I want to build a 300 blackout capable of precision shooting at either muzzle velocity (MV) range. Is there any difference in twist rate required to stabilize a 30 cal round when shot at subsonic versus "normal" MV. Back of my mind I know twist rate will generate bullet RPM that is proportional to MV - so 1050fps would spin less than half RPM of normal MV with obvious implications for rotational inertia of the bullet. I have never seen MV mentioned when choosing twist rates, only bullet weight which would say pick the twist for the projectile and go with it. Is this the right way to go?
Gunsmithing Twist rate question - 1050fps vrs 2600fps MV
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Re: Twist rate question - 1050fps vrs 2600fps MV
8tw will stabilize 220 and 240 SMK's from the 300BLK
The answer to your question is "Yes, the MV affects stability" though it turns complicated when subsonic vs. supersonic flight is discussed. It is well understood, but unless you're a math guy it's likely not going to be a purposeful discussion. If you want more detail on where to look for the actual governing equations take a look at Bob McCoy's book <span style="text-decoration: underline">Modern Exterior Ballistics</span>
8tw will stabilize 220 and 240 SMK's from the 300BLK
The answer to your question is "Yes, the MV affects stability" though it turns complicated when subsonic vs. supersonic flight is discussed. It is well understood, but unless you're a math guy it's likely not going to be a purposeful discussion. If you want more detail on where to look for the actual governing equations take a look at Bob McCoy's book <span style="text-decoration: underline">Modern Exterior Ballistics</span>
Re: Twist rate question - 1050fps vrs 2600fps MV
It also depends on which bullets you are shooting subsonic and if you're shooting through a suppressor. If you just want to shoot lightweights like the 150gr stuff than 1-10 may be fine. If you want to shoot the heavies like the 220 or 240gr matchkings than you'll need a faster twist. We normally run 1-8 in our 30-221's with great results. I've seen guys shoot 220's out of their 1-10 barrels with oval holes in the targets at 100yd. That tells me that they just aren't quite stable and the risk of baffle strikes is high. The longer the bullet the faster the twist needed to stabilize. The matchkings and other target bullets with the hollowpoint can make matters worse by moving the CG of the bullet back. That means you need more spin to keep them pointy end forward.
As far as supersonic goes we've had no problems with the faster twist 1-8 barrels and light bullets. 130gr flatpoint 30-30 bullets in the bolt action Rem 799 in 30-221 were doing 2300fps and shooting into an inch or less. I think the spin rate created some more spectacular effects when we shot waterjugs as well. We also shot 100gr plinkers at 2450 but the groups opened up a bit. They aren't really target bullets....
Frank
It also depends on which bullets you are shooting subsonic and if you're shooting through a suppressor. If you just want to shoot lightweights like the 150gr stuff than 1-10 may be fine. If you want to shoot the heavies like the 220 or 240gr matchkings than you'll need a faster twist. We normally run 1-8 in our 30-221's with great results. I've seen guys shoot 220's out of their 1-10 barrels with oval holes in the targets at 100yd. That tells me that they just aren't quite stable and the risk of baffle strikes is high. The longer the bullet the faster the twist needed to stabilize. The matchkings and other target bullets with the hollowpoint can make matters worse by moving the CG of the bullet back. That means you need more spin to keep them pointy end forward.
As far as supersonic goes we've had no problems with the faster twist 1-8 barrels and light bullets. 130gr flatpoint 30-30 bullets in the bolt action Rem 799 in 30-221 were doing 2300fps and shooting into an inch or less. I think the spin rate created some more spectacular effects when we shot waterjugs as well. We also shot 100gr plinkers at 2450 but the groups opened up a bit. They aren't really target bullets....
Frank
Re: Twist rate question - 1050fps vrs 2600fps MV
Thank you for the guidance. I want to shoot suppressed, so that is a factor for me. I am not interested in the math, only the answer. Sounds like I need 1-8. Thank you bohem and biffj.
Thank you for the guidance. I want to shoot suppressed, so that is a factor for me. I am not interested in the math, only the answer. Sounds like I need 1-8. Thank you bohem and biffj.
