Exactly what I've experience with my plain old 100 yd zero.
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If I read you correctly, you are saying we are seeing a POI shift and not a shift in zero. The POI shift requires sight changes, but the mechanical zero has technically not changed.
I guess we mix terms at times. Most High Power rifle competitors I have shot with over the past 30 years refer to their sight changes as their 200 yard off hand zero, 300 yard sitting zero and 600 yard prone zero. You don't agree that the mechanical zero has changed, just the point of impact due to distance and shooting position.
To the OP - you will see a shift in POI when changing shooting positions and hold. Zero the weapon from the shooting position you will use the most and understand the POI shift when moving to other positions.
I agree with your definitions of zero. A zero is a defined point of impact in relation to the point of aim at a specified distance with a defined consistent system, irrespective of specific/temporary environmental or user-induced factors. I have several known default zeroes for long distance work. Some of these zeroes are applied by changing the sights, some are applied by utilizing reticle holds, but both are known points of impact at a known range.
I also agree with Arctic1 in that a shift of POI in relation to POA due to an anomalous variable is not a "zero change", but rather a POI shift; i.e. wind, sling tension, applied force, etc.
Exactly... you end up moving your zero to chase your POI when going from bench to artificial support.
Can anyone give their typical POI shift at a fixed distance, caused by their changing from prone to standing, shooting an M4-like carbine? Obviously there are a million variables (as discussed in the thread), but it would be nice to have an order of magnitude estimate. Is it typically .5 MOA, or 2 MOA, or what?
Are there shooting disciplines which use different positions while shooting at the same range? If so, does anyone know their position dope (as distinct from range compensation) for a light 5.56 carbine? Or any 5.56?
NRA High Power/Service rifle shoots standing offhand and sitting at 200 yards. I don't have any data for 5.56 specifically. but for most rifles there is a different mechanical zero going from standing to sitting to compensate for the sling tension. Usually it is a couple of clicks in windage and/or elevation. On AR-style rifles this can be particularly significant and is one of the reasons free float hand guards are pretty much universal on AR competition rifles.
I would disagree, sort of, on the definition of "Zero" in that environmental factors and user induced factors all come into play.
My definition of a "Zero" is when the shooter's point of aim and the bullet's point of impact are exactly the same.
I have 800yd zeros for my rifles and changes in the environment will, with out a doubt change those zeros. There is an exactly perfect zero for every shooting position, range, altitude, wind condition, mirage condition, light condition, and temperature.
Often we are close enough to the target and the target is big enough that many of the factors are not so important but stretch the range and shrink the target enough and all these factors will effect your zero.
Personally I "Zero" from the most stable field position, prone with bipod, and then modify my other shooting positions to keep the "Zero" (shooter's point of aim and bullet point of impact) the same from those other less stable positions. Keep in mind that the average .270 Win hunting rifle ( 9lbs) will move over 1/10 of an inch while the bullet is still in the barrel and how the shooter controls that recoil will affect the bullet point of impact.
I cede that my stated definition of "zero" is actually a "true zero". The difference of course being that the "true zero" is for a theoretical perfect environmental condition and all other variables being the same, where a "zero" is the sight adjustment necessary to place a shot or a string of shots onto the desired point in the conditions present when the shot is occurring. Common use has pretty much removed the term "true zero" from use here, however, you are correct, and misusing terms without clarification causes confusion and disagreement where there may not be cause.
That's fine, and that's the usual perception of a "zero", however, one could also have a zero in which the group is offset from the primary sighting reference. The key aspect of the zero is that it puts the group at a distinct known point at a defined distance.Quote:
My definition of a "Zero" is when the shooter's point of aim and the bullet's point of impact are exactly the same.
Agreed.Quote:
I have 800yd zeros for my rifles and changes in the environment will, with out a doubt change those zeros. There is an exactly perfect zero for every shooting position, range, altitude, wind condition, mirage condition, light condition, and temperature.
