Month: July 2011

Sighting In

Posted by on July 18, 2011

When I asked y’all to cut me some slack I didn’t expect I’d need it so soon.   The following article is an extract from a book I’m writing about scopes.  It should contain four diagrams, but my blog won’t import in CAD format.  Then it screwed them up when converted to jpeg.  Obviously I have a lot to learn about blogging.  I have posted the article without the illustrations because I think it tells the story clearly enough, but I will gladly send it complete with diagrams to anyone who wants it.   Just contact me at boothroyd@polka.co.za.

A recent question about sighting in on a firearms related forum, reminded me that such questions are a recurring feature.   But not much is published, and what is published tends to be sketchy.   So it seemed like a good topic.     

Sighting in is so simple that it seems hardly necessary to explain it, but I have observed difficulty with it often enough to justify an explanation, and as it is the final step in scope fitting it must be included for completeness. 

It is not possible to zero a rifle dead on in the workshop, either by optical bore sighter or the expedient method.  It is possible to get quite close but not close enough for use in the field.  The final adjustments must therefore be done on the range.

The basics are simple enough.  It gets more complicated at long range because of the exponential increase in bullet drop as the range increases.  But let’s start with basic zeroing.   Because of the unavoidable approximations in workshop zeroing, the first shot might miss a small target at 100 metres completely.  It is then impossible to know whether the bullet went high or low or either side, or by how much.  The target should therefore be set up at 25 metres and the rifle fired from a solid dead rest.  Unless the workshop zeroing has been done very badly the point of impact should at least be on the target.

A good target is a sheet of white A4 paper divided into 25mm squares with solid black lines that are easily visible through the scope ( Fig 24 ).  Make the middle square solid black.  The importance of these squares is that 25mm at 25 metres is roughly four minutes of angle which is 16 clicks of the drum on most scopes.  Keep the original as a master and make as many photocopies as you need.  Fix one to a sheet of cardboard about 400 x 600mm and attach it to the target frame.  There’s no need to waste ammo in the early stages of the job.  Aim the first shot carefully and note the position of the bullet hole.  If it is two squares ( 50mm ) left of centre adjust the windage drum of the scope 32 clicks to the right.  Be careful to turn it in the right direction, but don’t worry about it too much, the position of the next bullet hole will tell you whether you’ve got it right.  If you haven’t, reverse it.

If the first shot is also high or low, as is likely, the same principle applies.  Suppose it’s one square high.  Turn the elevation drum sixteen clicks down.  If you’ve got both the adjustments right or almost so, the second bullet hole should be close to centre.  Another two or three shots accompanied by fine adjustments of the scope should be enough to get as near to dead centre as makes no difference.  There’s no need to fine tune it to dead centre, just close enough to ensure that the shots will be on the target at 100 metres.

Patch the holes or replace the A4 target with a fresh one, move the target to 100 metres and carefully fire a shot.  It will almost certainly be off centre laterally or vertically or both.  Adjust the scope in the same manner as was done at 25 metres, but remember that only one quarter as many clicks will be needed to shift point of impact one square than was needed at 25 metres, because one minute of angle covers four times the spread at 100 metres than at 25 metres.  One click ( 1/4 MOA ) that shifts point of impact just under 2mm at 25m will therefore shift it 7mm at 100m.  Note that some target scopes shift 1/8 MOA per click not 1/4 MOA.

It will take a few more shots to get dead on at 100m than it took to get roughly on at 25m, but 15 or 20 should be enough.  When you are satisfied that the rifle is shooting to point of aim, shoot a few five shot groups to confirm the rifle’s grouping ability.  For this purpose a similar A4 target with 25mm squares works well, except that there should be four black squares in different places on the target ( Fig 26 ), so that four groups can be fired without the need for walking out to the target to patch or replace it.

