Windage

[unclebuck]

My sako hunter .243 is zeroed at 100 yards for its 70 grain nosler ballistic tips homeloaded.
when I change cartridge to a 100 grain round nose Hornady interlock I expect to adjust the elevation. The 100 grain round fires about 3 inches lower , but why does it fire 3 inches to the left.
I have repeated this with a friends .243 and exactly the same difference appears.
The elevation difference seems quite obvious ,but why the Windage?

 

[Chops]

Unclebuck,

I think the problem may be the rifling in your Sako barrel. I have the same problem with my Sako when shooting 100g heads. The rifling in a Sako is 1in9", that is not enough to fully stabilise a 100grain head, I moved down to 90grains and starting punching 1" holes again!

I have never got my rifle to shoot 100grain heads well... though I know of some people who have got good results, they are still not compared to 90grainers.

Hope that this helps

Chops

 

[Bambi Basher]

The 100 grain round fires about 3 inches lower , but why does it fire 3 inches to the left.
I have repeated this with a friends .243 and exactly the same difference appears.
The elevation difference seems quite obvious ,but why the Windage?

Hi Unclebuck

One of the reasons why this occurs is because when a bullet travels up it sets up vibrations (think of a tuning fork when it is hit, the end vibrates) if you filmed it in slow motion the muzzle of your barrel it would be bouncing all over the place, these movements are small, but big enough to move your bullet impact point a few inches at 100 yrds.

Different bullet weights and the velocity at which the bullet is driven set up different harmonics in the barrel leading to different impact points.

When reloading you are trying to find the sweet spot in the barrels harmonics, this is the mid way between the extreme points at which the barrel vibrates. The bullet ideally needs to exit the muzzle at this point.

Conversely short fat barrels are more accurate than long thin ones as the “tuning fork effect” is reduced. Also that is why fitting a moderator makes a rifle more accurate as that great heavy piece of steel on the end dampens the vibrations.

I hope that makes sense if not I am sure somebody else will be able to explain it more eloquently.

B-B

 

[Watermain]

Wonderful posts. NOW I'm learning. THIS is the sort of thing I expected when I joined this forum.

Thankyou for treating us as adults. No smilie required: you know I'm pleased because my words tell you so.

 

[unclebuck]

Many thanks guys .I can only echo Watermains comments.
Do you think the fact that the 100grain bullets are round nosed compounds the problem ? if I switched to a 100 grain ballistic tip would it be better ? , I would like to be able to swap betwwen rounds with a few adjustment clicks on the elevation only.Or should I drop to a 90 grain BT and reduce the difference in weight between the two bullets ? I tend to use the 100gr for Reds and Fallow and the 70 gr for everything smaller.

 

[Bambi Basher]

Hi Unclebuck

IMHO the 100gn round nose could well shoot to a different point of aim than 90/95gn ballistic tips, this would be because the round nose bullet would have a greater surface area in contact with the barrel walls/rifling, hence setting up slightly different harmonics in the barrel.

I know of no way of predicting which bullets will have the same point of impact.
The only way is to keep trying different heads until if you are lucky you find one that matches your 70gn, also you can tweak your reloads a grain of powder less or more will alter the impact point slightly.

If you want a handful of 95gn Hornady SST or 95gn Nosler Balistic tips just to try rather than go out and buy 100 of this brand and a 100 of another brand only to find them unsuitable, then PM me your address if we live near enough to make it practical you are welcome to a hand full of each.

I personally use 95/100gn for everything, that way you have plenty for smaller species and if something bigger happens to step out then it is no problem.

I find keeping it simple works for me.

B-B

 

[unclebuck]

Many thanks
Bambi basher, I have sourced some Nosler 95 BT and will try those and let you know

 

[Tikka 260]

Just a thought on this issue.

"Barrel Twist Rate"

In the factory barrel that I had originally on the Tikka 595 I was never able to get a consistent result with 95 and 100 Grain bullet tips, however with 85 and lighter it was spot on and accurate consistently, I used the Sierra 85 Gn Gameking HPBT over 43 Gns of H414 with a Fed 210M primer.... obviously don't use that recipe verbatim, work up to it if thats any help.

You may find that the twist rate not suited to the heavier projectiles ?

ATB

Peter

 

[Bambi Basher]

Hi Tikka 260

Nice to see you here, I note with interest that your M595 would not shoot 100/95gn bullets, I use a M595 myself and have found that it shoots 100gn federal ammo, or hand loads with out any problems. The factory ammo in to a 1” but with reloads it comes down 5/8” at 100yrds.

Two of my friends have M595s in 243 and both of them shoot 100gn 243 factory ammo in there rifles, I could not say what groups there are shooting but they don’t seem to have any problems knocking deer over.

Strange that seeming identical rifles from the same manufacture do not shoot the same bullets equally well, just shows that you have got to spend a bit of time finding what suits your rifle best on the range.

B-B

 

[Tikka 260]

Amazing innit ? ..... different results , from the same (supposedly) barrels from the same factory , but the other variables are several. I am also of the opinion and it is only a personal opinion that 1MOA groups are good enough for jazz, and will certainly drop anything you wish, BUT and this was the reason that prompted me to 'customise' the original 595 , the action is just fine , but IT is capable of much better . In .260 with a Shilen barrel and a decent stock it will shoot 1/2MOA or less , always dependent upon the edjeet behind it. All a bit acedemic maybe, and It doesent kill any better or more often , but it gives me a great deal of fun to get the best out of it.

I know that customising a rifle is bound by the Law of diminishing returns, but in contrast to much of Life today, I still believe in the pursuit of excellence, and that applies as much to my stalking skills as to my indifferent shooting skills.

Ho Hum, whatever floats ones boat ..

Nice to participate in this non aggressive forum and so far no sign of root vegetables, or Lurcher enthusiasts.

ATB

Peter

 

[techman]

Unclebuck,
When I started Re-loading for my .243, I used N140 powder with 100grain Sierra bullets. I decided that I wanted more speed from these and went to N160 using the same bullet. To my surprise they landed 2" lower and 1" right at 100yards. When I asked my mentor why they hit lower when they should have been going faster, his reply was that the sonics cause the barrel to form roughly a figure of Eight as the bullet travels the length of the barrel. It depends on where on the figure, the bullet exits the barrel as to where it will land on the target.
Something that may be of interest.
The Lee Enfield .303 will give a 2" group at 200 yards. It will also give that same grouping out to 600 Yards. The reason is that with a 180 grain bullet travelling down the standard barrel at a MV of 2450 ft/sec the bullet exits when the barrel is at the bottom of its figure of Eight.
Use a different weight or speed and the whole thing goes out the window.