Thoughts on SCATT's F-Coefficient

There is a setting in the SCATT "shot parameters" menu with the name F-Coefficient, which appears to cause a lot of confusion for people. This parameter simulates the effect of muzzle movement on the outcome of the shot and should be set appropriately depending on which discipline you are training for. The "right" number for fullbore seems to be somewhere in the region of 30-40 however this is based on subjective experience and not a hard numerical analysis.



The F-Coefficient setting is found in the Shot Parameters menu.

Why is this important?
When a shot is fired the amount and direction of muzzle movement will have a significant effect on the fall of shot. If the muzzle is moving as the shot exits the bore then the bullet will continue to move in the same direction as it flies down the range (Newton's 1st law of motion.) The slower the movement of the muzzle, the less the shot will be pushed out from the centre of hold. Bullet velocity, distance to target and other factors will also affect how big the effect is.

SCATT attempts to simulate this effect. Manipulating the F-Coefficient allows the user to tune how far the shot is thrown to simulate their discipline.

F-Coefficient Effects
The three diagrams below show the effect on the fall of the same shot with F-Coefficients of 0 (zero), 37 (the value I usually use) and 75 (the default value, which is supposed to match 50mtr smallbore.)

The effect of different F-Coefficient values on the same shot.

As is clear from the diagram, increasing the F-Coefficient value increases the distance from the point of aim when the shot breaks (visible on the diagram where the grey-blue trace meets the red trace.) Where the F-Coefficient value is zero, the shot falls exactly where the rifle was pointing when the shot was fired. As the value is increased, the shot falls further away from this point. Note that the direction in which the shot moves is the same as the muzzle movement as the shot breaks, shown by the end of the grey-blue trace.

Suggested F-Coefficient Values
For fullbore I generally use an F-Coefficient value of 37, which seems to correspond best to what I would expect to see during a real shoot*; however any value in the range 30 - 40 would seem to be fine to me. Note that if you want to compare different SCATT files, you should set the F-Coefficient to the same value!

That said other shooters have alternative views on what F-Coefficient setting should be used. Most of these are simply a debate between which value seems to simulate a particular discipline the best; however one alternative method is to set the F-Coefficient value to 0 (zero) and use the "speed" tab to assess individual shots. I've never tried this approach as it seems overly complicated to me, but you may prefer it.

Get that SCATT out and start doing some training!

* The best way to determine the right F-Coefficient value for fullbore would be to do combined livefire / SCATT to compare the simulated fall of shot with the real fall of shot; however as this is not recommended with the SCATT USB Professional on a centrefire rifle (SCATT's newest model, MX-02 supports this but I don't have a GBP1200 to spare at the moment to try it out!)

Pragmatic Load Development

A Kiwi shooting friend recently asked me if I'd write a blog post on my load development process. I replied by Facebook message that I would be delighted to; although I did warn him that it might not necessarily be quite what he was expecting. Here's to you CG.

My Method
I find out the starting load for the powder I want to use, load 5 rounds each for loads starting from the starting load to the maximum load listed in the reloading manuals in 0.5 grain increments. I shoot them in sequence, checking for pressure signs* to determine the actual max load in my rifle.

Back off about 1 grain to allow a general safety margin and some headroom for temperature variation.  That's your long range load for 900x and 1000x. Back off 1 more grain. That's your short range load for 300x to 600x.

You're done.

If possible check the loads at any short range and 1000x before you use them in anger, and chrono them if you can, but in my experience it's rare that you will get a load that just won't perform acceptably at the very least if you're shooting a 30" barrel.

Rationale
1) 308 Winchester / 7.62 x 51** is a pretty easy calibre to load for, and when using a good quality bullet, case, primer and powder your loads aren't going to differ that much in accuracy in the normal range of loads listed in your average reloading manual. That variation in accuracy is probably going to be much smaller than you are able to detect, given that the best group that a "top 25 in the world" international shot will be able to hold consistently is about 1 moa at 1000 yards***.

