08-16-2011, 06:55 PM
High energy and high Ballistic Coefficient (BC) are desirable. Longer and heavier gives high BC allowing the bullet to retain energy at longer ranges. Lighter and faster gives higher energy. Since energy increases as the square of increasing velocity and momentum increases linearly, you get more by increasing velocity than you lose by reducing weight. As far as muzzle energy is concerned anyway.
Reducing weight in a military bullet of a given caliber that ideally can't have hollow cavities for for legal and barrier penetration reasons usually means a shorter bullet. shorter and lighter means lower BC's.
If we assume the military is going to want an M855A1 type bullet with a copper core and steel penetrator, then the length can be preserved as compared to a lead core bullet without voids.
We know that the Grendel starts to lose energy at above 120 grains or so grains of bullet weight because of powder, length, and pressure limits of the cartridge.
Since those factors are trade offs, plotted lines of Energy and BC on a graph as the bullet weight decreases would cross at some bullet weight. That would be the optimum, and it looks like that point is at about 110-115 grains.
If we look at designs with the steel penetrator tips, in addition to making the bullet longer, the steel tip is lighter than the copper/zinc alloys used in gilding metal. That shifts the center of gravity to the rear, which makes it more likely that the bullet will tumble on impact. That's a good thing for stopping ability.
I get about 2725 feet per second out of 107 grain bullets from a 20 inch barrel at less than a max load, so 2725 fps from a 110 bullet is doable. You should be able to get a BC of around .450 from a bullet like that.
That yields 1814 foot-pounds of energy at the muzzle compared to the 1742 ft-lbs I get with 123 grain bullets at 2525 fps in the same 20 inch tube. They have about the same energy at 275 yards with the 123 grain leading from there on out, if you believe the ballistic calculators.
If the 110 grain bullet could get it's BC up to .500 it would beat the 123 out to 325 yards. It may be possible, but I doubt it. The calculator I have shows the Barnes 110 grain bullet as beating the 123 Scenar in energy only out to 200 yards.
For a hypothetical 115 grain 6.5 mm with a .500 BC bullet at 2650 fps from the same tube, I get 1794 foot-pounds at the muzzle and there again about the same as the 123 Scenar at 300 yards.
So I guess it depends on what's more important to you, performance under 300 yards or over. The 115 drops to 500 foot-pounds at about 875 yards, and the 110 grain bullet at about 800 yards. The 123 grain bullet makes it to 925 yards with 500 foot-pounds. There again if you believe the calculators.
This is kind of a worst case comparison because I doubt you could make and M855A1 type bullet with a copper core and steel penetrator at a weight of 120-123 grains with a BC matching the Lapua Scenar and not have it so long that it reduces the powder load.
If you can get a 110 grain bullet up to .500 BC, it is going to have superior BC out past 1000 yards. With a more realistic BC 0f .475 the 110 grain bullet will be surpassed by the 115 at about 300 yards and will drop to 500 foot-pounds at about 840 yards.
Rick
Reducing weight in a military bullet of a given caliber that ideally can't have hollow cavities for for legal and barrier penetration reasons usually means a shorter bullet. shorter and lighter means lower BC's.
If we assume the military is going to want an M855A1 type bullet with a copper core and steel penetrator, then the length can be preserved as compared to a lead core bullet without voids.
We know that the Grendel starts to lose energy at above 120 grains or so grains of bullet weight because of powder, length, and pressure limits of the cartridge.
Since those factors are trade offs, plotted lines of Energy and BC on a graph as the bullet weight decreases would cross at some bullet weight. That would be the optimum, and it looks like that point is at about 110-115 grains.
If we look at designs with the steel penetrator tips, in addition to making the bullet longer, the steel tip is lighter than the copper/zinc alloys used in gilding metal. That shifts the center of gravity to the rear, which makes it more likely that the bullet will tumble on impact. That's a good thing for stopping ability.
I get about 2725 feet per second out of 107 grain bullets from a 20 inch barrel at less than a max load, so 2725 fps from a 110 bullet is doable. You should be able to get a BC of around .450 from a bullet like that.
That yields 1814 foot-pounds of energy at the muzzle compared to the 1742 ft-lbs I get with 123 grain bullets at 2525 fps in the same 20 inch tube. They have about the same energy at 275 yards with the 123 grain leading from there on out, if you believe the ballistic calculators.
If the 110 grain bullet could get it's BC up to .500 it would beat the 123 out to 325 yards. It may be possible, but I doubt it. The calculator I have shows the Barnes 110 grain bullet as beating the 123 Scenar in energy only out to 200 yards.
For a hypothetical 115 grain 6.5 mm with a .500 BC bullet at 2650 fps from the same tube, I get 1794 foot-pounds at the muzzle and there again about the same as the 123 Scenar at 300 yards.
So I guess it depends on what's more important to you, performance under 300 yards or over. The 115 drops to 500 foot-pounds at about 875 yards, and the 110 grain bullet at about 800 yards. The 123 grain bullet makes it to 925 yards with 500 foot-pounds. There again if you believe the calculators.
This is kind of a worst case comparison because I doubt you could make and M855A1 type bullet with a copper core and steel penetrator at a weight of 120-123 grains with a BC matching the Lapua Scenar and not have it so long that it reduces the powder load.
If you can get a 110 grain bullet up to .500 BC, it is going to have superior BC out past 1000 yards. With a more realistic BC 0f .475 the 110 grain bullet will be surpassed by the 115 at about 300 yards and will drop to 500 foot-pounds at about 840 yards.
Rick