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Old December 1, 2012, 03:16 PM   #19
Jimro
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Join Date: October 18, 2006
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Thanks for that insight Jimro. Never knew that about the Cheytac testing. Is it an inaccuracy on their part of the published BC or just the design of the bullet. In another post I asked about a ballistic calculator that used only velocity and 3 POA's at different distances to adjust the BC accordingly, therefore seeming more accurate, to me atleast. Would this be a better approach?
Ok, sometimes simple questions have complicated answers, I'll try to tell you what I know from my experience, but I'm going to have to draw a lot on some of my friends and co-workers who shoot beyond 1k on a more regular basis (last time I shot beyond 1k was about 4 years ago. 1400 meters with a Barret M107).

You can calculate BC in a number of ways, the "industry standard" is the old G1 reference as it produces the highest number.

The standard G1 projectile originates from the "C" standard reference projectile (a 1 pound (454 g), 1 inch (25.4 mm) diameter projectile with a flat base, a length of 3 inches (76.2 mm), and a 2 inch (50.8 mm) radius tangential curve for the point) defined by the German steel, ammunition and armaments manufacturer Krupp in 1881. By definition, the G1 model standard projectile has a BC of 1. The French Gavre Commission decided to use this projectile as their first reference projectile, giving the G1 name. Pulled from Wikipedia.

Note that the G1 model is good for flat based bullets with a double radius tangent curve profile. This is actually a very good reference for flat base spitzer bullets.

The G7, G7 (long 7.5° boat-tail, 10 calibers tangent ogive, for very-low-drag bullets) was designed to deal with the more modern spitzer boat tail design (which the US adopted in M1 loading of the 30-06 in 1925, 175gr FMJBT with 9 degree boat tail).

The boat tail angle is an interesting topic, the 13 degree boat tail of the 168 SMK "International" bullet (designed for 300 meter free gun) does not handle the transonic range very well at all (bullet destabilization). The older M1/M72 bullets with the 9 degree boat tail handled the transition just fine, as did the older 180gr SMK (which had an 11 degree boat tail, IIRC). The 175gr SMK was developed to replace the M72 bullet, and it did that quite well.

Ok, science aside, some match bullets work better than others. Flat base bullets are going to be the most accurate at short ranges. Boat tail bullets are going to be most accurate at long ranges. Bullets that handle the transonic range smoothly will be most accurate at extreme ranges. The transonic range really starts at about Mach 1.2 for some reason, I don't know why this is, and I don't know why some VLD design match bullets don't handle the transition well.

If you want to shoot long range, look at what other people are shooting long range. A 300 Win Mag will get you to a mile if you are shooting a 220gr SMK (which handles the transonic range nicely). Shoot the same 300 Magnum with the 210 VLDs (which have a higher published BC) and you will get erratic accuracy through the transition range to subsonic, but you will also get a flatter trajectory inside of the supersonic range (well beyond 1000 meters from a magnum).

A 338 Lapua (or any of the comparable 338 magnums, RUM, EDGE, etc) will get out out past a mile with a good bullet (250gr Scenar or lockbase is the "traditional" load). The Lost River 270gr bullet is probably the best compromise between velocity and BC, although there are a number of folks who swear by the 300gr SMK.

Sorry to ramble, but ballistics on paper are a lot different than ballistics in reality once you get beyond a kilometer. One of my snipers, (God rest his soul) had no problem making hits at 1400 meters with M118LR at the Yakima Training Center. The combination of high desert air and a bullet that handled the transonic range let him use the m24 very effectively to provide precision shots 200 meters shy of double the doctrinal range for snipers.

Heck, those old Brits shooting Enfields at Bisley range did amazing things with a cartridge that had a midpoint trajectory height of over 4 story building. Having a bullet that is stable down to subsonic velocities is absolutely key to long range accuracy.

Jimro
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