Sako 85 .243 100gr loads

I could never get my Sako 75 stainless to shoot decent groups with 100gn bullets. They were always ok, but nowt flash. I went to 90gn nosler BTs with H414 and shot a 0.2" 5 shot group off the back of the pick up. I stopped messing around with loads after that! Not what you wanted to hear though I guess, as I ended up rebarrelling in 260 as 90gn bullets weren't ideal for bigger fallow I was shooting.
 
I use 100 gr serria sp with 35.5 grns of imr 4895 and it shoots 1/4 inch groups no fuss
 
my sako 85 ss shoots 100g norma factory loads very well excellent rounds also shots 87g vmax with n150 with great results too
 
My Sako 75 .243 shoots under an inch at 100 metres with 105 grain Geco. So 1 in 10 twist can still shoot heavy bullets with a decent group.
Mark
 
I shoot a 75 and 85 in 243, both love 35.5gr of ADI 2208 (Varget) behind 100gn Sierra Boatail and Hornady flatbase pills, shoot 1/2-3/4 groups. I load for my Finnwolf also in 243 using 36.5gr 2208 and 87gr Hornady flatbase pills, 3/4 groups, magazine length limits distance to lands with this rifle. Brass is Lap and CCI200 primers.
Jerry
 
What barrel length and twist rate ?
For 100gr you need a 1:8 twist.
Simply not true, I have two 1:10 twist 243's - that is true 1:10 - and they both shoot 100gn bullets without any problem. I have measured some 1:10 at 1:10.5-9.
You might find that a flat base will work better if the Boat tail is 'marginal' (shorter bullet).
One other wrinkle that I have noticed is that most handloaders do not use a chronograph when working up loads they just pray that the book velocities are good enough - false assumption.
If you have a 'marginal precision' load at or very near to max book velocities you prolly need the faster twist, if on the other hand your loads are a little like a slug on prozac there is likely some head-room to speed them up so that they might stabilise.
Bullet RPM is proportional to velocity, bottom line need a chrono!

Remember Bullet length vs twist is what matters.
 
Simply not true, I have two 1:10 twist 243's - that is true 1:10 - and they both shoot 100gn bullets without any problem. I have measured some 1:10 at 1:10.5-9.
You might find that a flat base will work better if the Boat tail is 'marginal' (shorter bullet).
One other wrinkle that I have noticed is that most handloaders do not use a chronograph when working up loads they just pray that the book velocities are good enough - false assumption.
If you have a 'marginal precision' load at or very near to max book velocities you prolly need the faster twist, if on the other hand your loads are a little like a slug on prozac there is likely some head-room to speed them up so that they might stabilise.
Bullet RPM is proportional to velocity, bottom line need a chrono!

Remember Bullet length vs twist is what matters.

Thanks Sauer 90; which chronograph do you favour?
Thanks
Andrew
 
I only ever used 100g federal powershocks in my 85 .243 and they were one of the most accurate bullets I've shot.
 
My Sako 75 .243 shoots under an inch at 100 metres with 105 grain Geco. So 1 in 10 twist can still shoot heavy bullets with a decent group.
Mark

Glad you posted this, just bought some to try but had it at the back of my mind that 105gn would never stabilise in my 1 in 10 Sako A7.
 
+1 for Magnetospeed. Seems to be the most accurate of all the chronos I've tried, plus its hard to destroy!
 
It's not bullet weight that primarily determines the rifling pitch rate needed, rather bullet length and specifically length in calibres. That's why the less dense and longer for weight Barnes copper bullets need to be spun faster than same weight but shorter lead core models. In fact, if two bullets in a calibre have identical lengths but different weights, the heavier model actually requires less spin, hence gets away with a slightly 'slower' rifling twist rate.

Then on top of that, boat-tailed and flat-base bullets of similar / identical length have different requirements, the BT model need faster rotation for full gyroscopic stability. In 6mm if both types are the same weight/length, the flat-base model needs around a full 'inch slower' rifling. In the real world, 100gn 6mm PSP and BTSP bullets are rarely if ever the same length, the flat-base type being shorter, and hence managing with a considerably slower twist barrel.

