Jim Beckley wrote:This post has been fun to look at and even informative, but as I look at the names who have shot a 40/40, Kurreck, Lawton, O'Neil, Sanchez, Winstead-Severin, and Perry (SB Standard Rifle), I wonder if any of this BS crossed their minds while on their way to 40, somehow I doubt it!

You're right Jim.... It's the rest of us that need excuses that are taking the notes

Jim, I don't think thoes shooters get into it that deep. They all have great holds and trust their hold. Their spotter says you were a 1/2 minuete low and they crank up their sights without trying to figure it all out. It dosen't take a physics major to realize when you are not hitting on call that you need to change your sights.

Kevin: "Can a scoped rifle be set into a rest and aimed in the AM. Each hour the POA is noted and a round fired w/o touching the rifle. Does the POI change? Does the POA change? Do they change the same amount/direction? "

If you read my earlier posts, that's about what I do with my rail, and I have found that my POI does indeed move, with no change in POA. I have to click that out in my scope or I'm holding too low on the target for my taste. But that's just from what I have seen.

I have a match this sunday, and I will pay particular attention to this and see what happens.

s.

I have not had the time to read every reply on this topic but there is a variable that has not been mentioned. That variable is the contraction and expansion of: (1) the metal that the "rail gun" is made of, (2) the metal of the "rest" that the rail gun is sitting in, (3) the metal or wood bench that the rail gun rest is sitting on and (4) the stand that the target is placed on. I can imagine how much a wood or metal target stand upright can "grow" or "shrink" in length during daylight hours due to humidity, temperature and how much sunlight is hitting it. There is a reason that really accurate machine shops have temperature, light, humidity and other variables controlled as much as possible. There is a reason the talented machinists handle metal in ways that do not allow the hands to cause expansion or contraction of the metal. If my wife takes too long to "mike" a part the part will become smaller. If I take too long to "mike" a part the part will become larger. Her hands are very cold and mine are very warm just as an example. I have very poor temperature control in my home shop but at times have still achieved .0002" tolerances over a 5 inch length of cast aluminum.

Bobby R. Huddleston

(1) the metal that the "rail gun" is made of: Hall action made of 416, bolt body of 17-4 SS. Barrel 416SS, vac heat treated, button rifled, 1.450" tapering down to 0.950", 29.5" long, hand lapped (Hart 6 groove)

Alum barrel block, epoxy bedding between the barrel and block. Block is 6" long. Stock is Shehane epoxy soaked laminate. SS steel rails machined into bottom of stock, epoxied in place.


(2) the metal of the "rest" that the rail gun is sitting in: Lots of alum. 3/4" thick alum plate stock, about 40 lbs of it. Intermixed with tool -steel inserts, carbide strike plates, steel elevation posts, delrin guides.


(3) the metal or wood bench that the rail gun rest is sitting on: Re-enforced concrete top 8" thick on group filled concrete blocks, epoxied and lagged in pace on large concrete pad.


(4) the stand that the target is placed on: Wood 2x4" construction. This is an interesting consideration, and somthing that never occurred to me. Wood does indeed move when heated, but like most materials, it wants to expand. So, it could either expand to the right, left, forward, or back, and even like a pipe, it could expand up. But, if it did so, then the shots would be striking the target low, not high. If expanding, it would be doing so in the wrong direction.

I see your direction, but with all the materials and surface interactions, it would be impossible to calculate directional growth, and it to be a linear growth, very unlikely. Also looking at so many different shapes, which would move in different directions.

With all that said, I still see my POI moving straight up.

I also think the materials in my rail are too thick to cause such a shift over a 30 degree change. The actions are built so tight the bolt would not operate, the epoxy surfaces would not hold, and the very fine adjustment in the rest would bind, especially since they are a blend of materials that have different thermal expansion rates. We are looking at the entire unit moving 1/2" at 50 yards - far too great of movement for material expansions.

