270 and Long Distance

[rusty815]

Taken for Longrangehunting.com's article about Berger moving to the G7 BC model:

"the BC of a modern long range bullet that’s referenced to the G7 standard is constant for all velocities! In other words, a trajectory that’s calculated with a ‘G7 BC’ doesn’t suffer from the same velocity dependence problems and inaccuracies as calculations that are made with a G1 BC."

[helotaxi]

Yep. The G7 model essentially accounts for the variability of the drag coefficient as part of the model. The G1 model does not. The G7 model also better models the drag profile of a long profile boat-tailed bullet compared to the G1 model.

I'd forgotten that the new Noslers had the G7 listed. I'm curious how they derived them. The BC can be derived by computer modeling or measured. Most bullet makers derive their BCs and they end up inflated, at least slightly. Sierra measures theirs. Berger has measured values for some of theirs as well.

[FUBAR]

Taken for Longrangehunting.com's article about Berger moving to the G7 BC model:

"the BC of a modern long range bullet that’s referenced to the G7 standard is constant for all velocities! In other words, a trajectory that’s calculated with a ‘G7 BC’ doesn’t suffer from the same velocity dependence problems and inaccuracies as calculations that are made with a G1 BC."

Someone left off the word relatively in front of constant above... Here is a short condensed version of this topic on the JBM website: http://www.jbmballistics.com/ballist...pics/bcs.shtml

I only took issue with the statement that "G7 was not velocity dependent". ALL G drag functions are velocity dependent.

I completely agree that for this bullet selection the G7 drag function model is the better choice...

G7 BCs, for certain bullet types, are more constant over the useable velocity spectrum for the particular type, but so is G1 for certain types of bullets.

Note: roughly speaking we are talking about supersonic Longrange shooting, where the bullet maintains supersonic-transonic velocities.

There are Longrange shooters that for well over 100 years have been lobbing heavy low BC bullets into a target aroun 1/4 mile away

However their bullets quickly go subsonic, and at subsonic the design parameters change, here is an excerpt:

"Subsonic (below about 1090 fps depending on air density) bullets have their BC more affected by the shape of the base than the nose. A very blunt bullet can have its BC improved by nearly 40% by changing from a flat base to a rebated boattail shape...."