This book is about fitting and zeroing scopes, not about accuracy per se, but a few words in that direction will not be out of place.  Careful zeroing is academic if the rifle isn’t respectably accurate in the first place.  I would define accurate as 1 MOA for a hunting rifle, and 1½ MOA is acceptably good for most hunting purposes.  2½ MOA is OK for ranges up to 150m.  A rifle that won’t group better then 2½ MOA  ( 70mm at 100m ) needs attention, and can usually be improved with a little work.

            Fig 24 25m Target      Fig 25  100m Target

                        Missing test target diagrams

                       

Long Range Zeroing

The previous section on basic sighting in is sufficient for rifles that will be used up to 150m.  A typical medium calibre sporter in the 30-06 class zeroed at 100m will print more or less to point of aim at all ranges under 100m and about 50mm below point of aim at 150m.  All hunting up to 150m can therefore be done without the need to allow for bullet drop. 

Longer ranges are another matter.  As an example let’s look at a 308 Cal 165 Grain spitzer fired at 2600 FPS .  Zeroed at 100 yards, point of impact will be 4.7″ below point of aim at 200 yards and 17.2″ below at 300 yards ( Fig 26 ).  Of course anyone hunting at those ranges should know the trajectory of his ammunition and hold over accordingly.  The problem comes with range estimation.  Between 100 and 200 yards there’s no problem, but if the range is estimated at 300 yards but is actually 250 yards, bullet drop will be about 10″ not 17″ as expected.  If the rifle itself doesn’t group better than 2½ MOA and shooter error is factored into the equation, that could be enough to miss an animal or worse, wound it.  It’s even worse if the range is estimated at 300 yards but is actually 350.  Bullet drop is then 27″ rather than the expected 17″.

At longer ranges the problem is exponentially greater because of much greater bullet drop and difficulty of accurate range estimation.  With the same ammunition, bullet drop ( 100 yard zero ) is 17″ at 300 yards, 39″ at 400 yards  ( Fig 26 ), and 72″ at 500 yards.  It is obvious that significant errors in range estimation at these ranges will result in hopeless accuracy. 

A partial solution is to zero the rifle at longer range.  This ammunition zeroed at 300 yards will print 6″ above point of aim at 100 yards and  7″ above at 200 yards  ( Fig 27 ).  That’s a lot easier to handle in the field than the first example above.  Note however that impact will be above point of aim at all ranges under 300 yards and will require hold under rather than hold over.  But, as long as the amount of hold under is more or less right, significant errors in range estimation will not make enough difference to the point of impact to matter.

A better method however, is to zero the rifle at a range somewhere in the middle of the maximum and minimum ranges likely to be used.  If it is known, for example, that shots under 150 yards will be unlikely and shots over 300 yards not desirable because of the high risk of missing or wounding, the best scope setting will be somewhere in the middle.  More precisely, it should be zeroed at that distance at which the hold under at 150 yards and the hold over at 300 are the same.  With this particular ammunition that would be 260 yards.  Hold under at 150 and hold over at 300 will be the same at about 5″, less at ranges nearer 250 yards, and the effect of errors in range estimation will be much less ( Fig 28 )

Missing trajectory chart 

Comparison Between Calibres

It is a common belief that some “flat shooting” calibres have trajectories so much flatter as to significantly reduce errors in range estimation.  While there is some truth in that, it has often resulted in poor calibre choice.  So let’s take a closer look at two calibres, one of which is considered as fairly pedestrian and the other quite hot in the velocity stakes.  To make a fair comparison, let’s assume “standard” bullet weights and typical velocities for calibre.  As flat base bullets are also more used than boat tails, let’s use those too.

So let’s compare the 165 Grain spire point at 2600FPS in the 308 Win with the 130 Grain spire point at 3000FPS in the 270 Win. 

We have already shown in Fig 28 that the 308 equalises maximum hold over and hold under at 5” each from the muzzle to 300 yards when zeroed at 260 yards.  Fig 29 shows that the 270 equalises hold over and hold under at the same 5” to a range of 340 yards when zeroed at 290 yards.  It thus extends the range by only 40 yards at the same 5” hold over.

