When someone says that "Subsonic ammunition utilizing heavy, boat-tail projectiles do not stabilize adequately to avoid baffle contact in barrels with a twist rate slower than 1 turn in 8 inches." - do they mean that the "slower" twist rate would be like a 1 turn in 6 inches, or does a slower twist rate mean a higher twist, like a 1 in 12?
Twist rate question
- Thread starter Greywolf
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nitesite
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Slower twist has higher second numbers in the ratio.
i.e.- a 1:7 is faster than a 1:12
i.e.- a 1:7 is faster than a 1:12
nitesite
Member
Greywolf said:
Forgive me for my ignorance but the only boat-tail bullets that I know of are fired from center-fire rifles. No handguns, no rimfires. And to my knowledge, there is no such thing as a subsonic centerfire cartridge.
And what does stabilizing in a barrel to avoid baffle contact mean, exactly? I guess I can always learn something new.
griz
Member
There are subsonic centerfires, the 147 grain 9 mm is one example.
nitesite
Member
Okay, I guess I didn't state my case very well.
"... to my knowledge, there is no such thing as a subsonic centerfire cartridge." I should have inserted the word "rifle" in there.
As I was thinking about centerfire rifle bullets being the only "true boat-tailed projectiles", it never occured to me about such things as the 147-gr 9mm. I knew 'bout that one. But heck, so then is the .45ACP, .38 Spl, and on and on and on... And none of them are boat-tails. See where I'm going with my train of thought?
My post was only focusing on centerfire rifle boat-tails, and I did not know of the 300 whisper. I did say that I could learn something on this thread!
Sometimes I open my mouth (or type) without proofreading.
I still would like to learn more about what the original authors question is about.
"... to my knowledge, there is no such thing as a subsonic centerfire cartridge." I should have inserted the word "rifle" in there.
As I was thinking about centerfire rifle bullets being the only "true boat-tailed projectiles", it never occured to me about such things as the 147-gr 9mm. I knew 'bout that one. But heck, so then is the .45ACP, .38 Spl, and on and on and on... And none of them are boat-tails. See where I'm going with my train of thought?
My post was only focusing on centerfire rifle boat-tails, and I did not know of the 300 whisper. I did say that I could learn something on this thread!
Sometimes I open my mouth (or type) without proofreading.
I still would like to learn more about what the original authors question is about.
Tag
Member
a 1:7 twist rate is faster than a 1:12. (the bullet spins all the way around every 7 inches as opposed to 12)
I wonder if they are talking about the bullet making contact with the flash suppressor or whatever the thing on the end of an M60 is called?
I wonder if they are talking about the bullet making contact with the flash suppressor or whatever the thing on the end of an M60 is called?
ConcernedCitizen
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Actually, they are referring to SOUND suppressors, not FLASH suppressors. What they mean by a baffle strike is the bullet hits an internal part of the sound suppressor. Imagine a normal washer, a flat plate with a hole in it, that the bullet has to pass thruough. The bullet passes through the washer, or baffle, but the gas and sound bounces off, and gets reflected around inside the suppressor chamber. I'm sure there is more to it than that, but it's the best way I can explain it at the moment.
Now, a baffle strike is when the bullet hits the baffle, instead of going through the hole perfectly straight. This can severly damage the suppressor, and possibly cause injury to the shooter, I imagine.
If the bullet isn't stable enough, and wobbles, it can cause a baffle strike. That is why they recommend a higher twist rate with slower subsonic bullets.
As for subsonic ammunition, there are companies that make this in most of the popular calibers, such as .223 and .308. It's basically a really light load, with a heavy bullet. I can't find a link to any of them right now, but I have seen them.
Now keep in mind that I am not an expert by any means, so you should always do your own research. I hope this helps.
CC nailed it.
As one might expect there are significant disadvantages to subsonic rifle rounds. The only reason to use one is to allow the effective use of a silencer/moderator/suppressor. While it's possible to suppress the BLAST from a supersonic round, the bullet still makes a crack from exceeding the speed of sound.
Shooting a subsonic bullet eliminates the "crack".
However, shooting a bullet at fairly low velocity means that it's not spinning as fast as it normally would be when it exits the muzzle. If you don't have enough spin the bullet will wobble or even tumble. That means you need a faster spin (smaller second number) than you normally would for a given bullet.
For a suppressor to work well, the bullet needs to be a close fit to the baffles--in pistols for very close work sometimes rubber "endwipes" are used. These actually contact the bullet for the ultimate in close fit. Of course they wear out rapidly and touching the bullet wrecks long-range accuracy.
For obvious reasons endwipes are not acceptable in rifles so close fitting metal baffles are used to allow the bullet to pass while bleeding off the high-pressure gases behind the bullet.
Any wobbling or tumbling of the bullet is very hard on the suppressor, the bullet, and possibly the gun and shooter.
As one might expect there are significant disadvantages to subsonic rifle rounds. The only reason to use one is to allow the effective use of a silencer/moderator/suppressor. While it's possible to suppress the BLAST from a supersonic round, the bullet still makes a crack from exceeding the speed of sound.
Shooting a subsonic bullet eliminates the "crack".
However, shooting a bullet at fairly low velocity means that it's not spinning as fast as it normally would be when it exits the muzzle. If you don't have enough spin the bullet will wobble or even tumble. That means you need a faster spin (smaller second number) than you normally would for a given bullet.
For a suppressor to work well, the bullet needs to be a close fit to the baffles--in pistols for very close work sometimes rubber "endwipes" are used. These actually contact the bullet for the ultimate in close fit. Of course they wear out rapidly and touching the bullet wrecks long-range accuracy.
