Bullet lot BTO Variance Effect on Precision.

[jlow]

Would like to know what others ACTUAL experience has been with this. If you have done the OCW and seating depth optimization and the load is shooting well (under half MOA) and you run out of bullets and have to changed bullet lot. The new lot requires you to dial back significantly I.e. 8.5 thousands on the micro-adjust seater because the old setting is seating the bullet too deep. Do you find:

1) The new lot of bullets shoots just as well and you had no need to re-optimize.
2) Lost precision and had to redo seating depth optimization.

If optimization is lost with the new bullets, why do you think this happened? Is it because:

1) Distance of ogive to throat is less important than case volume which is also affected by seating depth?
2) BTO of a bullet is not always reflective of distance to throat?
3) Other dimensions of the bullet not measured by BTO is affecting load?
4) Other reasons?

 

[dkhunt14]

I see loss of precision and have to do seating depth change. It seems to vary with bullet type and whether you are in the lands or jumping. The thing I see is when the bullet gets shorter it gets fatter. I believe that might come into play also. I believe the most critical change is in the placement of the bullet to rifling. Matt

 

[jlow]

Thanks Matt, I am jumping and that’s what I appear to be seeing. Your theory makes good sense to me, I will re-run the seating depth optimization.

 

[jlow]

I guess the other obvious question I should ask is does my previous seating depth a good indication of where the new optimal depth is? For example, the original setting was 1.5 and that old setting seats the bullet deeper based on BTO measurements suggesting as you say the ne bullets are fatter. To get the same round round BTO measurement, I have to dial it back to 10. So now my question is which direction is the optimal seating depth? Deeper or shallower? Any experience there?

 

[barefooter56]

jlow,
BTO measurements between lots can vary due to wear (within tolerances before replacement of the die) on the die used to make the bullets. But the bullets in THAT LOT should be very close. Now that dosent mean that once the lot of bullets you have dialed the rifle in with is go that it will no longer shoot as accurate as before with another lot. It just means you have to adjust the bullet seating depth to get back to your seating depth sweet spot and maybe adjust the powder charge up or down a small bit to get back to your velocity sweet spot depending on if the new lot of bullets is longer or shorter in BTO than the original lot. Its the same for readjusting your seating depth for throat wear as the rifle gets some rounds on it. Nothing stays the same you have to keep good notes and adjust.

 

[jlow]

Barefooter56 – Agreed.

 

[jlow]

Hi Tom, no quite sure what you mean.

In my own case, this is a .308 with rounds are loaded to mag length (shorter actually), so no concern at all as it relates to possible over/under jamming.

 

[jlow]

Thanks Tom. So here is the dilemma. With the Hornady bullet comparator which is what I use, the actual ogive point use to determine length of BTO is likely different that the area of the ogive that the seater push the bullet down, and also likely different than where the bullet ogive would touch the lands since the three touch points are likely different in diameter. What are accurate with this type of measurements are their relative lengths. Are you saying the Bob Green tool is different in some ways.

Sorry but still not clear what you are saying.

 

[noload]

http://greensrifles.com/New_Products.html

Above link explains his tool.

 

[jlow]

http://greensrifles.com/New_Products.html

Above link explains his tool.

Thanks for the info. I guess if you know and have the specific throater for your factory rifle, that might be the trick but that is highly unlikely. My guess is this type of tool and strategy really works for custom barrels with a known specific reamer which does not apply here.

 

[jlow]

Now you have me lost... the Bob green tool just sorts ogive datum points, and it does reflect your touch point. I guess that's all I am saying. Now the Bob green comparator might work together better also, but I don't have one. I still use my hornady bodies, and a ss one my buddy made. I fire a lot of test groups anyway, so rechecking when changing sub lots is no big deal to me.

Tom

That's pretty much my plan. Just wondering if others do it (now confirmed), and knowing that the new bullet lots are fatter, wondering which direction and degree of distance I should start my testing.

My thinking now is just to try both directions by reloading at the range and increase group size when I find something promising.