Re: Twist rate question - 1050fps vrs 2600fps MV
if the pill stabilizes in the subsonic, it will stabilize in the supersonic. go with that thought process and you'll do well. some bullet skins are thicker than others, so you might have to play with supersonic combos.
if the pill stabilizes in the subsonic, it will stabilize in the supersonic. go with that thought process and you'll do well. some bullet skins are thicker than others, so you might have to play with supersonic combos.
Re: Twist rate question - 1050fps vrs 2600fps MV
We have found that by reducing the bullet weight by 13% to 15%.
will provide a stable and accurate load.
In the 1:10 twist 308 the 168SMK and 175SMK's are very accurate. Trying to shoot either of those bullets at 1055fps will cause keyhole strikes at 100 yds. By droping to a 150 gr bullet the 1:10 twist barrel will stabilize a subsonic. In order to maximize impact damage we use the Sierra 150 gr. SP.
Thru our suppressor we can maintain a sub MOA group at 100 yds, but there is a 13" drop in point of impact. Easy to compensate for the drop with a mil-dot reticle and zooming down the scope power.
Hope that helps.
We have found that by reducing the bullet weight by 13% to 15%.
will provide a stable and accurate load.
In the 1:10 twist 308 the 168SMK and 175SMK's are very accurate. Trying to shoot either of those bullets at 1055fps will cause keyhole strikes at 100 yds. By droping to a 150 gr bullet the 1:10 twist barrel will stabilize a subsonic. In order to maximize impact damage we use the Sierra 150 gr. SP.
Thru our suppressor we can maintain a sub MOA group at 100 yds, but there is a 13" drop in point of impact. Easy to compensate for the drop with a mil-dot reticle and zooming down the scope power.
Hope that helps.
Re: Twist rate question - 1050fps vrs 2600fps MV
The required twist is based more on the length of the bullet and the location of the CG. This is why round nose bullets will stabilize where pointy ones won't. The longer a bullet is the more it has to be spun to keep it from wobbling. With the round nose bullets the CG is forward rather than back so they are more stable inherently. Reducing the weight as Howard suggested will help but only because the lighter bullets are shorter.
Frank
The required twist is based more on the length of the bullet and the location of the CG. This is why round nose bullets will stabilize where pointy ones won't. The longer a bullet is the more it has to be spun to keep it from wobbling. With the round nose bullets the CG is forward rather than back so they are more stable inherently. Reducing the weight as Howard suggested will help but only because the lighter bullets are shorter.
Frank
Re: Twist rate question - 1050fps vrs 2600fps MV
My 300 Whisper/BLK is a 7.5 twist. Only thing I have put thru it is the two loads from Hornady in Whisper. Both stabilized.
My 300 Whisper/BLK is a 7.5 twist. Only thing I have put thru it is the two loads from Hornady in Whisper. Both stabilized.
Re: Twist rate question - 1050fps vrs 2600fps MV
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: biffj</div><div class="ubbcode-body">The required twist is based more on the length of the bullet and the location of the CG. This is why round nose bullets will stabilize where pointy ones won't. The longer a bullet is the more it has to be spun to keep it from wobbling. With the round nose bullets the CG is forward rather than back so they are more stable inherently. Reducing the weight as Howard suggested will help but only because the lighter bullets are shorter.
Frank </div></div>
C of P to C of G..When it's spinning in the barrel it's 'stabilizing' on a different center location than when it's spinning outside the barrel.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: biffj</div><div class="ubbcode-body">The required twist is based more on the length of the bullet and the location of the CG. This is why round nose bullets will stabilize where pointy ones won't. The longer a bullet is the more it has to be spun to keep it from wobbling. With the round nose bullets the CG is forward rather than back so they are more stable inherently. Reducing the weight as Howard suggested will help but only because the lighter bullets are shorter.
Frank </div></div>
C of P to C of G..When it's spinning in the barrel it's 'stabilizing' on a different center location than when it's spinning outside the barrel.
