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What's Wrong With .30 Caliber? By Bryan Litz
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<blockquote data-quote="BryanLitz" data-source="post: 262224" data-attributes="member: 7848"><p>LR3,</p><p>I'm not <em>sure</em> what you're asking re: 6mm br and larger cases, but I'll comment anyway!</p><p>What you may have been getting at is how much vertical dispersion a small caliber will have from a given muzzle velocity variation compared to a larger caliber.</p><p></p><p>Consider the 6mmBR shooting 105VLD's at 2800 fps. At 1000 yards, a 20 fps variation in muzzle velocity will cause the vertical impact to shift by 5.5".</p><p></p><p>A .300 Win Mag shooting 210's at 2800 fps will have 5.0" shift in vertical POI from a 20 fps variation in muzzle velocity. </p><p></p><p>The comparison favors the higher BC round, but only slightly. In order to achieve the 5.0" of vertical spread, the 6mmBR would have to cut the muzzle velocity variation from 20 fps to 18.7 fps. In <em>real life</em> the 6mmBR would probably be capable of much lower velocity ES than a larger magnum, so the actual results would probably favor the 6mmBR.</p><p></p><p>In general, higher BC bullets will be less sensitive to all variables (including muzzle velocity variation) than lower BC bullets. This isn't a caliber specific statement, but is a general trend.</p><p></p><p>As for military munitions, they certainly obey the same physics as our small arms stuff, but they push against different corners of the envelope. An artillery shell is mechanised with moving parts, fuses, threads, etc. It's a nightmare to make something like that balanced. One of the biggest challenges in artillery shell design is making sure the round will <em>trace</em> when fired at high angles of elevation. Depending on the stability characteristics, the round may not be able to trace. If it doesn't, it will fall short of the target and fail to fuse (probably a good thing for the friendlies that sometimes occupy the ground <em>short of the target</em>)</p><p>Modern systems are being outfitted with GPS guidance which changes everything.</p><p></p><p>It may be a surprise that many large scale cannons (20-30mm) have a certain level of dispersion <em>built in</em> to them intentionally to give a shotgun effect. If you're trying to gun someone down in an air-to-air engagement, you don't necessarily want to be shooting small groups. The dispersion is controlled with the bore diameter. The rounds are generally steel body with bronze or plastic rotating bands. The rotating bands are quite narrow, and they engage the riflings while the body of the round <em>augers</em> down the tube. The specific clearance between the projectile body and the bore controls the dispersion quite predictably.</p></blockquote><p></p>
[QUOTE="BryanLitz, post: 262224, member: 7848"] LR3, I'm not [I]sure[/I] what you're asking re: 6mm br and larger cases, but I'll comment anyway! What you may have been getting at is how much vertical dispersion a small caliber will have from a given muzzle velocity variation compared to a larger caliber. Consider the 6mmBR shooting 105VLD's at 2800 fps. At 1000 yards, a 20 fps variation in muzzle velocity will cause the vertical impact to shift by 5.5". A .300 Win Mag shooting 210's at 2800 fps will have 5.0" shift in vertical POI from a 20 fps variation in muzzle velocity. The comparison favors the higher BC round, but only slightly. In order to achieve the 5.0" of vertical spread, the 6mmBR would have to cut the muzzle velocity variation from 20 fps to 18.7 fps. In [I]real life[/I] the 6mmBR would probably be capable of much lower velocity ES than a larger magnum, so the actual results would probably favor the 6mmBR. In general, higher BC bullets will be less sensitive to all variables (including muzzle velocity variation) than lower BC bullets. This isn't a caliber specific statement, but is a general trend. As for military munitions, they certainly obey the same physics as our small arms stuff, but they push against different corners of the envelope. An artillery shell is mechanised with moving parts, fuses, threads, etc. It's a nightmare to make something like that balanced. One of the biggest challenges in artillery shell design is making sure the round will [I]trace[/I] when fired at high angles of elevation. Depending on the stability characteristics, the round may not be able to trace. If it doesn't, it will fall short of the target and fail to fuse (probably a good thing for the friendlies that sometimes occupy the ground [I]short of the target[/I]) Modern systems are being outfitted with GPS guidance which changes everything. It may be a surprise that many large scale cannons (20-30mm) have a certain level of dispersion [I]built in[/I] to them intentionally to give a shotgun effect. If you're trying to gun someone down in an air-to-air engagement, you don't necessarily want to be shooting small groups. The dispersion is controlled with the bore diameter. The rounds are generally steel body with bronze or plastic rotating bands. The rotating bands are quite narrow, and they engage the riflings while the body of the round [I]augers[/I] down the tube. The specific clearance between the projectile body and the bore controls the dispersion quite predictably. [/QUOTE]
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