Often we are close enough to the target and the target is big enough that many of the factors are not so important but stretch the range and shrink the target enough and all these factors will effect your zero.
I prefer to zero from the most used, most stable position that shows the greatest commonality with other positional zeroes. For 5.56 carbines this is a magazine monopod position with slight downward pressure on the handguard and firm rearward pressure on the grip. With 7.62 carbines that have a precision requirement, it is (usually) a neutral bipod with firm rearward pressure seating the stock to the shoulder. Loading the bipod generally drops my group by 0.2 Mils with 175gr projectiles at 2400 to 2500 f/s.Quote:
Personally I "Zero" from the most stable field position, prone with bipod, and then modify my other shooting positions to keep the "Zero" (shooter's point of aim and bullet point of impact) the same from those other less stable positions.
Seeing the numbers or high-speed video of recoil does a lot to illustrate the necessity for consistency behind the gun for precision.Quote:
Keep in mind that the average .270 Win hunting rifle ( 9lbs) will move over 1/10 of an inch while the bullet is still in the barrel and how the shooter controls that recoil will affect the bullet point of impact.
Different perspectives, definitions and/or applications might cloud the discussion a bit.
I still don't agree that shooter and/or environmentals affect your zero. Zero is achieved when your optic/sight line is adjusted so that the trajectory of the bullet will intersect it at a specific point/s.
So, if you zero a specific weapon/optic/ammo combo at 200 meters, then you are zeroed at 200 meters. Granted, this zero applies for the specific set of environmentals applicable at the zero location(temp, barometric pressure, humidity). A shift here will affect your POI, so if you are going to shoot under vastly different environmental conditions than when you zeroed, then you should re-zero so you are set up for that specific environment.
Wind and/or spin drift do not affect your zero, but again result in a POI shift. If using a fixed/variable power scope with windage/elevation adjustment turrets, one can make windage adjustments to compensate for spin/wind. Other sights require that you hold L/R for wind/spin.
Inconsistencies or changes in shooting position, eye relief, force applied to weapon etc. also do not affect zero unless, as markm pointed out, a novice shooter who is not grouping consistently constantly adjusts his sights to compensate for what is in effect shooter error. Recoil effects on POI are definitely dependant on a good/bad shooting position, but under normal circumstances it will usually only affects grouping characteristics; ie. not shooting hole in hole groups. The same with variations in muzzle velocity due to inconsistencies in powder shape and weight.
I'll exemplify my point, though I will not include temp, barometric pressure or humidity. I'll base it on my last issue weapon, shooting issued ammo.
HK416 w/Comp M4
NM255 (62gr 5.56 FMJ Lead-Free)
Ballistic Coefficient (G1): 0.329
Weight: 62 grains/4grams
Muzzle velocity: 901.5m/s - 2957,67 f/s
Optic height over bore: 71mm/2.79"
If I zero this rifle at 200 meters, the projectile will also intersect the sight line at 49 meters. The maximum ordinate of this specific zero trajectory in relation to the sight line is 46mm, and stretches over 9 meters (122m - 131m).
If I zero this rifle at 300 meters, the projectile will also intersect the sight line at 30 meters. The maximum ordinate of this specific zero trajectory in relation to the sight line is 162mm, and stretches over 17 meters (164m - 181m).
If I hold POA for 200 at a 250m target using the weapon zero'd at 200m, the POI is 89mm below the sight line.
If I hold POA for 300 at a 250m target using the weapon zero'd at 300m, the POI is 105mm above the sight line.
The actual trajectory required for the projectile to the same POI at 250 meters would be identical, regardless of the zero distance, but the actual POA/holds will differ between the two zero's.
I hope this example helps explain what I base my opinion on, and that it can perhaps clarify my definition of a zero.
I don't see someone zeroeing their weapon at 200 meters from the prone, mag supported, and when they change to a sitting position shooting from a bench their zero is more akin to a 300 meter zero.