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Old September 28, 2012, 06:54 AM   #11
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
Sierra Bullets also tests their bullets for BC using time of flight between two points at different starting velocities. They've been doing that for decades. Sierra uses the G1 drag model while Berger uses the G7. Sierra lists more than one BC for their bullets in several velocity bands. Berger uses only one. They're both very precise for their software and therefore excellent for calculating drop and drift numbers.

Some ammo companies use SAMMI spec test barrels clamped in solid mounted receivers for muzzle velocity and pressure numbers. Others use whatever factory rifle barrel they've at hand fired from a bench. The differences in muzzle velocity between two people shooting the same rifle and ammo from a bench can vary as much as 100 fps. And solid mounted test barrels always give higher muzzle velocities; they don't move backwards in recoil while the bullet goes through them. And one's personal rifle probably has different bore and groove diameter and length dimensions than the factory barrel has. To say nothing of the differences in primer, powder, case neck tension and a few other things involved.

Scope makers use slightly different mechanics for their click values. Some use the original MOA numbers of 1 inch per hundred yards estabilshied by the shooting sports over a century ago. Others use the more recent trigonomic value of 1.047 inch per hundred yards. Either will vary as much as 5 to 10 percent depending on the exact focal length of all the lenses in each of several lens groups in a scope as well as the mechanics used to move the target image on the reticule. Unless one accurately measures their click values, they should be prepared to accept some inaccuracies.
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