Some .308 brain vomit

[kopcicle]

The more I learn the more I remember what I've forgotten.

I just remembered ...

The effect on midrange trajectory (MRT)and maximum point-blank range (MPBR, 5" in this case) by line of sight (LOS) distance over line of bore (LOB)

Definition of terms
midrange trajectory (MRT) Highest point of travel over line of sight (LOS)
maximum point blank range (MPBR) 5" in this case. No more than 2.5" over or under line of sight (LOS) for a total of 5" .
line of bore (LOB) bore of the rifle.
Zero, distance in yards the rifle is zeroed at.
Near field zero (NFZ) Where line of sight first crosses bullet trajectory. More on this handy number later

Sighting in a rifle to maximize the effective range where a dead hold will place an impact no more than 2.5" above or below line of sight is a 5" MPBR. Others may use the term differently or choose a different MPBR depending on size of target, game, or ego.

308 Winchester (7.62mm NATO), Federal Sierra MatchKing BTHP, 168gr 1.5" over bore, 2650 F/Sec
NFZ 25 yards, MRT @ 119 yards, Zero 210 yards, MPBR 246 yards

308 Winchester (7.62mm NATO), Federal Sierra MatchKing BTHP, 168gr 3" over bore, 2650 F/Sec
NFZ 46 yards, MRT @ 138 yards, Zero 227 yards, MPBR 259 yards

I'll supply data on request as it's commonly available. It's just numbers, far too many to post here.
I just wanted to show raw data to support the notion that the distance between LOS and LOB does make a difference when calculating MPBR.

Also keep in mind that these numbers are derived from a computer model and real-world results will vary noticeably but it's still a good baseline.

NFZ comes in handy when you don't have a range where you can set a target an arbitrary distance like 227, etc. For me it's just a way to avoid wasting ammunition.
The 308 Winchester (7.62mm NATO), Federal Sierra MatchKing BTHP, 168gr 1.5" over bore, @ 2650 F/Sec having a NFZ of 25 yards is purely coincidental.
The 308 Winchester (7.62mm NATO), Federal Sierra MatchKing BTHP, 168gr 3" over bore, @ 2650 F/Sec having an NFZ 46 yards is kind of inconvenient so use 1.3" low at 25 yards (It's in the data)


I was just overthinking ballistics again and thought I'd share. Again, I do not solve unsmooth integrals in my head. I just get into some nerdy crap on occasion.

 

[RichCoyle]

copcicle,

Maybe twenty years ago I actually did a test with an accurate .223 with a sight height of 1.68" and 2.2". It stirred up a hornet's nest on 24hourcampfire.com. I fired ten shots at 100 and 300 yards after sighting in for 200 yards. The small difference in height resulted in enough difference in apparent trajectory one would need about 100 feet per second more velocity with the lower sight height to match the higher sight height. Both at 100 yards and 300 yards the higher sight hit closer to the bullseye.

By the way, no one refuted it based on actually firing a test like I did. Many just did the ad hominem thing. A couple months later Barnes bullets verified my results with a real world test.

 

[kopcicle]

I did/have tested this but the test platform was a uniquely accurate FN-FAL.
Although affected by various commercial ammunition, once fired, neck sized and shoulder bumped loads held a consistent water line through several tenths of grains of powder and seating depths.
In short, if loaded for the rifle, vertical stringing was near non-existent. The usual general dispersion of the tilting bolt lock up however did apply.

It wasn't much of a sample size and verification was a mix of targets placed at the odd distances as well as extrapolation via digital caliper conformation at conventional distances. Still nothing I saw dispelled the above crude hypothesis.

Thank You for the reply in any case. This thought when shared on the interwebz usually falls on deaf ears.

@RichCoyle , Just curious. When attempting to explain sighting in for a 3" , 4" , 5" MPBR to a shooter that is fixated on a 100 yard, 200 yard, 300 yard zero, do you tend to run into "deer in headlights" , "why bother" , "makes no sense", "can't be done" , "not invented here" , "makes MOA, Mil, iron sights, useless" , kind of responses that I do?

Asking for a friend.