zeroing your scope

[jrs]

wrad -
There are no dumb quesitons! But here's a dumb answer or three.
Interior ballistics begins at the breachface and ends at the muzzle.
Exterior ballistics begins at the muzzle and ends at the terminus of the trajectory.
In a match the firing line is the line beyond which no part of the shooters body (or equipment - check the rule book) may extend. Tends to keep everyone behind the danger zone - has next to nothing to do with the distance to the target (from an engineers point of view), and as pointed out by others has very little to do with point of impact at 'normal' ranges.
So to answer your first question put your total station datum on the muzzle (crown) of your rifle at the very last point that will contact the bullet as it exits, which could be a few thousanths before the end of the barrel. Now measure from there to your target the exact distance you desire. Don't forget to allow for Earth curvature which becomes significant at some point.

The bullet follows a curved trajectory BECAUSE of gravity, and the positions of the shooter and target are on a curved surface (actually also because of gravity), so if you don't measure the lineal distance ALONG that curve you'll have the wrong range (distance normal to the Earths surface that the bullet travels to the target) - ie a laser will tell the straight line distance to the target not the distance over which gravity acts. So now for the $64,000 question ... which distance is right? It's very similar to the high angle shot problem where the right answer is the gravity distance to target.

One engineer to another ...

 

[jwp475]

When speeking of .25 vs .2, I was talking 100 yard groups.

I knew that......

 

[wrad]

jrs -
Thanks for the good info. Sounds like you're a Civil Engineer also, trained in surveying as one element in our bag of varied skills. I know about earth curvature corrections. And I concur with the station datum starting at the end of the muzzle where the bullet has last contact. Assuming that a bullet does not touch the walls of a muzzle brake, there could be a few inches of brake located ahead of the datum. Right?

For the rest of you, please, we understand that our discussion has degenerated into purely academic issues. Don't waste your time reading me and jrs unless you just love to know stuff too trivial to affect your accuracy outside of a testing lab that's 1,000 or more yards long.

 

[jrs]

Wrad -

Electonics Engineer, with strong Civil background, Navigator, and author of Coast Guard Search and Rescue software, who once owned a construction company -- hows that for mixed bag?

Muzzle brakes, suppressors, and any other component that extends beyond the last point of contact between the barrel and the bullet don't count. CLOSE is not CONTACT, and those devices are NOT IN CONTACT with the bullet, but they are very close to contact. Off hand can't think of anything that contacts the bullet after it leaves the rifling.

A simple rule (mine anyway) is that if "it" can affect the path of the bullet, and the shooter can directly affect "it" then "it" is analyzed as a componet of Interior Ballistics, if on the other hand only nature can affect "it" then "it" is a component of Exterior Ballistics.

Hummm... Ported barrels, the porting is behind the last point of contact, that sort of muzzle brake has rifling and does contact the bullet. Original thought was of the screw on the end type ... anyway rule still applies.

Another way to look at it would be the last point at which gravity does not affect the trajectory - except that gravity does bend the barrel a measurable amount - refer to Vaughn's accuracy work for how much.

Anyway I still like the muzzle for the demark.

Want to try something really fun? While you are trying to get your rifle on target at a mile, try it with a T/C Contender with a 10" .44Mag barrel, iron sights. Trajectory is more like a mortar, but FUN in the Extreme! If you can get out past 600 you'll have exceeded the maximum I've tried ... let me know what sort of sight picture you get?

jrs

 

[wrad]

jrs -
Ha, ha - gravity bending the barrel. I get it, but you are going to make some of these other guys crazy.

Talk about mixed bags, how about this one: Air Force War Plans Officer, Amusement Park Ops Director, Roller Coaster Builder (The Beast in Cincinnati, the one on New York - New York Casino in Vegas), Waterpark manager & owner, management consultant in China, Vice Pres of a Japanese/American manufacturing company, farmer (1,400 acres), pilot, rifleman, cosmologist (not, as my wife says, cosmetologist), photographer, avid Tea Partier & 2nd Amendment rights protector (March on D.C. on Monday), and other stuff.

Life's too short,
wrad

 

[jrs]

Wrad -

Yeah, we should start a thread - call it - DETAILS or somesuch.
Shoot me an email and we can compare notes on our personal histories.

jrs
[email protected]

 

[Hired Gun]

I'm no engineer but I shoot a lot and in every shooting discipline I have ever studied the distance to the target is defined as from the farthest forward part of your body on the ground. If standing it is your toes. When prone it is your front rest. Off the bench it is the front edge of the bench. I guess if you have a 99 yard long barrel you are still 100 yards from the target if are still behind the 100 yard line. In the field I range it and the distance from my rangefinder to the target is my input that I use for the correction. Your field rangefinder is only accurate to + or - 1 yard anyway. As the CRSO for my range this is SOP.