The Applications of different twist rates

[Mattnall]

Thank you Ian

As you say none give a definitive answer, I even had some .224" 1:7" barrels that would stabilise a 69gn but nothing heavier. Nothing wrong with the barrels we could find but about half a batch failed to shoot he heavier bullets but shot the lighter ones like a dream.

[Montana]

I shoot 1000-1200 pretty much every week with a 1:10 using 175gr SMKs.

Thats a 20" barrel.

I also have a 26" Remington 700 PSS that has the 1:12 twist, again I am shooting over 1000yds with it. (although maybe only once every 3 months or so)

The biggest issue with the Remington you are looking at is unlikely to be the twist rate, but more applicable will be the distance between the ogive and the lands with your standard rounds. The absolutely massive jump will kill the ability of the bullet to be accurate, and it seems like 99% of modern Remingtons have excessive throats.

The older ones (Like the Police PSS) are shorter and there work a lot better with factory ammo.

However, as a general rul you will be good to go with anything 1:10, and anything up to 168gr should work great in a 1:12, but the issue there is 168s are not fantastic for 1000yds.

You could get round both issues of course by shooting 155gr scenars.

[vikkz]

Many Thanks for the replies Will take all this in to account on what I buy next :)

[rox]

but the issue there is 168s are not fantastic for 1000yds

That's a bit of a generalisation; as a rule 168s are fine at 1000 yards - as long as you avoid the ones that were designed specifically for short range use.

The difference is in the boat tail angle; there's nothing generically wrong with the 168 grain weight (after all, 155 works well, and so does 175), but people often mistakenly state that 168s simply don't work on the basis of the Sierra 2200 168 grain HPBT Match King, which was designed for 300m shooting (before 6mm became dominant) and has a sharper (13°) boat tail angle that can cause instability when used at longer ranges (unless driven hard enough). Some of the Sierra 2200's clones, such as those from Nosler and Speer, share the same boat tail angle, but other 168s like the Berger VLD or Hybrid and the new Sierra 7768 TMK are definitely designed for long range, having boat tail angles in the 7°-9° range.

..

[Countryman]

Guys, hope you don't mind me pulling this back up.

My Remington is a McQueens machine but I'm just not getting there at 1000 yards which bugs me.

I've just agreed to buy a rifle with a 30" Truflite 1:12 barrel. I have 155gr Lapua Scenars as well as 155gr Palma match and 175gr SMK's. What's it going to like?

Can anybody help me with some load data for that elusive 1000yard Vbull?

[Sim G]

A friend has stopped trying to shoot his .308 "for score" at 1000. He has a 29" barrelled RPA and was launching 155gn Palma in front of large doses of medium to slow double based powders. He reckoned they were the wrong side of SAAMI specs for his piece of mind. Fine when everything goes to plan, but when Murphy becomes involved, like he will undoubtably do, he doesn't fancy being at the centre of an investigation.

There are far better cartridges than the venerable old .308 for 1000 yards.

[Countryman]

Great Sim G I just wasted my money. Thanks for that help buddy.

[Laurie]

The primary determinant of the required bullet spin-rate which in turn determines rilfing twist pitch is bullet length, or more accurately bullet length in relation to calibre, modern stability formulae starting by dividing one by the other to calculate a bullet's length in calibres. So, a 90gn Berger 0.224 VLD which is nominally 1.263" has a length of 5.64 calibres whilst the 0.308" calibre Berger 185gn LRBT Juggernaut which many believe to need an inordinately fast twist is 1.353" long and divide that by 0.308 and you get a length of 4.39 calibres and it needs a substantially slower twist rate than the 22.

At 2,800 fps MV, both consistent with 30-inch barrel FTR or TR type rifles in .223 Rem and .308 Win, their optimal twist rates are 90gn / 22 = 1 in 6.75" and 308 / 185 is 1 turn in 11.75". That's at standard ballistics conditions - 29.92 inches mercury air pressure, 59-deg F air temperature. If you do all you shooting at Thule AFB in Alaska in winter, you'll want a tighter twist; if at the US NRA Shooting Center, Raton in New Mexico in summer, some 7,500 ft ASL, you get full stabilisation with considerably slower twists.

When I say 'optimal' that is twist rate / bullet / MV combinations that produce stability factors (Sg) of 1.5 or above. Theoretically, any bullet that exceeds 1.0 is stabilised, but traditionally 1.4 was recommended as the desirable value. Recent research has shown that whilst bullets given Sg values of say 1.2 and above are apparently stabilised and may group very well, their aerodynamics as quantified by a BC (ballistic coefficient) figure are degraded. So, run the 185gn Juggernaut in a 13" twist barrel (which was a common practice amongst American sling shooters in 'any bullet weight' matches) at 2,800 fps and you get an Sg value of 1.23 which experience has shown 'works' extremely well at 1,000 yards, but the bullet's manufacturer says drops the actual average G7 BC from 0.284 to 0.261, an 8% drop in efficiency, so you get a bit more movement in a wind change.

