Twist rate question

[IMMORTAL]

Possibly looking at getting a 6SLR rifle with a 1-10 twist barrel. Would I be able to shoot the 105 AMAX out of it or do I need a faster twist?

Thanks,Dave

 

[jscandale]

Bullet stability comes from rotational velocity. A 6mm SLR is probably capably of launching a 105-107 at about 3000-3100 fps. A 10" twist would yield about 223,000 rpm at the muzzle, most likely resulting in a 14" group at 200 yds. A 105-107 gr bullet truly needs about 240,000 rpm at the very least to stabilize over the long haul. A 9" twist would yield about 245,000 which is right on the cusp. A 1:8" would be just right.
The calculation is:
(12/bbl twist) x 60 x muzzle velocity = RPM

JS

 

[bdale]

1 in 8

 

[IMMORTAL]

Thanks for the info guys. I'm new to this long range stuff so please don't flame me if I ask what seems like a stupid question.

What do you think would be the heaviest bullet I could run in a 1-10? Rifle will be primarily used for long range ground hogs.

 

[Nomad47]

I have never measured the plastic tip length on a 105 AMax. According to the JBMBallistics stability calculator, if the tip is .175" or longer, a MV of 3100 fps will stabilize the bullet. The OP is inquiring about a 30" barrel. I have a 28" 8 twist in the 6SLR cartridge and 3150 MV is easily obtainable.

 

[jscandale]

You'll be able to use a 95 gr bullet just fine and they are a very good choice for varmints. A Sierra 80 gr blitz bullet is fantastic and turn chucks into red mist.

JS

 

[IMMORTAL]

Which powder would you guys recommend for the 6SLR?

Dave

 

[mikecr]

To clarify something important; Bullet stability does not come from rotational velocity(RPMs).
Gyroscopic Stability here, from a dynamically stable bullet, comes from ROTATION per DISPLACEMENT(1 turn per 8" displacement of ICAO air density).
The 105 Amax will not reach full Sg from a 1:10tw barrel no matter the velocity or RPMs resulting.
At sea level, & 59degs, 1:10tw, even 5,000fps/360,000RPMs would only bring this bullet to marginally stable(if it hadn't blown up at 4,000fps).
Yet it is fully stable, same conditions, 1:8tw, at only 2,000fps/180,000RPMs (half the rotational velocity)

I just think folks should know that RPM notions in stability fail tests, and therefore do not represent truths.

 

[LCazador]

To clarify something important; Bullet stability does not come from rotational velocity(RPMs).
Gyroscopic Stability here, from a dynamically stable bullet, comes from ROTATION per DISPLACEMENT(1 turn per 8" displacement of ICAO air density).
The 105 Amax will not reach full Sg from a 1:10tw barrel no matter the velocity or RPMs resulting.
At sea level, & 59degs, 1:10tw, even 5,000fps/360,000RPMs would only bring this bullet to marginally stable(if it hadn't blown up at 4,000fps).
Yet it is fully stable, same conditions, 1:8tw, at only 2,000fps/180,000RPMs (half the rotational velocity)

I just think folks should know that RPM notions in stability fail tests, and therefore do not represent truths.

I will second this post. After reloading the 220 Swift for almost 50years, those 55grain bullets still won't completely stabilize out of that 14 twist barrel, no matter what speed ! And todays 55 gr plastic tipped bullets are even longer and even more difficult to stabilize in a 14 twist. It's the oveall length of the bullet that must be taken into consideration when twist is calculated!

 

[jscandale]

Mike,

I also agree with your post. The proof of your assertion is proven with intermediate calculus. I always refer to rotation velocity in posts like this since the velocity window is relatively small and it almost like referring to a table to get a quick answer.

JS

 

[bdale]

90

 

[IMMORTAL]

Guys... I talked to Preacher (who did the chamber work) and he said the 6SLR barrel in question is a 1-8 twist so it looks like the 105 Amax's are GTG!! ;D

Thanks to all here.

Dave

 

[Nomad47]

That sure is good news, Dave. Did you get the range card info I sent?

 

[IMMORTAL]

That sure is good news, Dave. Did you get the range card info I sent?

Got it Bill. Thank you very much. Got a Sightron SIII 8-32 coming for the rifle. Now I'm trying to scrounge up some reloading components so I can start load development.

 

[Nomad47]

I think you already know, but for the 6SLR, Mr Whitley recommends Winchester or Remington 243 brass, but I use PRVI that I bought at Graf's. I also use 6XC dies, but since the 6XC f/l die does not size the bottom .122" of the case, I use a 243AI body die for my final sizing.
Good luck finding 105 AMax bullets. Actually, there is a gun store in Mt Vernon that has at least a couple hundred (when I checked two weeks ago), so if you get desperate, let me know and I will pick them up for you. I believe he was asking $24 per box.

 

[IMMORTAL]

I think you already know, but for the 6SLR, Mr Whitley recommends Winchester or Remington 243 brass, but I use PRVI that I bought at Graf's. I also use 6XC dies, but since the 6XC f/l die does not size the bottom .122" of the case, I use a 243AI body die for my final sizing.
Good luck finding 105 AMax bullets. Actually, there is a gun store in Mt Vernon that has at least a couple hundred (when I checked two weeks ago), so if you get desperate, let me know and I will pick them up for you. I believe he was asking $24 per box.

Will do Bill. Thanks again for all you have done.

Dave

 

[IMMORTAL]

I think you already know, but for the 6SLR, Mr Whitley recommends Winchester or Remington 243 brass, but I use PRVI that I bought at Graf's. I also use 6XC dies, but since the 6XC f/l die does not size the bottom .122" of the case, I use a 243AI body die for my final sizing.
Good luck finding 105 AMax bullets. Actually, there is a gun store in Mt Vernon that has at least a couple hundred (when I checked two weeks ago), so if you get desperate, let me know and I will pick them up for you. I believe he was asking $24 per box.

Bill...sent ya a PM!!