How much variance should I accept in my brass?

[JustC]

I am trying to decide how much variance in weight I should accept as a "LOT" for my Norma 280 brass. My chamber is tight,..but not necessarily a tight neck that requires turning to fit. So,..if have from 180.5gr to 182.6gr as a spread (with the high and low numbers only being a few cases) how should I seperate them? Do I use the high and low weighted cases as fouler rounds,...and just have one "lot",..or should I split down the middle and use all of each of the resulting two "lots" as match loads.

 

[jcpython357]

JustC, The weights you have are all in range, If you really want to get picky you could cut it down the middle like you said and have two separate lots so to speak, but IMO it's not going to make a difference. Take two shells from the heaviest and two from the lightest and load them up, assuming you have a load of known accuracy. I got my money on they all group together. Is this for match shooting or for hunting? What range will you be shooting? Over 500yds, then I'd have them within a grain of each other. like you said, Two separate lots. Jay

 

[BountyHunter]

JustC

Weigh them out and put in rows behind each other. You should find a normal bell curve. Most people are sorting to 1% weight variance and if you want to tighten it go to .5%. Chances are you will find two major groups with a few odd pieces on the bottom ends of curves to be used for other purposes (adjust neck turners, sample rds etc)

BH

BH

 

[JustC]

Thanks guys.

Jay,..this is a 28" barreled 280AI on a 700 action. I will use it for crop damage at ranges to 500-600yds,...but would like to use it in some informal 600yd benchrest matches for practice/learning in preperation for trying my hand at the 1000yd game. It is not "specifically" built for benchrest,.i.e. no benchrest stock,..but it does group very tightly.

BH,..my curve seems to be all within the 181-182gr range with most being mid range. I could probably get a fairly large lot with maybe 15-20cases pulled out of the curve,..but with this brass I was going to try to get as much of ti as I could to work with and only have the high and low outlier cases to foul with or test wind.

So,..that being said,..you guys think I should split them down the middle?

Thanks for the help so far.

 

[Brent]

Like Jerry said, volume is what matters, not the weight of the brass. The wt of the brass is only a "loose" indicator, at best, of what the internal capacity might be.

I suggest you weigh a bunch of cases first, then again weigh with water and subtract each cases weight from it's total weight to see the internal capacity in water weight. I use an eye dropper to fill them to a "consistant" level to the top.

Wright the spread of the case weights down and the total spread, you'll likely see a big variation in the case wt, but "not" on the internal capacity water weight.

Good luck.

 

[JustC]

So then you guys would suggest fireforming all of the 100 cases and then measuring internal volume with the expended primers still in place,.correct?

Then cull the outlier cases with volumes that are < or > 1% of the mean? Or just stay within 1gr of powder capacity across the lot?


Thanks

 

[Brent]

You can do all 100 rounds, it should take an hour or so is all. I doubt you'll be even close to or over a 1% spread in internal capacity though, probably closer to .5% if that.

My 300 Ultra brass from 4 different lot #'s were +-.2gr is all. Later I weighed some from the last 100rd lot and it fell in line with the first three lots also. I still haven't checked my 308win Lapua brass or the 243win Lapua brass, just haven't had time yet.

Let us know what you find too.

 

[jcpython357]

JustC, I myself wouldn't even bother ****in' around with the water thing, the cases are close enough and IMO will shoot just fine, and I don't think your going to find a variance more than .5% anyways, If it would make you feel better, do it. Norma brass is good enough to not worry about stuff like that. Jay

 

[Savage99]

Jerry Teo makes a point that I had not thought of but having read it on the net a month or so ago I agree with his point.

One variation that I think that does matter is neck wall thickness variations. Sorting this way and hopefully not having to turn may be more effective. Such variation must have a major effect on neck tension and also runout.

 

[JustC]

Savage99 I am in the process of buying a tubing mic for the exact measurement of neck wall thickness.

Brent,.I will let you guys know what I come up with as these cases are fireformed for the ackley shoulder,..and while the primeres are still keeping them water-tight.

Thanks for your replies

 

[Mysticplayer]

JustC, the weight of the cases are close enough. What you might want to consider is fireformed volume. This matters a lot more then difference in extractor groove dimension.

I measure my brass by filling one with a fine powder. Then dump into other cases. What I have found is that even though there are slight difference in weight, the volumes are essentially the same.

Case volume determines pressure or variations in pressure. This affects vel and spreads.

I also shoot the brass for group. Any brass that shows up as a flyer gets marked and reshot. If it remains a flyer, it gets pitched.

I have found that with the above culling techniques, I have gotten all types of brass to shoot very well. Of course, all are sized properly and bullets seated straight.

To me, brass weight does not matter...case volume does.

Jerry