Mt 7/08 will group 11 out of 12 140gr NBT into 1 3/8 inchs @ 100yds, 5+ inchs @ 200yds, and no measureable group @ 300yds (cause I miss the paper). My 7mag will group a 160 Sierra around the same 1 1/4 to 1 3/8 at 100yds, and 4 inchs @ 300yds.
I use the same shooting technique for each rifle, but don't understand why the difference in accuracy when the 100yd groups are pretty identical? Any thoughts?

Pretty much all bullets become unstable at a certain range that depends on such things as bullet contour, barrel twist, B.C. and muzzle velocity.

quote:

Can bullets be stable @100yd but not @300yd?

To answer the question; yes.

Larry Gibson

AND your rifle/load might shoot good at 300 and terrible at 600 yards.

AND your rifle might shoot good at 600 yards, and shoot TERRIBLE at 1000 yards.

Actually the bullet is unstable. It just didn't have enough time to express any gross problems at 100.

Bullet RPM is a factor in stability.
As velocity decreases, RPM decreases. If RPM decreases below a certain point, they may become unstable.

can? yes...
likely, with a conventional bullet? no..

for example, the plastic core 7.62x39 rounds are pretty accurate to 100M... can't hit a city block at 200M, and are lucky to hit the ground at 300.

Sniper,
RPM's don't really decrease all that much, in relation to down range velocity...

if you have a 1:12 twist barrel, an launch it at 3000 FPS .. at the muzzle, its spinning, RPM, of what, 180,000 RPM?

there's been a good deal of research on this, in that the RPMs aren't facing the same draw forces as the forward velocity... gravitation and air resistence (bc) aren't applicable as much.

the work was done to test bullets are long range, and they found that results were NO WHERE NEAR consistent with launching them at lower speeds.

i THINK, and that's been 10+ years ago reading this, than RPM at 500 yards has only lost less-than-10% spin, but vels are north of -40%...

so, lets just say its 40% loss of vel..
2400 FPS, with 162,000 RPM

vs launching at 2400 FPS, resulting in RPM of 144,000

but can it be more unstable, farther? almost certainly, and CERTAINLY when it recrosses the sonic barrier

Jeff, you are correct.
It would have to be right on the edge at the muzzle for it to matter in a few 100 yards.

Hmm, wonder how much Newtons laws are effecting what you are describing, something about inertia?

I loaded some 60gr speer gold dot 32acp bullets in my 308 at 2600fps would shoot one hole groups with them at a hundred passed one fifty you didn't know where they went seem them strike many yards of to the side.

Could hear the hollow point whisle as they went side ways.

But at 50 yards they would turn a rabbit/p dog ect inside out.

Fun shooting varmints with them just vaporized them.

Yes they can.What you need to do is try several different loads,bullets and powder.Find loads that shoots well at a 100yds and then try those loads at 200 and see if it still groups well.It is quite surprising to see how loads that seem to shoot about the same at 100yds,compare at 200yds or farther.It will really open your eyes at just how good your load is at all ranges.

RPMs are important but I don't think they decrease much because the bullet follows the law of angular momentum. The issue should be the linear momentum is decreasing at a great rate compared to the angular momentum which magnifies any instability the bullet was experiencing from differences in center of mass vs center of form until it is going ass over tea-kettle.

How well does the bullet fit the throat? It could be entering the bore with a cant. That'll cause it to start out in a straight path then lose it's stability.

According to the spin stability tables the velocity does not influence stability much unless you get down to near sonic velocity where the spin needs to be faster. The faster a bullet travels the higher the destabilizing forces acting on it and therefore the faster it needs to spin. So, twist rate is virtually independent of velocity.

If these are both 9 twist barrels, there are other issues with the accuracy than the bullet being stable.

Absolutly, but 99% of the time if you try some different bullets you will find stablization with some of them...A lot of ifs and ans can be involved in stabilization such as twist that relates to rotation and bullet weight. But to cut through the gab just try some different weight and brands of bullets.