up some tests and compare groupings for the lights, the slightly underweights, the zeros and the heavies. But before I get there, I have to build up an entirely new load because the bullets I had been using (*) became scarce or my google-fu went bad (even before COVID).

(*) Hornady 75 gr BTHP with cannelure.

EDIT: LOL -- NOW when I google them I find them!
EDIT1: never mind, out of stock. ;-(

ageingstudent · Apr 19, 2020

awshoot said:
I too like my IMI brass and have no issues with it though as a caveat I would say I load to plinker spec (I drop powder from a powder measure which testing for that powder, gives -0.1 to + 0.3 grains of charge) and shoot it from an el cheapo rifle I got for fun. So this was mostly just a curiosity thing.

As for the spike, I don't think picking a different case would have changed the spike in its relative position -- for example if I chose one of the cases that currently measure at -1.0 grains and called it zero, I'd have a spike at +1.0 grains.

My thoughts are several actually. First, It seems odd to me that I did not have any case that was either -0.1 or +0.1 grains. That result is just weird and I wonder if there is some quirk to my scale that caused that. Secondly, the ones that did measure "zero" in the majority displayed as "0.00" -- which seems odd too. Once I got away from zero, the results in any particular tranche seemed fairly distributed, for example, I'd see negative 0.60, 0.62, 0.64, 0.66, and 0.68 quite a bit. So in the end, I'm questioning my scale.

The other thing I wondered was about the manufacturing process -- presuming it was not my primer pocket trimming that caused variation (a bad presumption I think) -- what would have to be true for most of the cases to weigh X, and for most of the ones that varied from that weight, to weigh less than X rather than be somewhat evenly distributed between more and less? It almost seems like IMI is aiming for a maximum weight of the case and will accept variation below that to some degree, but not above it. I wish I knew more how cases were manufactured and the machines used to make them.

Anyway, that's what I've been thinking about. I did take the zero cases and segregated those, then put the cases from -0.1 to -1.0 grain cases in a separate bag (and put the heavies and the light ones in separate bags too). At some point, I'll load up some tests and compare groupings for the lights, the slightly underweights, the zeros and the heavies. But before I get there, I have to build up an entirely new load because the bullets I had been using (*) became scarce or my google-fu went bad (even before COVID).

(*) Hornady 75 gr BTHP with cannelure.

EDIT: LOL -- NOW when I google them I find them!
EDIT1: never mind, out of stock. ;-(

If you want to check your scale get another scale or even better two and check them all against each other. I have a PIA lee safety scale and two digital scales and they all agree within .01 grain. If I need to be closer than that I quit this s

.

DizzyJ · Apr 20, 2020

That's great! I tend to weigh all of the brass and segregate if it's something I'm looking to get extreme long range accuracy from.

One of my best groups in .223 came from weight sorting LC brass and Nosler 77gr CC.

.103" center to center, and held 1/2 MOA out to 600 yds furthest I tested it. Did weight sorting cause this accuracy? Don't know. I do know it didn't hurt.