Longer bullets destabilizing downrange?

[QuazyQuinton]

OK, I definitely don't know everything about reloading and bullet selection, but I'm trying to figure this out. I understand that longer bullets require a faster rate of twist to be stabilized in flight. I've also heard that higher velocities help to stabilize bullets that might otherwise be marginal at a certain rate of twist. This implies that as a longer bullet loses velocity downrange, it might destabilize quicker than a shorter bullet. I'm wondering if I need to worry that a bullet that works well at 100 yards might slow down and not be flying as straight at 300 or 400 yards?

Specifically, I'm looking at the 175gr Nosler Partition for my 7mm Rem Mag. I know there are other 175gr bullets that are not recommended with the standard 1:10 twist I have, and I also know that bullet is heavy enough that I won't be pushing it real fast. It looks like it has potential, but I'm concerned about losing stability downrange.

QQ

 

[44hunter45]

Google "Don Miller's Formula". You will find some good explanations of optimum twist.
There are free downloadable calculators tailored to various type of bullets. (monolithic, lead/copper, etc)

 

[junior71]

We have played with seating depths, powder charge, and primers with barrels that didn't have enough twist. Its a pain!! We have got them to drive tacks at a 100 yards only to watch the group open up to 10" at 500. Then we just concentrated on our 500 group, getting to a 3" group at 500. Great right? we though!? Shoot it at a hundred and omg its shooting a huge group. What we realized was, we were actually controlling stability. These were berger 180s in a 10 twist 7mm, they have a sleek round boat tail.

Now on to your question and the benefits of using the partitions. We have shot the same bullet in a 7mm/10 twist, with awesome results! The partition has a flat base and imo is what stabilizes a bullet that some times shouldn't be. That being said, all guns are different and you need to test your gun. Get it shooting 1" group at 100, then move to 3,4, and 500. The groups at those distances will show the stability. At 300 yards you should be at or near 3" group, at 400 a 4" group, and so on. I think you will be fine.

 

[Fire_9]

If you’re concerned about it why wouldn’t you just drop a weight class? At 3-400 yards I don’t see a 175 partition in any scenario having an advantage over a 150.

 

[mtmuley]

If I had a 7mm I'd feed it 160 Accubonds. mtmuley

 

[88man]

No worries at 300 or 400. Your MOA grouping might grow a little but nothing drastic at that range. Have fun> don't theorize shoot the gun at 300 and 400.

 

[VikingsGuy]

My first choice for a 7mmRM would be 150 grain TTSX. Will take anything from coyote to moose with ease.

 

[std7mag]

My 7mm rifles all have either 1:9 or 1:9.5 twist.
That being said, i'm using the 150gr ABLR.

 

[cahunter805]

Most bullets tend to begin to destabilize when they hit transonic speed(1340FPS). A 175gr partition at 300-400 yards from a 7mm Mag won’t be anywhere near destabilization.
Are you sure your rifle is a 10 twist? Either way I’d recommend like others have said and just load some and shoot them at distance.
Also there are many other factors that can affect group sizes at longer distances.

 

[ImBillT]

Velocity only improves stability because it increases RPM. Increasing velocity does not increase SG as dramatically as twist rate. This is because although increased velocity increases gyroscopic forces, it also increases the use forces imposed upon the nose of the bullet by. The increase in gyroscopic forces is greater than the increase in force due to air resistance, but only slightly, so the net stability factor increases, but not much. So...as the bullet travels downrange the RPM of the bullet doesn’t decrease very much, but the force of the air on the bullet’s nose decreases substantially. Bullets get more stabile as they travel downrange until they drop down into the transonic range.

All that said, there are many other reasons that a load that shoots good groups at 100yds might not at 500yds. Generally stability isn’t one of them.

Use a stability calculator. An SG of around 1.1 or below starts to get pretty sketchy even the bullets aren’t keyholing. Above that, I wouldn’t worry about stability in air until your velocity becomes transonic. On an animal I prefer a little more just to be safe, but don’t have any evidence that it’s important. Your BC will drop slightly with SGs below 1.5.