Perfect stability factor

[MOOSE39465]

Well, things that get missed often are cutting the chamber neck to fit the brass or cutting the brass to fit the chamber neck, cutting the throat to match the bullet and COAL. Most people just go with the chamber they get which has a neck sized to swallow any brass you find, brass from box ammo or bought for price and a throat designed to avoid overpressure.

It's surprising that any guns are accurate with a situation like that.

Good point. My new venture is custom dies made to match my chamber, but im still reading and learning.

 

[Mikecr]

The stability factor being talked about is really a transonic issue. If you read that on bergers website or through Bryan litz's books, it's said that the 1.5 stability factor is to make sure the bullet is stable using in reference to specific type of bullets maintaining stability from transonic to subsonic. If you don't shoot in that range, ie; 1000y or less, this level of stability in unwarranted. As long as you have a marginal stability, and are supersonic, you will be fine out to 1000y.

I disagree with everything you said here.
Gyroscopic stability matters from the instant bullets are muzzle released, from bullets snapping to sleep to bullets immediately tumbling, and certainly w/regard to accuracy. Why do you think a bad crown hurts accuracy?

In contrast, Sg in no way affects transonic issues (which are matters of dynamic stability, Sd). A bullet can be at a relative Sg of 7 by transonic, and if it's dynamically unstable, it will still tumble there. The infamous Sierra 168gr MK perfectly demonstrates this:
Released at 2800fps, 12tw(12" per turn), std. ICAO conditions, Sg=1.47
By 1100yds the bullet's effective twist rate has dropped to 5.4"per turn, relative Sg=7.17, and velocity at 1029fps (entered transonic)
The bullet consistently tumbles..

You did qualify your notions with <1000yds, and most bullets do not have an Sd issue, but being 'fine' with marginal initial stability is in fact -less than optimum. The reason for this is less than full gyroscopic forces to overcome the huge disturbance that is muzzle release.
Things have happened with bullet flight even before they've made it through your chrono screens.

 

[P7M13]

You would think a well built rifle, proper component selections, and proper reloading practices would be enough to obtain sub moa accuracy all of the time, but that's not always the case. I have a few factory rifles that will out shoot my customs. You just never know.

ROFLMAO!
One *would* think. I suspect many members here have experienced otherwise.

 

[L.Sherm]

I'll give you 2 examples of what I have.
1- 20 BRAI with a 9 twist shooting Berger 55's with a SG of 1.33 at 500 yards shoots awesome nice little round 20 cal holes.
I know a guy who has shot on at 1,000 yards with a 9 twist and shot under a 4" group in competition.
2- Multiple 30 Noslers and 30-28 with 10 twist running 3100FPS with 210-215 Bergers all have shot great. Just about all the BR guys I know run 10 twists in there 300wsm with 210-215 one record holder at Williamsport told me hes tested them all and the 10's shoot smaller at 1,000

 

[Buck Fever]

Good point. My new venture is custom dies made to match my chamber, but im still reading and learning.

That won't do much if your brass, chamber neck, throat and bullet are not matched up well.

 

[dfanonymous]

 

[L.Sherm]

I dont know what custom dies have to do with throat and bullets, it all about how your brass fits, never had them ask me what bullet or throat I have when I ordered custom dies.

 

[dfanonymous]

I disagree with everything you said here.
Gyroscopic stability matters from the instant bullets are muzzle released, from bullets snapping to sleep to bullets immediately tumbling, and certainly w/regard to accuracy. Why do you think a bad crown hurts accuracy?

In contrast, Sg in no way affects transonic issues (which are matters of dynamic stability, Sd). A bullet can be at a relative Sg of 7 by transonic, and if it's dynamically unstable, it will still tumble there. The infamous Sierra 168gr MK perfectly demonstrates this:
Released at 2800fps, 12tw(12" per turn), std. ICAO conditions, Sg=1.47
By 1100yds the bullet's effective twist rate has dropped to 5.4"per turn, relative Sg=7.17, and velocity at 1029fps (entered transonic)
The bullet consistently tumbles..

You did qualify your notions with <1000yds, and most bullets do not have an Sd issue, but being 'fine' with marginal initial stability is in fact -less than optimum. The reason for this is less than full gyroscopic forces to overcome the huge disturbance that is muzzle release.
Things have happened with bullet flight even before they've made it through your chrono screens.

Your right.

 

[josip89]

I disagree with everything you said here.
Gyroscopic stability matters from the instant bullets are muzzle released, from bullets snapping to sleep to bullets immediately tumbling, and certainly w/regard to accuracy. Why do you think a bad crown hurts accuracy?

In contrast, Sg in no way affects transonic issues (which are matters of dynamic stability, Sd). A bullet can be at a relative Sg of 7 by transonic, and if it's dynamically unstable, it will still tumble there. The infamous Sierra 168gr MK perfectly demonstrates this:
Released at 2800fps, 12tw(12" per turn), std. ICAO conditions, Sg=1.47
By 1100yds the bullet's effective twist rate has dropped to 5.4"per turn, relative Sg=7.17, and velocity at 1029fps (entered transonic)
The bullet consistently tumbles..

You did qualify your notions with <1000yds, and most bullets do not have an Sd issue, but being 'fine' with marginal initial stability is in fact -less than optimum. The reason for this is less than full gyroscopic forces to overcome the huge disturbance that is muzzle release.
Things have happened with bullet flight even before they've made it through your chrono screens.

I dont understand how twist rate ,on 1100y ,can be drooped from 12 to 5.4"
If I understand You,bullet with distance have more spin?

 

[skipglo]

Kind of like women. You can treat one exactly like another and get a totally different result.
From a womans perspective, I suppose you could say the same about men,

Nice comeback!

 

[skipglo]

Kind of like women. You can treat one exactly like another and get a totally different result.
From a womans perspective, I suppose you could say the same about men,

I would have to disagree here...men are more constant.....let's face it ...a woman buys us a new gun, 40 Rds of ammo and says "Honey...go have fun" we'd say Babe I LOVE YOU and pretty much all be out the door at the same time! Buy a woman a new purse and she's gonna ask you what you did wrong and torment you for the next hour!

 

[Steve gavette]

I dont understand how twist rate ,on 1100y ,can be drooped from 12 to 5.4"
If I understand You,bullet with distance have more spin?

In the barrel, the twist per inch is a physical property regardless of velocity. Once outside they are independent, and the spin tends to stay relatively constant while forward velocity decreases. So the turns per forward distance changes with speed.

I am assuming rpm stays relatively constant. I am not a ballistics expert, but this explains change in turns per distance.

 

[Tiny Tim]

I dont understand how twist rate ,on 1100y ,can be drooped from 12 to 5.4"
If I understand You,bullet with distance have more spin?

Spin rate decay is much less than velocity decay. The effect is that the bullet becomes more stable as the bullet velocity decreases. Thus, the effective gyroscopic stability (Sg) increases. It has the apparent effect of an increase in barrel twist from the original 1 in 12" to 1 in 5.4". Hope I'm explaining this correctly.

 

[josip89]

I know that velocity decreases much fuster than spin.
I know that bullet makers have thighter twist barrels to test with reduced load (simulated long range shoots).

But I dont understand how bullet can became more stable if they lose speed.
I have seen few rifles which shoot good and straight on 100 but on 600 they keyhooled and grouping is much bigger.