Ballistic programs,100 yards, chronographs, zeroing & clicks

[Dangermouse]

It is often said that computers will give out what is put in (Sh*t in, Sh*t out).
This is very true of ballistic programs but I had not fully realised how much difference a few fps or yards out would make to a graph or firing solution.

Let’s start at the beginning 100 yards.
It has already been mentioned a number of times that the 100 yard Short Siberia range at Bisley is not 100 yards. From memory the tables are around 110 yards and the gravel prone firing line is only a couple short of that. To shoot at 100 yards would take you well on to the grass in front of the firing point and clearly is not possible if others are shooting behind you.
Those 10 yards may not be a problem if you only shoot short distances but for those of us who are looking beyond 800 yards; it all of a sudden becomes an issue. At 800 yards your program will be 80 yards out. At 1200 yards 120 yards out, depending on how flat a trajectory your chosen calibre shoots, that is quite an error. For an F Class target, with a 5 inch V Bull you are going to be outside the points from shot 1. For any other "target" you are probably going to miss.

I am guilty of setting up a chronograph poorly. Apparently a cheap chrono can be out by 100 fps and expensive ones can only get to around 15 fps of real figures.
If you were to add the figures from a good chrono with a good variation of 15 fps to ammunition that had an acceptable 15 fps variation, then all of a sudden you are looking at up to 30 fps variation error in the figure that you put in to your ballistic computer. That can easily equate to a 10 inch +/- at 1000 yards all by itself. Just think what a cheap chrono with a larger error will be doing to the calculations.

I had not fully appreciated the need to set up the chronograph flat / square to the target and shooter or in other words parallel to the line of sight. The tripod that I use is far from fit for this purpose with it moving in the wind and hastily set up whilst others wait to shoot. I need to be thinking more about a solid platform to assist. A 5 degree variation will create another 13 fps error in readings.
If the sensors are supposed to be 2 feet apart and they are 2 feet 0.1 inch apart that will create an error of 13fps for a projectile travelling at 3000 fps - see above for results down range.
We can clearly be our own worst enemy when using these things and I will need to book the range to myself or other like minded people when doing my next round of testing.
I wonder if two tables were dragged out to the correct distance and placed one in front of the other if they could be used to fire from and rest the chronograph on. Anything further would mean building something specific.

When shooting from a table like this it would be acceptable for checking muzzle velocity etc, but shooting from tables should only be done if you intend to shoot on the day from a table. When zeroing it is essential that you zero in the position and with the equipment that you will be using. For example, no point zeroing prone with the rifle resting on a sandbag if you will be standing supported come the day. Likewise for me, no point zeroing from a table if all my shooting will be prone with a bi pod.
I need to zero from 100 yards and would like to play around with different positions. Right leg up, leg down. Right thumb gripping, right thumb relaxed. Pull stock in tight, let it sit naturally, cheek touching stock and not and so on. A, to see what difference these little changes make and B to find what works best - which is what we are all after at the end of the day.

Finally the values of the clicks on the turrets are often taken as read that they are correct and that your ballistic computer is at error.
To check the values you need to shoot for simplicity at 100 yards (or meters if your scope works in meters). This will prevent you having to do additional maths and making simple errors.
You need to set up a target with a line on it that is perfectly horizontal. The use of a plumber’s line is probably the only way to do this. I know the target frames at Bisley rock and sway so a little time is needed to set this up, possibly with a shooter checking from the firing point with a spotting scope or high powered scope capable of seeing the string.
Once set up, fire a group of 5 shots at the bottom of the line. Then raise it 10 MOA repeat and do again twice more so that you have covered 30 MOA of travel.
You then need to check the average distance between the first and last group to confirm how accurate your clicks are compared to the advertised figure. And back down again if you want to check the consistency of the tracking.
1 MOA is 1.047 inch at 100 yards
The 4 groups should have 30 MOA between them or 31.41 inches anything else needs to be worked out and added to your ballistic programs to get the best firing solutions.
Say you measured 29.5 inches between the group, that is only 94% of its marked value or 6% less than it should move. So when we dial that scope for elevation we need to dial it 106% of the intended adjustment. So if we have a long range target that needs a 30 MOA, we actually would need to add (30 x 1.06) 31.08 MOA.
Should you find that your groups vary away from the straight line on the target one of two thing are happening.

