Scope height ballistic program question to ponder and consider.

[Jeff In TX]

A lot of us use ballistic programs to assist us with our long range shooting. I was at the range on Sunday and ran into a shooter who was having fits with his ballistic program trying to match is actual data from 100 to 200 yards.

I brought out my PC and opened my RSI program and was getting basically the same results he was getting.

He was shooting a 30-06 and had two chronographs set up. One for the muzzle velocity, the other of 100 yards. We measured any and everything and our results were pretty close on our programs, which weren’t close to his actual range data.

Then I discovered the problem and had to do some reverse engineering to get the computer data to agree with his actual data. It all came down to the scope height, which we measured a couple of times to make sure it was correct.

In a perfect world the scope is parallel or almost parallel to the centerline in the bore. Using the actual scope height our ballistic program data would have been close to his actual data. But he was using 20-degree angled mounts to gain more elevation. This setup really threw the results off on the ballistic program. Once we tweaked the program and really lowered the actual scope height on the program (I think we put in .70") the program matched his actual data.

Anyone else ever experience this with there b-programs matching there range data using angled mounts. I don’t have angled mounts on my rifle so I've never really run into this one. Damn I love working out a puzzle!

Just food for thought.

[JBM]

Assuming you meant a 20 moa base, I don't really think it had much to do with it. The barrel is always at an angle to the line of sight. Any angle in the base can be included in that angle giving the same answer. Errors in scope height give an angular error -- so many moa per yard. It's usually pretty small.

What were you trying to measure at 100 yards with a chronograph that scope height would have an impact on? That part, I don't understand.

I'd be more inclined to suspect the BC you were entering.

[Chrismadrid]

It might be soemthing like effective SCOPE HEIGHT. (I like problems too). Where did you measure Scope height - OBJECTIVE o FOCAL PLANE?. I'm just guessing.

[Jeff In TX]

First, I meant to say 20 MOA bases and not degrees. My mind was saying MOA, but my fingers were typing degrees.

He was using 2 chronographs to figure out his exact BC. I was actually done shooting and ready to leave when I saw him setting up. I thought I was the only one who used two chronographs so I hung around.

He was pretty anal about his set up. He had a 100 yards surveyor tape measure.

His actual range data was off his ballistic program at 200 yards by almost 1.75". Still not sure what was going on as I've never been off that much on my b-program, but using two different b programs gave us the same results.

I thought it was the 20 moa mounts, but I stand corrected on that. The only thing I can think of is, one of his chronographs was way off, but his new BC was pretty close to the published BC of the bullet manufacture.

As for the scope height, we measured at the objective, we also measured at the tube trying to find something to work with.

I couldn't hang around as I had an evening flight to catch, but changing scope height brought his results in line with the program.

Jeff, if he chrono'd on a cloudy day with the diffusers installed, his readings are probably incorrect. For his poi to be off 1.75", either his reticle is sticking, or his scope is improperly mounted (meaning he did not lap his rings and the tube is binding ~ the interior clearances have been compromised) or his powder charge isn't what he thinks they are. For a sticky reticle, rotate the turrets 2 moa past where he wants to be, then come back down and then rap hard on the top of the turret with your knuckle.