bullet destabilization

[Screwey1]

What would cause a bullet to destabilize suddenly? Can shoot a pretty consistent 4-5 inch group at 850 yards , at 900 yards, flight gets erratic and with 0 consistency. I thought maybe the bullet was going subsonic , but according to my ballistics program that should not happen until around 1300 yards.
Shooting 270wsm with 1 in 10 twist, 129 grain bullet with a .463 BC at 2950 fps. I am a little confused.

 

[Mateo]

My guess is not enought twist for that weight of bullet at that range.

 

[Mikecr]

If it was twist he would not be shooting well at 850. He would not even be shooting well at 25..

 

[Screwey1]

I guess I should clarify just a bit. At 900, that's where it begins to fall apart. It's not that it just completely can't hit anything at all, it's just that accuracy begins to collapse. At 1000 yards, It can only hit a 10x10 target a couple times.
At 100 yards I can cover a 4 shot group with a dime.
I have looked at a lot of the posts here trying to find answers and most seem to also believe that a stabilized bullet at 700 will be a stabilized bullet at 900 or 1000.
For that reason, I can not understand what we are seeing. For now, cut-off for distance is 850 yards for this rifle.
In talking to a friend , he told me of a similar situation with a factory rifle " falling apart " at 600 yards.
Any idea what could be causing this? What can I try to remedy?

 

[d11r529]

????? I've got a factory Remington Sendero 270 and am experiencing the same thing. Good groups at 850 yards but all bets are off at 1000. I've gotten the same results with 4 different bullets running from just over 3000 fps to just under 3200 fps??????

 

[FearNoWind]

270wsm with 1 in 10 twist, 129 grain bullet with a .463 BC at 2950 fps. I am a little confused.

Actually, Mateo may very well be headed in the right direction. Assuming your bullet is .277 @ 129 grains and a naked length of 1.3 inches with the MV you listed and assuming your atmospheric pressure is around 30 inches of mercury with a temperature somewhere between 65 and 80 degrees I suspect you'd do better with a 1:9 twist. For your 1:10, try a 150 grain bullet (suggest Berger Hunting Bullets in 270 caliber at 150 grains VLD HPBT) and see if that works better for you.

For purposes of discussion, how long is the naked bullet and is it tangent or secant ogive?
Just try to remember that bullet weight isn't enough by itself to determine optimum twist rate. Bullet caliber, material, weight, shape, length, bearing surface, are all factors to be considered.

 

[Mateo]

This is very interesting. I'm really curious what the solution is and why it's doing this.
I'm just guessing here. But I would think as the bullet slows down and maybe spinning slows too? And 1/10 twist isn't that fast. Kinda like a football. The slower the ball is thrown the more the spin helps. You can throw a fast ball and it don't take much spin to keep it flying. A slow lob needs a bit more spin to keep it straight.
I'd be really curious what a different weight bullet would do. Shoot as light a bullet and as heavy a bullet and see? You can buy 10 pack bullets here in Canada for load developement. A good cheap way to find out?

 

[bigngreen]

What is your load shooting for velocity spread? usually if they are running out of twist it shows up by shooting and some group and some drop dramatically low and I mean by feet.
I shoot 165 and 175 gr Matrix bullet in my 270 WSM with a 1-10 twist and they are very stable to 2000+ yards so a 129 should be no issue.

 

[Mikecr]

Who makes a 270 129gr bullet?

 

[FearNoWind]

Who makes a 270 129gr bullet?

Barnes .. they are solid copper and have a BC of around .365 - .375 (depending on who you talk to; some claim 463 but BC changes with velocity and I'm skeptical about the .463 figure. Barnes' own technical data is inconsistent depending on where you find this bullet advertised) and are often set with a jump of up to .50; sometimes more. OP doesn't say if that's what he's shooting though.

 

[Screwey1]

I'll try to answer as many questions as I can remember.
Yes, Barnes bullet, length of 1.345 velocity spread average 18fps. tangent ogive. I am confident , in my case , that the BC is accurate as advertised (.463). when punched into ballistics calculator , I get accurate shooting solutions out to 850 in different temps at different altitudes on different days. Altitude spread has been over a 5000 ft difference. What happens to BC after 850, I have no idea.
Barnes gives no minimum twist rate requirements for this bullet. Maybe a call to manufacturer is in order.
Does anyone know common twist rate for a 270wsm? The logic I have heard on that so far is the easiest to wrap my mind around at this point. However, when I hear that someone is pushing a 175 gr bullet from the same twist rate, it confuses me yet again. I was under the impression that heavier bullets require a tighter twist.

 

[Screwey1]

In reference to bullet jump, this is set at .015. Don't know if that would have a difference or not when it shoots as well as it does at the distances it shoots well, but as I am looking for all available info, I feel all available input may be useful.

 

[elkaholic]

As far as rotational velocity slowing down too much, that is highly unlikely at any reachable distance! Rotational velocity decreases far more slowly than forward velocity.The b.c. COULD be dropping off much faster than lead core bullets, but that still doesn't explain loss of accuracy. Normally, a 10 twist would be PLENTY with a 129 gr. .270, as has already been noted...........Rich