VB, the "formula" (really just a barrel-makers' rule of thumb) is that the rifling button or cutter leaves 25% of the bore circumference standing proud as so many lands while it swages or cuts the other 75% into so many grooves. This means that the fewer grooves there are, the wider the lands are.
<br>
<br>In a bore as small as a .224 or a .257, the lands in a six-groove barrel wind up being pretty dang narrow. In any bore, the fewer the lands and grooves, the fewer are
<br>(a) the sharp land edges that score the bullet jacket
<br>and
<br>(b) the tiny "gutters" at the bottom of each groove for the toughest fouling to collect.
<br>
<br>The less the bullet jacket gets scored, the less likely that bullet is to come apart prematurely. The less tough fouling there is, the easier it is to clean that barrel.
<br>
<br>Figuring roughly, the land widths in a .25 barrel would total about 0.2 inch. Divide that by six, and you get six lands about 0.03 inch wide.
<br>
<br>In a five-groove .25 barrel, each land would be about 0.04 inch wide.
<br>
<br>In a four-groove .25 barrel, each land would be about 0.05 inch wide.
<br>
<br>In a three-groove .25 barrel, each land would be about 0.067 inch wide -- just a tad more than the thickness (0.060 inch) of a 1982 Denver penny here on my desk. That's still not wide enough to be terribly rugged, it seems to me.
<br>
<br>War production called for some short-cuts, one of which was two-groove .30 barrels. They surprised a lot of folks with how accurate they were.
<br>
<br>My new .220 has a three-groove bench-rest barrel with a nine-inch twist. If I were building a .25-.284 (perish the thought!), I'd order a three-groove barrel.
<br>
<br>Ask your barrel-maker for the exact width of each land in his .25 barrels -- for each number of lands. If he doesn't know, doesn't care, or won't say, I wouldn't buy a barrel from him.
