frnkeore
posted this
29 December 2012
this is kind of a odd question for me as it limits things to “factory” chambers and it's not asking what maybe best for accuracy.
the best factory throat is the 308 but it's lacking in that it has a 3.5 geg included leade and a huge neck dia.
as i see it the best way to attain accuracy, be it fixed or breech seated, is to start the well cast bullet off in the best alignment you can with the least distortion.
would anyone disagree with that theory?
we will start with the leade and go back to the shoulder.
the bullet hits the lands with great force and the bullet has to “take” the lands. the slower it does that, the less it will be able to distort the bullet when it impacts the bands and a longer leade (smaller angle) i.e. 1/2 deg vs 3 1/2 deg. included or slice instead of hack. i don't think 1/2 is nessasary but, i do believe 1 deg or less is. matching the bullet taper to the lead should be helpful, too for alignment.
next the freebore or ball seat (the military term for the 7.62x51. .310 isn't horrible but, you have to remember the bullet has to “seal” the entire bore to keep from gas cutting and the larger it is, the more sizing and distortion the bullet has to go through to get into the barrel proper so, .3085 - .309 would be better. or even PT&G .3082 palma freebore. here is were your sealing has to be done and you want to push the bullet into that area or have the next band touching it and be .0002 - .0005 larger.
the transition from the case mouth to the freebore, i like to be 30 deg. but, i don't think the angle is a big deal but, it does need to be smooth and deffect and burr free.
then we have the case neck, here we want it to be concentric and .0005 - .001 slip fit in the camber neck with the i. d. the same as the base band of the bullet or .0005 smaller than the base band.
add all these things together and you have a near prefectly aligned bullet (with in .0005 t.i.r. possible) w/o having to worry if the neck sizer goes off a little to one side. as most know, the fixed rule only means you have to hold the case upside down w/o the bullet falling free.
case capacity plays another roll but i won't go into that but, to say you should have over 65% loading density.
after that, you only have your load to worry about :)
frank
ps
for hunter and mil-surp, your stuck with what you have but, you can work in these directions with your cases and molds.