Figs 29 & 30            Equalising hold over & hold under

Missing trajectory chart

If zeroed at the same 260 yards as the 308, the 270 would reduce the maximum hold over and hold under to 3.25” from muzzle to 300 yards.

While the trajectory advantage of the 270 over the 308 is thus apparent, it is debatable whether it makes a material difference in the field.  The difference in remaining velocity at 300 yards is about 300FPS but the difference in energy is only 50 foot pounds.

To add a bit more perspective, the trajectory of a 30 calibre 180 grain spire point fired from a 30-06 at 2700FPS is about half way between the 308 and the 270 but the velocity at 300 yards is only 170FPS behind the 270 and the energy is considerably greater.

The 175 Grain spire point fired from a 7mm Rem Mag at 2700FPS is less than 100FPS faster than the 180 grain 30-06 bullet at 300 yards, and the trajectory is only half an inch flatter at all ranges out to 400 yards.

This shows that a great deal of nonsense is talked about “flat shooting” and that the advantages of the hot high velocity calibres are more imaginary than real.  On top of that, most game is taken at ranges under 200 yards, at which the hot calibres destroy more meat than the more modest velocity cartridges without noticeable improvement in killing power.

Point Blank Range

Above we discussed zeroing to a maximum hunting range and establishing what the high and low points of the trajectory will be at that range.  Point blank range is the same principle but the emphasis is reversed.  That is, we first take the size of the vital area of an animal, then calculate the maximum range at which a shot will hit within that vital area without allowing for bullet trajectory. 

To make that clearer, let’s use the same 308 load used in Figs 28 and 30 and assume that the vital area of the animal is 6”.

The method is similar to that described on pages 25 & 26 but is done in reverse, that is, the maximum hold over and hold under are first decided, which will be 3” above and below the line of sight in our example (Fig).

The trajectory diagram establishes that the point blank range of the example is 255 yards for a 6” target area.  Obviously it will be longer for a bigger target area and shorter for a smaller target, and will be different for each and every calibre and velocity.  It can easily be calculated with a simple diagram constructed from the ballistic tables in manuals such as the Hornady Handbook.

Fig 31   Point blank range

            308 Win, 165 grain, 2600FPS

Missing trajectory chart

Theory versus Practice

The above explanations of the min/max range and point blank principles are of course theoretical, because there is no other way to explain them.  But the reader will no doubt realise that a point blank circle of  6” diameter only holds good if the rifle will shoot one hole groups and if a good dead rest is available in the field.

If the rifle groups in 1.50” at 100 yards it will group in 3.75” at 250 yards.  When other factors like nerves, fatigue and lack of a convenient improvised dead rest are added to the equation, the 6” circle can be reliably hit only if the hunter is himself capable of shooting into one MOA or less in hunting conditions.  I am willing to bet good money that no hunter alive can do that.  The net effect, therefore, is to reduce the maximum range to the distance at which the 6” vital area can be hit reliably.  If that happens to be 150 yards it makes that the point blank range for that type of animal no matter what the theoretical range might be.

Therefore, while the point blank principle is a useful aid for scope zeroing, shooter ability (or lack thereof ) can render it theoretical and to be used with due caution.

Fresh Start

Posted by on July 16, 2011

This blog was set up for me by my good friend and IT guru Wouter De Waal, because he figured, rightly, that I needed one.   He started it with a bunch of postings I had posted to an e mail based firearm related forum in South Africa.   Because of my lack of computer literacy, I never learned how to drive the blog, so it has remained unchanged and unserviced since then.   That is about to change, but please cut me some slack as I learn how to use it properly.     Later, I will post something about my personal history, but the first post after this will be about sighting in rifles, as there seems to be a lot of confusion about it.