All of these values are calculated using modified 'shortcut' versions of the Miller's Twist Rule formulae. A full twist rate / stability calculation as done for say artillery shells is a long multi-task business, but the shortcut is easily close enough to reality for rifles. The Greenhill Formula is a complete waste of time unless you're a BPCR shooter using short, heavy, large calibre, flat-base bullets. The addition of a boat-tail rear end alone makes Greenhill unreliable as it requires a 1" 'faster' twist or thereabouts than the identical weight / length / calibre / MV bullet with a flat-base.

If you want to play go into Berger Bullets' ballistics webpages, click on 'Ballistics' and scroll down to 'Twist Rate Calculator' and you get a menu driven calculator that covers all Berger BT / VLD bullets with separate advice on flat-base models:

http://www.bergerbullets.com/twist-rate-calculator/

If you're not shooting Bergers, but have or can borrow a sample bullet, measure its length and simply overwrite Berger's database value.

So far as a 12" twist 30-inch barrel rifle goes for 1,000 yards this limits usable bullets a bit, but nothing like as much as many people seem to think.

Taking Berger's bullets, all 155s and the superb 155.5gn BT Fullbore @ 3,000 fps are easily full stabilised with Sg values ~1.6

the 168gn Hybrid, a superb long-range bullet at 2,900 fps = 1.50 exactly

The 175gn LRBT, (a much more aerodynamic design than the competent but hardly sparkling 175gn Sierra MK) at 2,850 fps = 1.47 marginally slow (in theory, but not in real life)

The 185gn LRBT Juggernaut at 2,800 = 1.44 which in real life also works superbly.

The heavier Hybrids, 185gn and above, are very long bullets for their weights and really need an 11 inch or 10 inch twist to give their best. Likewise, the excellent 210gn LRBT at 2,650 will work in a 12 twist, but is better with a faster version, 12-inch producing Sg in the low 1.2s, 11-inch running at 1.49 in effect optimal; 10 inch giving just over 1.8.

So what works in a 12" twist 30-inch barrel 308 at 1,000 yards? Any 155 on the market. The higher BC models are the newer Sierra MK (#2156), Lapua Scenar, HBC, Berger 155.5gn BT (and Hybrid, but it's not nearly so easy to 'tune').

The Berger 168gn Hybrid (seat so it's into the lands a modest amount)

175 and 185gn Berger LRBTs - superb performers and easy to tune.

The newer 178gn Hornady 'Match' (hollow-point BT, not AMAX) looks promising.

The 175gn Sierra MK is a good tolerant design but rather short of 'legs' at 1,000 yards. At 2,900 fps, it's running at 1,275 fps at 1K and moves 0.89-MOA per 1 mph change in a true 90-deg crosswind.

The 175gn Berger at the same speed is 1,371 fps / 0.8-MOA

The 185gn Juggernaut at 2,800 fps is 1,400 fps / 0.75-MOA

The higher BC 155s at 3,000 MV run at ~1,330 fps and 0.85-MOA per 1 mph wind change.

Why is speed important? Ideally we want to stay above the top end of the transonic speed range which is somewhere around 1.2 MACH or 1,350 fps. In practice, experience has shown the critical boundary is ~ 1,250 fps some 125 fps above the speed of sound. The 175gn SMK at 2,900 fps is getting very close to that value. Drop significantly into transonic speeds and groups enlarge and wind effects are enlarged, nearly doubled in a US Army investigation some years back with the old FA 173gn FMJBT match bullet. Drop further into some bullets being supersonic at the target (~1,125 fps), some not, and you are potentially in trouble even with a good natured design like the 175gn SMK.

MV wise, you need

155s - 3,000 fps

168 Hybrid - 2,900 fps

175s - 2,850 fps

185 Juggernaut - anything above 2,650 fps but they really shine when you get into the 2,750-2,825 fps MV bracket.

[Countryman]

Laurie, thank you for a fantastically well put together answer.

My struggle with the 175gr SMK in my Remington was I figured that I just wasn't getting enough muzzle velocity from the shorter barrel.

I'm delighted that this should be an interesting gun to shoot with a wide range of bullets. In bulk, I have the items listed but I also have a few boxes of heavier bullets that I had earmarked for plinking with my .300 win mag sporter.

Right away I go to have a nose at my loads.

[snayperskaya]

The Greenhill Formula has no mention of weight, it assumes a uniformly dense projectile but works quite well with the variable density of modern jacketed and composite bullets.

It is true that most bullets of a given calibre get longer as they get heavier, but tracer are longer and lighter than many of their FMJ/solid contemporaries and will require a faster twist than an FMJ of the same weight. That is why a 63gn 5.56 tracer will need a twist rate similar to an 80gn 5.56 match bullet to stabilise (and the much shorter 63gn FMJ stabilise just great in the fast twist the military use).

Bit of a sidetrack but for example Russian 54r tracer bullets (T-46) are a fair bit longer than its light ball counterpart but are the same weight, the later T-46-M bullet (designed to ballistically mimic the light ball round more closely) is shorter than both but is still the same weight.

The Russians altered the twist rate on the Dragunov,my barrel has a twist of 1:320mm or 1:12.6 inches which is the original SVD twist before they tightened the SVD to 1:240mm or 1:9.4 inches to better stabilise the longer tracer and armour piercing bullets.