88 TMK Loading Thread

[Nine Banger]

think of it this way.

freebore is additional case capacity. the bullet doesn't encounter meaningful resistance until it hits the lands..235" is nearly 2½x the .100 freebore in this comparison.
either you have a huge jump or you load the bullet out longer and get more useable case capacity, either way you can add powder to get to pressure.

so the +P is a modified land transition specifically to reduce the pressure spike from the freebore/bore transition. This allows more powder to be added to get back to the operating pressure which increases velocity by lengthening the pressure curve.

Weatherby is always referenced as being an early example of long freebore/high velocity.

My experience is freebore/jump/capacity will make a more dramatic difference in velocity than barrel dimensions in a comparison like this.

 

[Thegman]

think of it this way.

freebore is additional case capacity. the bullet doesn't encounter meaningful resistance until it hits the lands..235" is nearly 2½x the .100 freebore in this comparison.
either you have a huge jump or you load the bullet out longer and get more useable case capacity, either way you can add powder to get to pressure.

so the +P is a modified land transition specifically to reduce the pressure spike from the freebore/bore transition. This allows more powder to be added to get back to the operating pressure which increases velocity by lengthening the pressure curve.

Weatherby is always referenced as being an early example of long freebore/high velocity.

My experience is freebore/jump/capacity will make a more dramatic difference in velocity than barrel dimensions in a comparison like this.

True...but...his chamber is 2.50 + .100 = 2.60" with the 88 TMK and an AI to boot, which adds even more volume.

Mine is 2.340 + .235 = 2.575" with the 88 TMK and standard 5.56 chamber, so he definitely has more overall chamber volume in his chamber. I'm limited to OAL by the AR-15 platform (using a windowed magazine).

The idea of a different transition to the rifling is interesting...and who knows what else, I guess.

 

[longrangelead]

True...but...his chamber is 2.50 + .100 = 2.60" with the 88 TMK and an AI to boot, which adds even more volume.

Mine is 2.340 + .235 = 2.575" with the 88 TMK and standard 5.56 chamber, so he definitely has more overall chamber volume in his chamber. I'm limited to OAL by the AR-15 platform (using a windowed magazine).

The idea of a different transition to the rifling is interesting...and who knows what else, I guess.

it's possible other factors combined could add up to such a difference.
I know my 73ELDM/LVR load is reduced 100 fps from 40 degrees to -15.

I haven't tested temp variability above 40 because it doesn't seem to be a big enough variable to matter.

powder variation and slow/fast barrel could influence the gap, there is a lot going on there.