I think something worth considering would be the scope heating up and POA shift. We shoot under a covered roof, so there is no direct sunlight, but still the scopes have alum bodies and that could be moving. My scope is a Nightforce 12x42x56 BR.

Here is a cool link showing the different linear expansion rates of materials. http://www.engineeringtoolbox.com/linea ... -d_95.html

And another link showing the math of the changes. Notice how large the temperature changes are in these calculations:

http://www.engineeringtoolbox.com/therm ... d_283.html

finally, i think the roots of this are in one of two places:

A: fluid dymanics. i'm working on the math of drag related to air density.

or

B: scope body movement due to heat expansion.

s.

steve b., My post was not knocking your rifle, rest or ideas. My humble apologies if my reply offended. I do not think anyone except maybe an organization such as NASA or Eley could do anything more to technically research this phenomenon than you have done.

Two "country boy" examples of humidity problems I have experienced at our Club range:
(1) I build steel target stands out of 1 X 2 inch tubing. Cheap 1 X 2 furring strips fit the target great under all conditions. I picked up a bundle of discarded new dimensional 1 X 2's from a neighbor's remodeling job. I shaved them down to fit the metal stands. They fit in the dry air in my shop but when at the range the dimensional wood swells to the point you cannot get them out of the stands. My wood strips have got to be growing in length also.
(2) Our club range is in a swamp in the floodplain of a nearby river. Paper targets pick up so much humidity that staples will not easily hold them to our corplast backers.

While collecting this POI data during the day, would it be possible to put some kind of strain guage or even a simple dial indicator on a part of the rifle, rest or table to see if any of it moved due to some external condition such as heat or humidity without influencing the function or accuracy. My idea would be a thick, heavy steel plate sitting on the bench with a magnetic dial indicator holder with a inverted V shape tip on the dial indicator if you wanted to indicate near the end of the barrel. Being out in the elements I would not use a digital dial indicator but just a cheap mechanical indicator. If you think that would be enough force to act as a bad tuner on the barrel then what would you think about using a indicator with a regular small tip on some part of the rest that would indicate vertical travel in at least some part of the mechanism? I have no doubt that it is not the rifle, bench or rest but an external influence. Could the ground or concrete that the bench is on be changing with the heat and humidity? Concrete can do some strange things during the day in Mississippi. If money is no object a 3 axis DRO system could easily be built to fit the rest and feed data to a computer recording any movements caused by heat or humidity during the test period. Then it would be easy to plot a graph of each of the planes of movement per a measure of time. If they can do it in Charlotte on a moving race car, it should be possible on a rifle rest or bench.

On second thought maybe a small 3 axis DRO and/or strain guage could also be put on the scope body since you think this may be where the movement is coming from.

Excuse me if some of my terminology is not precise. I have been out of the machine tool business since September of 1995.

Air rifles are looking less troublesome to me as many of these powder-burner variables are eliminated.

Bobby R. Huddleston

Yesterday I was attending a biathlon at the Pemi F&G club. In the morning when I arrived the targets were set in such a way that half were in the shade and half in the sun. This was also true of the firing line. The temps were pretty cool and there was no wind. Shooting prone, I zeroed my Izhmash on sunny targets and immediately moved to a shaded set and shot five rounds. The POI of the group (3/4" at 50 meters) for both the sun and shade was essentially the same. The rifle has aperture sights so this doesn't answer the scope question.

The light in the shade is more diffused that the direct sunlight so not sure if this qualifies and a change in sun angle. I didn't have a chance to repeat the experiment after the sun rose when the wind and temps picked up. Not sure how relevant this is to the discussion but I expected more change between sun and shade.

In my few expermints, If you set the X of a survey instrument on a target in the sun and then shade the target, everything looks the same. If you set the X on a target in the sun and then come back 2 or 3 hours later in the sun, the immage has moved in relation to the X in the scope.

The old position shooters had a saying; light's up, sights up. Light's down, sights down.

I found the same to be true with a 20x survey instrument. Should be the same for scopes on a rifle.