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Old December 19, 2018, 10:51 AM   #36
Join Date: March 4, 2005
Location: Ohio
Posts: 19,791

Note that in a larger diameter projectile a same-weight bullet has lower sectional density than its narrower counterpart. That means it takes less pressure to achieve any specific rate of acceleration. Thus, to achieve a specific velocity, less powder is needed with the wider, lower SD bullet.


I don't know what you mean by optimal. If you read the Houston Warehouse experiments you will conclude that 21¾" empirically seems to be best for rifle accuracy regardless of chambering, but clearly not best for long range shooting. If you mean, what barrel length will extract the maximum amount of energy from the powder? that will be the length at which acceleration just reaches zero. For a .22 LR, that is 16 to 19 inches, depending whose load you fire, the high speed being at the longer end and standard velocity being at the shorter end. After that, the bullet gradually starts to slow down in the bore due to friction, though not enough to matter in a 24" .22 LR barrel. For high power rifle cartridges with their much greater powder capacity, though, a barrel long enough to achieve that is impractically long for many cartridges. With long barrel friction activated, QuickLOAD puts it at 45.6 inches for the 30-06 load, above, dropping to 45.0 inches with a 175-grain bullet and the same powder loaded to the same pressure. Switching to IMR4350, a slower powder loaded to the same 60,000 psi, it grows the barrel to 47.8", so you can see this doesn't change a lot. Maintaining accuracy with a barrel that long whipping around would be difficult, at best. It only gains about 100 fps over a 32" Palma barrel, which is long enough for most folks.
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