Firstly and cheapest to fix, assuming that your scope has been mounted correctly, is to ask yourself are you canting your rifle? The fact that you have a line on the target that is perfectly horizontal should be stopping you from doing this but a rifle that is leaning to one side or the other will never shoot consistently and will confuse the pants of you when trying to make changes for elevation and wind. Many people use the tops of the targets to get a horizontal from, beware doing this; many are not as level as you think. The best tool would be a spirit level attached to your rifle.

If after checking the above your scope is still placing shots away from the line then you have to ask if the tracking in the scope is faulty. Possibly for disciplines where by the turrets are never touched this is no great issue, find your zero and let it be. But for other disciplines where you are constantly turning the turrets between distances this would be an unworkable scenario and the scope would need to go back to the shop etc.

Maybe the next time you curse the ballistic soft wear that you are using you might just stop and wonder if perhaps some of the information you entered might actually be out and that you are to blame not it.

Why is all this important? Well to me it is important, too many times I have looked at my graph for a distance and on the first shot been well outside the scoring rings. When you see others getting 5’s and V’s from the off you start the competition mentally already having lost. Whilst others are watching the wind, I find myself still trying to find the 5 and can spend the whole 12 shots chasing my shots all over the target. I want to be able to, as best I can, get in the scoring from shot one and give myself a better chance of climbing some places.

So, anyone up for doing some of the above? Say Short Siberia 100 yards,mid week , all day, all to our selves in January? Get the season off to a good start etc,

DM

[EagerNoSkill]

Hi DM

Concur!!

I did NOT know Siberia was 110 yards at table!!!!

Solid Chrono Test - Will bring the PVM21
I would be in subject to a trip to MikMak to swop my barrels

Will keep in touch about it!

[kennyc]

I always thought Short Siberia was a 100m range? and all the others were measured in yards?

[meles meles]

Many military ranges are also out. When the army went metric it generally just involved sending some unfortunate oik around to convert the markers from 'yards' to 'metres' by the expedient of a paintbrush rather than re-siting the firing points and markers.

[tackb]

where we shoot at barton road it's metres back to 600m(checked with my leica) then yds from there back to 1200 although they do helpfully put metre markers up so you can set your sights at 1054m(my memory may let me down there) for example, and if shooting into the sand(to spot fall of shot) it's another 50 m on that.

[ovenpaa]

Tackb - I have ranged Barton Road out to 1200 with my Leica and it seems fairly close all the way, the real problem is the mantlet is not quite level so an ACD can be of assistance to some. Also I do wish people would realise that shooting into the sand at 1200 may well give you an indication of how you are for wind but (As you point out) it is 50 yards further out which is probably another 4MOA for a .308, so if the perceived fall of shot is level with the centre of the frame you will need to make quite a change to your elevation to get on target.

Better to know your true zero and use decent tables that allow for the main variable at BR which is temperature.

[TJC]

I think you are very much on the right line. This is a well regurgitated and expanded version detailing sources of ballistic inaccuracies.

http://www.arcanamavens.com/LBSFiles/Sh ... /Programs/

I like the idea of dedicated session and maybe we can try and get a few chrono's out to compare and verify. Ultimately, whatever your output the final test comes after the chrono stage and that is verifying your data in the real world and making the inevitable adjustments that have to be made.

[tackb]

Tackb - I have ranged Barton Road out to 1200 with my Leica and it seems fairly close all the way, the real problem is the mantlet is not quite level so an ACD can be of assistance to some. Also I do wish people would realise that shooting into the sand at 1200 may well give you an indication of how you are for wind but (As you point out) it is 50 yards further out which is probably another 4MOA for a .308, so if the perceived fall of shot is level with the centre of the frame you will need to make quite a change to your elevation to get on target.

Better to know your true zero and use decent tables that allow for the main variable at BR which is temperature.

it is close but the point i was making was that its metres to 600m then the firing points are still at yds back to 1200?

[The Cupcake Kid]

If you can organise a trip to the range with a friend who also has a chrono, it's well worth setting up two in line with each other so you get a reading from both with the same shot. It will give you some idea as to the accuracy of either/both and whether one (or both) may not be calibrated correctly. Even though you won't know whether either of them is correct, wildly differing readings would ring alarm bells.

Of course, you really should have a third also in line to give you enough data to give the whole exercise any proper meaning.

There's only so many hours in the day, however...

[Steve E]

I'd be up for a day (preferably a Friday) for a session with a cronograph. I have some .303 loads that need crono'ing and I don't have one.