Gyroscopic stability is measured against a coefficient of stability (Sg) under standard ballistic conditions (59-deg F, 0% humidity, 29.92 inches mercury atmospheric pressure) where an Sg value of 1.000 theoretically just stabilises the bullet. For years, ballisticians' advice was to achieve a value of 1.40 or more to cover all conditions and for optimised external ballistics performance. Recent tests for HPBT match type bullets have raised that value to 1.50 as lower values were found to give increased in-flight air drag and hence a lower BC. That's much less of an issue for short range (by F-Class standards) deerstalking, but somewhere around 1.25 as a minimum is an excellent idea, and getting up to the 1.4 mark is needed for use in exceptionally cold and high-pressure conditions. (The original prototype 5.56mm AR-15 / XM16 assault rifle had a 14 inch pitch twist for what became the M193 55gn FMJBT until it was taken to Greenland for Arctic winter warfare trials. Whilst the 14 rate barrel apparently worked OK in summer continental US conditions, it could hardly hit a barn door at 100 metres in minus 20 temperatures.)

Bryan' Litz's book Ballistic Performance of Rifle Bullets provides full data on over 250 models including optimal and minimum twist rates, but is skewed towards long-range, primarily match models and therefore includes few flat-base types. Looking at his 6mm examples though he does cover the 100gn Sierra PSP Pro-Hunter whose OAL is 1.025", and is likely typical of such 100gn models. Litz gives a 'nominal case' Sg of 1.56 in a 10-inch twist barrel, ie optimal, and a 'worst case' (very cold and high pressure) value of 1.35 which will still work more than well enough in stalking. Hence, the oft-quoted advice that many 100 grainers work in 10-inch twist 243s.

But not all 100gn bullets. Find a longer Boat-tail model, say the 1.083" length Hornady 100gn BTSP which despite a very short boat-tail section has the effect of depressing Sg values and hence needing faster twists. The 'nominal case' 10 inch twist Sg value reduces to 1.33 and 'worst case' to 1.15. S0 - should still work in most conditions, just.

If you take a still longer 105gn match bullet such as the Lapua Scenar at 1.25" length, the two Sg values are now 0.92 and 0.79, ie definitely unstable as soon as they leave the muzzle.

Because Litz is writing primarily for L-R match and specialist varmint / game shooters, his 'case' assumptions generally involve longer barrels and higher MVs than are seen on the UK scene with cut-down 18 and 20-inch barrel moderated rifles. If you want to play with Sg values, it's very easy to get a calculation yourself providing you have the bullet length and some idea of your actual MV with that bullet. Just Google 'Millers Twist Rule' and you'll eventually find a site where you can download a simple little Microsoft EXcel spreadsheet and you do it yourself.

QuickLOAD suggests 2,750 fps MV isn't unreasonable from an 18-inch barrel and input this for the 100gn Sierra 1540 FB (QL says 1.040" OAL) into Miller and you get:

Caliber0.243Inches
Bullet Weight100Grains
Bullet Length1.04Inches
Barrel Twist10Inches/turn
muzzle velocity2750fps
Temperature59degrees Fahrenheit (59 is standard)
Pressure29.92inches of mercury (29.92 is standard)


Sg =1.48
Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may
gain some accuracy by going to a slower twist barrel.


Even at well below freezing, say 20F, Sg is still good at a calculated 1.37.


But the 1.150" OAL 95gn Berger Hunting VLD gives

Caliber0.243Inches
Bullet Weight95Grains
Bullet Length1.15Inches
Barrel Twist10Inches/turn
muzzle velocity2750fps
Temperature59degrees Fahrenheit (59 is standard)
Pressure29.92inches of mercury (29.92 is standard)
Sg =1.05


under standard conditions which is at a level where performance can't be guaranteed and may only work in some conditions. In winter ... not a chance. Several ballistic software packages such as JBM provide this information and Berger Bullets' website has a menu driven twist rate calculator with a database of all Berger models.


 
It is quite simple, really. A flat base bullet, and especially a round nosed bullet, has more of its mass distributed near its outer circumference, than does a long, boat-tail bullet. So it requires a lower rate of revolution to generate the rotational inertia of the bullet, which is the center ( or integration ) of the mass times the square of the radius. So, as this mass is moved towards the perimeter, the inertial force increases exponentially.

Shifting the weight to the rear also makes the bullet settle down more quickly, dampening out any wobble around the axis of rotation. A bullet with a large plastic tip distributes more weight to the rear and closer to the rifling, so if you want to shoot a boat tail bullet, try something like a Nosler BT or SST, and stay with jacketed lead rather than solid copper like the Barnes, unless you have a long barrel, more powder and a faster rate of rifling, like the 6mm Remington has.

In general, expect to have to push 100 gr bullet to 2,900 fps to get it to settle down quickly enough for a set of them to track identically enough for small groups on target.
 
I use RWS100 grain soft points for red deer in myT3 lite .they shoot very accurately in friends T3s both varmint and RPA.they shoot to same point of impact as 87grain v max in my rifle,covering all bases
 
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