brickeyee claims this major difference between those two rounds:
Quote:
The major difference is in the length of the leade.
The rifling in 556 is further out that in 223.
That is the only significant difference.
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I can think of others, too, that would make any one rifle shoot a given ammo more accurate than another one. One with bullets too small for the bore will typically shoot less accurat than one with bullets larger than bore diameter.
The length of the lead varies only by its angle; it's the slope from bore diameter back up to groove diameter plus a bit for bullet clearance. And shallower angles deform bullets less as they're they're fired into them so they shoot more accurate than shorter leads. Doesn't matter much how long the throat is from case mouth to start of the leade as long as the ammo's pretty straight.