For obvious reasons endwipes are not acceptable in rifles so close fitting metal baffles are used to allow the bullet to pass while bleeding off the high-pressure gases behind the bullet.
Any wobbling or tumbling of the bullet is very hard on the suppressor, the bullet, and possibly the gun and shooter.
Tag
Member
thanks for the explaination guys, learned my new thing for the day 
now... what is a 'boat-tailed' bullet? Is that were they taper off on back?
now... what is a 'boat-tailed' bullet? Is that were they taper off on back?
ConcernedCitizen
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Exactly, it's tapered or beveled in the back, for better aerodynamics. Less drag.
If you ever have a chance to pick up a decent reloading manual, I would highly recommend it, even if you don't reload. Lots of good info and drawings, as well as info on the cartridge history itself. Good reading.
Ahh, JohnKSA
Newbe here. I believe the bullet is spinning the same twists/ft. independent of it's speed. I also believe the reason for using a faster twist rate for subsonic rounds is since the bullets are going slower they have to weigh more to be effective. What determines the twist rate you need is the mass of the bullet. The heavier the round the more twist you need to stabilize it. 55gr. .223s would give pretty poor results going <1100ft./sec. Just my opinion. savoo
Newbe here. I believe the bullet is spinning the same twists/ft. independent of it's speed. I also believe the reason for using a faster twist rate for subsonic rounds is since the bullets are going slower they have to weigh more to be effective. What determines the twist rate you need is the mass of the bullet. The heavier the round the more twist you need to stabilize it. 55gr. .223s would give pretty poor results going <1100ft./sec. Just my opinion. savoo
It has the same TWIST (revolutions per foot) but since it's traveling MUCH slower (feet per second) the SPIN rate (revolutions per minute) are much slower.The bullet will still make the same number of spins on the way to the target, but it will take much longer to get there.
Stabilization is dependent upon SPIN rate not TWIST. But, in a given muzzle velocity range the spin rate for a given twist rate will be pretty close so people talk about a given twist stabilizing or not stabilizing a bullet. When you have a bullet travelling a third of normal velocity when it leaves the barrel you get a lot slower spin rate and therefore bullets that might have been stable with the twist rate of that barrel may not be stable.
Example.
Twist rate is 1:10
Muzzle velocity is 3000fps
Spin rate is 18,000 rpm
Twist rate is 1:10
Muzzle velocity is 1000fps
Spin rate is 6,000rpm
Stabilization is dependent upon SPIN rate not TWIST. But, in a given muzzle velocity range the spin rate for a given twist rate will be pretty close so people talk about a given twist stabilizing or not stabilizing a bullet. When you have a bullet travelling a third of normal velocity when it leaves the barrel you get a lot slower spin rate and therefore bullets that might have been stable with the twist rate of that barrel may not be stable.
Example.
Twist rate is 1:10
Muzzle velocity is 3000fps
Spin rate is 18,000 rpm
Twist rate is 1:10
Muzzle velocity is 1000fps
Spin rate is 6,000rpm
Jim March
Member
"Twist rate" has a different meaning in San Francisco.
There, it's the speed at which a new resident to the city picks up kinky behavior.
There, it's the speed at which a new resident to the city picks up kinky behavior
.Brian Dale
Member
We interrupt briefly for the following announcement:
Welcome to The High Road, savoo!
Now back to the technical discussion.
Welcome to The High Road, savoo!
Now back to the technical discussion.
Shawn Dodson
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The Hornady 9mm 147gr XTP-HP is a boat-tail handgun bullet.
45crittergitter
Member
Greenhill Formula for rifling twist, assuming gilding metal jacketed lead core bullets:
150 x dia x dia
Bullet length
Note that the Greenhill Formula does not approximate the optimum rate of spin, but approximates the required (minimum) rate of spin.
150 x dia x dia
Bullet length
Note that the Greenhill Formula does not approximate the optimum rate of spin, but approximates the required (minimum) rate of spin.
savoo,
Yes. And actually, as shown in the formula, it's not the weight but the length of the bullet that is the critical feature.
Also, if you search around a bit on Greenhill's formula, you will find that there are different forms (different constants) based on the range of muzzle velocities to be dealt with.
(Sorry about the delayed response--I've been out of town.)
Yes. And actually, as shown in the formula, it's not the weight but the length of the bullet that is the critical feature.
Also, if you search around a bit on Greenhill's formula, you will find that there are different forms (different constants) based on the range of muzzle velocities to be dealt with.
(Sorry about the delayed response--I've been out of town.)
John, your info in your 11/14 post is good, but the examples are flawed. Taking the first one, for example:
Twist rate is 1:10 [revs:inches]
Muzzle velocity is 3000fps [feet per second]
3000 ft/second X 12 inches/foot X (1/10 revolutions/inches) = 3600 revolutions per second
Spin rate is 60 sec/minute X 3600 rev/sec = 216,000 rpm
(18,000 rpm wouldn't stabilize almost any bullet you can name.)
Twist rate is 1:10 [revs:inches]
Muzzle velocity is 3000fps [feet per second]
3000 ft/second X 12 inches/foot X (1/10 revolutions/inches) = 3600 revolutions per second
Spin rate is 60 sec/minute X 3600 rev/sec = 216,000 rpm
(18,000 rpm wouldn't stabilize almost any bullet you can name.)
yep, goofed on the math (forgot to convert feet to inches), but the principle is still sound. Dropping the mv by a factor of 3 drops the spin rate by the same amount--at some point the bullet won't be stabilized. Conversely increasing the mv enough can stabilize a bullet by increasing the spin rate.
Problem with the latter is that most calibers are already operating near max mv for obvious reasons.
Problem with the latter is that most calibers are already operating near max mv for obvious reasons.
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