how much base to ogive variance is too much?

[panzer075]

I have been having ES issues with a new 338 NM. I started for the first time measuring Base to ogive BTO of the bullets prior to loading. I had 2boxes of 338 hybrids from the same lot left...and upon measuring I found BTO variances of the bullets to be .0125 from shortest to longest..and I now have 6 boxes of culled bullets.

2 questions..
How do you guys sort them? Groups of .002? Smaller, bigger?

And a can difference of .0125 make a difference in ES?

I am thinking this also explains the troubles I was having seating them to a consistent overall length by ogive in

 

[spclark]

Seating to a dimension measured BTO - in this instance that's base of cartridge head to bullet ogive diameter where your seating depth gauge contacts each bullet - then measuring loaded round OAL has two (or more) flaws: first, bullet lengths will vary base to meplat (pointy tip) and second, your seating die likely doesn't contact the bullet ogive at the same diameter as your seating depth gauge. Any differnce in length between those two diameters will affect your loaded round OAL adversly.

Theoretically if you measure seating depth only with the depth gauge even if bullets vary base-to-ogive they should seat consistently. Bullet bases will vary in depth however once seated which leads to the ES problem as pressures likely will vary some as well.

Do you anneal your case necks? Inconsistent neck tension will also affect how consistent you get bullets to seat in those cases. Will also affect ES.

Oh yeah, your question:

I'd segregate bullets to 0.005" BTO & see if different groups perform consistently within a given group. If they don't then try 0.003" or 0.002".

Testing lots of rounds in a 338 anything's gotta be a spendy endeavor. Hope you find a fix relatively quick....

 

[panzer075]

Spclark...

Yes I anneal every firing. Benchsource annealer. I weigh each charge to .02 grns. I just never bothered to sort bullets before..I am planning to try grouped to .002 variance and see.

I was trying to get some quick answers as yes its getting expensive..
Gotta order more bullets and I hope this lot of bergers was just a fluke.

 

[FeMan]

That is a big BTO spread. I can typically get my cartridges to within a 0.002" BTO with a good anneal. I anneal after every firing. The length does start to vary if I don't anneal.

I use competition seaters and measure each round's BTO after seating. If they are long, I dial in what I need to get them the same. I aim for rounds that are within a half thousandth.

In my experience, the BTO variability will affect group size and sometimes POI, but I wouldn't expect it to affect velocity in any meaningful way.

 

[panzer075]

I think there is some confusion. The BTO number I gave .0125 was actually for the bullets themselves...before loading I measured them. Not the loaded round measurement. The BTO of loaded RDS was just .001-.003 max.

 

[dkhunt14]

I have not seen a difference in .003 in a bullet cause a difference even at 1000 yards. The difference of your ogive to base isn,t caused by the bullet. Either the seating stem is hitting the point before ogive or it is inconsistency in the seating. I believe in that big of a bullet you could easily go .005 difference. Matt

 

[spclark]

The BTO number I gave .0125 was actually for the bullets themselves...before loading I measured them.

That's how I understood your statement: in a box of bullets you've found 0.0125" difference between the shortest and longest BTO dimension among bullets you've measured.

I'd have to believe as case volume increases over a range of cartridge sizes, changed in seating depth will have less effect on velocity spread across rounds loaded with slightly different base depths. From my experiences shooting 6HAGAR (600 yards) and 308WIN (for Palma) seating depth variation (though I strive for consistency anyway) has less effect (understand that I mean not over a too wide a range, no more than +/- 0.003" or about half what you're seeing with that lot of 338's) than changes in "jump".

Which Berger bullet shows the variance you're seeing? Not too many 338 shooters out there but maybe somebody here's using the same lot# & can offer their perspective.

 

[panzer075]

Lot# 6122
338 OTM hybrids 300 grn

I am hoping this helps the ES problem. I will have 7-8 shots with an ES of 4-6 fps then 1-2 shots that open it up to an ES 17-25 fps

 

[mikecr]

I've yet to see a test in which bearing variance alone(if that's even what you have) has actually affected anything.
And I seriously doubt you'll define affects, if any, with a 338.

You have consistent seating, so shoot a spread in BTO.
I'm predicting no difference in results.