Questions on 168 past 600 yards

[1stmar]

I've frequently read on this board that the 168gr bullet becomes unstable after 600 yards and accuracy deteriorates significantly. I cannot recall if this phenomena is unique to all 168s or Sierras smk. That is the first question, is it all 168s ? Secondly what is it about the 168? I haven't seen anyone state this about 150s or 175s.

[Bart B.]

Accuracy gets worse when any spitzer boattail bullet slows down to the speed of sound. They start buffeting and change directions. Just like jet planes did as their speeds were fast enough to approach the speed of sound.

It just so happened that it occured often when 7.62 NATO match ammo with 168-gr match bullets with a lower BC than the former 173-gr ones were used in long range events past 600 yards with 22 and 24 inch barrels. Bullets slowed into the subsonic range about 800 to 900 yards. This started in the early 1980's in the USA.

Military rifle teams would replace the 173-gr arsenal match bullet in M118 match ammo with Sierra 180-gr match bullets. The Sierra's were more accurate and so used, won a lot of matches and set some records. In the mid 1980's, Sierra changed that bullet's boattail from a long, 9 degree one to the shorter, 13 degree one like their 168 has as a cost cutting scheme. That new 180 shot out the same 2600 fps from 22 inch M14 barrels but it's lower BC let it go subsonic about 900 yards down range. Just like Sierra 168's did. The U.S. Army contracted Sierra to make their 180 HPMK bullets with the original boattail then they started winning again.

In the British Commomwealth countries shooting 7.62 NATO service ammo with 147-gr bullets at 2800 fps from 22 and 24 inch barrels in their long range matches in the early 1960's, same problem happened. As their rules only allowed arsenal ammo (no handloads nor commercial ammo), their solution was 30 inch barrels. They shot those light, low BC bullets out at 3000 fps. That kept them supersonic through 1000 yards.

When Sierra released their new 155-gr 30 caliber Palma bullet in 1991, people had to shoot them from 30 inch barrels from 308 Win cases. They learned what the British learned 30 years earlier. 175, 190 and 200 grain match bullets shot from 24 and 26 inch barrels left fast enough to stay supersonic though 1000 yards.

[1stmar]

So keep spbt bullets above subsonic out to the distance you want to shoot..I know there are ballistic tables, is there a formula I can use ie.. bc x muzzle velocity x bullet weight

[Bart B.]

I suggest spitzer boattail bullets be shot fast enough to stay at least 50 fps above the speed of sound to the target.

Ballistic tables or software is the easiest way. You'll need to factor in atmospheric conditions as they effect the speed of sound. JBM and Berger Bullets web sites have free software that's pretty good.

There are math formulas to figure it out like computer software does. Maybe on the Internet's a site that has them.

[4EVERM-14]

The 168 was supposedly originally for 300 meter international matches. As a high quality bullet it was a good choice to replace the 173 in govt. match ammunition. M852 became the go to Service Rifle load for XTC shooting. The short coming was the lower BC for long range. Pushed hard enough the 168 would make 1000 yards sometimes entering the 10 ring sideways. But the pressures were borderline especially in gas guns. The 175 was as the long range replacement.

[1stmar]

Went to http://www.shooterscalculator.com and Sierra's site. I found the initial BC of the 168SMK is .462 with velocities above 2600, it drops to .405 with velocities below 1600. Using their calculator and starting with a BC of .462 and a velocity of 2700 (all other factors are default). It looks to stay above 1100FPS all the way to 1000yds. If I change the BC to .405, it appears to stay above 1100 out to about 900 yards. Recognizing that all other factors wont be the "default", ie atmosphere, temp etc. It doesn't sound like it would be hard to push it to stay above 1100 out to a 1000. I was conservative on the velocity, 2700 is about mid to mid low, should be able to get to 2800 or maybe 2900. At 2900 and a starting BC of .405 it remains above subsonic to 1000. Granted all other values are default. I thought I read that beyond 600, 168's were unstable. So even if you aren't going to 1000, should be able to get well past 600 even at different atmospheres and elevations...
Cartridge is 06

[T. O'Heir]

The 168 was supposedly originally used in .30 AP ammo. Found to be extremely accurate in matches, Marines, I think, just after W.W. II. Like 1946 or '47. In any case, it's not so much that 168's are no good for past 600 as it is 175's are better. It's a physics thing.
Palma Matches and their 155's don't count as the rifles are very different.
"...Commonwealth countries shooting 7.62 NATO..." The DRCA(Dominion of Canada Rifle Association) used single shot, bolt actions(usually, but not always, No. 4 Lee-Enfields) with iron sights(usually PH). Ammo was provided by DND. There's an Australian company that made barrels that DA and IVI really liked. Forget the company name .

[Bart B.]

Speed of sound in air at 70 deg F is 1128.17 fps. So the low safety limit's 1178.11 fps by my preferences. The bullet will stay well stabilized at that speed. Any slower and it starts buffeting which changes its direction.

A .30-06 can easily keep a Sierra 168 moving over 1178 fps at 1000 yards leaving at 2800 fps from a maximum load.

[Mobuck]

accuracy deteriorates significantly.

How much "significantly" is depends on the actual accuracy requirement. If an additional 1/2 MOA puts you outside the X-ring, that's significant(but not significantly inaccurate).

[1stmar]

Need to get out an chrono my loads. My best loads are on the light side but my barrel is 28".

[EchoM70]

The 168gr SMK fired out of .308 1000 feet above sea level with a 2650 muzzle velocity will still be traveling 1353 FPS at 800 yards. Which is about where I stop using a 168gr bullet. But up to 800 yards its a great bullet.

One the biggest things to look at when choosing a bullet for long range is where does it enter a transonic state? Meaning when a bullet goes from super to subsonic. If this happens before your intended range your accuracy will suffer greatly as the bullet will be unstable.

[30Cal]

The 168 is a good bullet, but for 600+ yds, all of the other match bullets available today will beat it in the wind.

[Bart B.]

T.O., the AP bullets in 30-06 service ammo were 165 grain, flat based bullets. Some lots were as accurate as M72 match ammo and service rifle teams often used them; all the way to 1000 yards with decent accuracy. It depended how well centered the steel core was in the bullet.

Starting in 1958, Sierra's 168 Int'l Match bullet was used in .30-06 bolt action match rifles as well as Garands and produced better scores through 1000 yards than M72 match ammo if the barrel's groove diameter was less than .3080".

[mavracer]

When Sierra released their new 155-gr 30 caliber Palma bullet in 1991, people had to shoot them from 30 inch barrels from 308 Win cases.

The current 155 Sierra palma bullet has a .504 BC that shouldn't require a 30" barrel to get to 1K am I missing something.