Shoulder growth in service rifle rounds?

[mallc]

Guy's,

I'm not sure I'm using the proper terminology here so bear with me please...

I'm working on loads for M1 Garand. 45 gr IMR 4895 with 168 gr SMK.

I trimmed and sized once fired cases of the same headstamp in a Dillion 1200 size/trim unit. The cases fit the case guage perfectly and I established a zero point using a Redding dial comparator. Then I fired the rounds and remeasured the cases. The case shoulders had blown forward by 0.003" to 0.007"

1) Is this a normal growth range?

2) Why did they not all end up with the same growth?

3) If I'm setting the bullet ogive at 0.010" off the rifling, how much does the forward growth affect accuracy?

Thanks for your assistance,
Scott

 

[cdrt]

I get the same type of stretching with my M1 in .308. Not sure if it's endemic with the M1 overall. Maybe someone out there has the answer.

I using 40.5 grains of IMR 4895 with the 168 grain Sierra match bullet in LC match cases. After resizing, I still need to trim back quite a bit to get within specs.