2) Provided that you're getting over 2925 fps then you're probably going to be supersonic at 1000 yards with most sensible bullets, and the additional velocity you're going to get from using a double-base powder isn't going to make that much of a difference to the wind-bucking characteristics of the round. Using double-base powders will give you a velocity boost, but at the cost of a seriously reduced barrel life.

3) It probably costs you something like GBP1.50 - 3.00 or NZD2.50 - 5.00 every time you pull the trigger when you account for barrel life, ammunition cost, target hire, fuel and accommodation. Would you prefer to spend that money finding the perfect load, or getting some focussed range time in?

In summary, the time, cost  and effort of working out the perfect load for your rifle just isn't worth it for the vast majority of TR shooters because the difference between the accuracy of the rifle/ammunition combination and the accuracy of the shooter is too great (although it is probably worth it for our F-TR and F-Open shooting cousins.)

Example Short/Long Range Loads
VihtaVuori N140 - 45.0/46.0 grains
VihtaVuori N150 - 47.0/48.0 grains
TR 140 - 45.5/46.5 grains
Varget / ADI AR2208 - 45.0/46 grains

Case: Neck sized RWS brass
Primer: Federal 210 Match or Remington 9 1/2
Bullet: 30 Calibre Sierra MatchKing #2156 (40 thou jump)

NB - These loads have been proved to be safe in my rifle, but they may not be your rifle so please work up your loads responsibly. If you fail to do so, you might get lucky and not have a problem, but then again you may end up amputating various portions of your left hand when your barrel lets go and/or potentially blinding yourself with the gas and brass fragment blowback. If you're lucky. That's not to mention the damage you could do to your fellow shooters and innocent bystanders. It could also trigger nuclear conflict between previously friendly nations. So there. Don't be daft.

* Pressure signs include, but are not limited to: Excessive muzzle blast, cratered primers, flattened primers, sooty marks from as leaks around primers, loose primers, blown primers, and possibly most importantly case head expansion. This list is not exhaustive.

** Yes. I know that these aren't quite the same thing, strictly speaking.

** I know this to be a fact because I spent quite a few hours in the summer of 2008 measuring, recording into Excel and then analysing the details of all ~1500 shots fired by the winning GB Palma Team in the 2007 Palma Match. The shooter with the smallest statistical group held 0.96 minutes of arc with a confidence interval of 95%, the shooter with the biggest was about 1.60 minutes.

Get those shots away quickly to avoid retinal fatigue

I was on Facebook the other day when a friend posted a negative picture of a woman with a blank slide next to it, the idea being to stare at the picture for 30 seconds and then stare at the blank slide, blinking a few times. Miraculously, a ghostly positive image of the woman is reproduced on the slide.

It is critically important to understand that the reasons behind this optical illusion are also the reasons why you shouldn't stare down the sights forever and an age before letting a shot off.



Stare at the three coloured dots on the woman's nose for 30 seconds, then look at a brightly lit white piece of paper or wall and blink. You should see a photo-positive image.

The answer to the riddle of why we see a positive image of a woman, having stared at the negative image lies in the chemical magic of how our eyes work. When light shines on certain chemicals in the cells of the retina, a reaction is triggered which results in nerve impulses being fired. Our brain interprets these nerve impulses and we see an image as a result. These photosensitive chemicals are then regenerated for further use.

The problem is that there are finite quantities of these chemicals in our eyes and the chemical process which recycles them takes a little time. Normally this doesn't matter because your eyes are constantly moving and the patterns of light falling on your eyes change also, so the regeneration process can keep up; however if you're very still and looking at the same image for a long time, the supply of photosensitive chemicals is exhausted in the retinal cells. When you look at the blank piece of paper, the overstimulated retinal cells don't react in the same way whereas un-stimulated cells do, producing the same pattern of reactions that you would get if you were looking at a positive image. The image has been "burned" into your retina and you are no longer seeing what is actually there.

Just as in the Facebook post where you see a ghostly afterimage of a woman, when looking through the sights of your rifle the image of foresight and target becomes distorted. Holding on to shots for too long before letting them off is not a great recipe for phenomenal accuracy. Let the rifle settle naturally on target, breathe in, breathe out, acquire the sight picture and then fire the shot within 2 - 8 seconds.