BTO .015 +-

[Link]

I just got a new run of 30 cal bullets and the BTO is .015 longer in this run than last years. Both runs are within .001 +- so that's good. How much difference is this going to make on the target at 300 yd ?

thanks

 

[Keith Glasscock]

Assuming your load is in the node, and not on the edge: If you can measure the difference, you are better than me.

 

[T-shooter]

If you are seating the bullets to jump a specific amount, then the .015" longer is restricting the volume of the case a bit. The smaller the case, the more pressure and velocity. Here is a chronograph test (average of only 5 shots each) in a .308 using Berger 200.20X bullets and 42.5 g of Varget powder.

Jump .030" 2,506 fps
.020" 2,518 fps
.015" 2,526 fps
.010" 2,530 fps
.005" 2,533 fps

I also did some velocity test on 5.56 with a 55g LeHigh bullet. I run my AR mags to accept a 2.300" round and did others for a friend at the standard 2.260" length.

28.4g of CFE223 gave an average of 3,045 fps at 2.260". With the longer 2.300" round with more case capacity, 28.4g only provided 3,007 fps. It took 28.8g of powder to get the velocity back and it averaged 3,050 fps.

 

[Zero333]

Whatever cartridge base to bullet ogive measurement you were using with the old lot do the same with the new one. You'll be fine and unlikely you will need to adjust anything unless this is for benchrest where every thousands of an inch in group size matters.

 

[mikecr]

Let's see, Base-to-Ogive,, That's base length + bearing + ogive radius.
Have you determined what is different from batch to batch?
Is the OAL different?
Has the bearing itself changed?
Have the ogives you're taking datum off been qualified yet?

You should know; it all begins with a bullet. So changing bullets could mean nothing all the way to starting over.