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Thread: Question about rifling and stabilization

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    Default Question about rifling and stabilization

    I was just watching a show discussing the original M16 and I have a question. They mentioned the rifling was changed from 1:14 twist to 1:12 twist after the Air Force determined that the 1:14 was not stable in severe cold weather. Would one (or more) of you smart people explain how the tighter twist makes a difference in severe cold?

    Forgive my ignorance but I just don't get it.

    Thank you!
    BEE

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    Member Ryan J's Avatar
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    Air is denser?

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    Cold air is more dense. To stabilize a bullet, it is calculated based on standard atmosphere. (29.92 Hg, 27 C., MSL, etc) changing these parameters works this way. A more dense atmosphere, needs faster spin to stabilize, less dense requires less spin. The 223 @3100 fps was marginally stable with 1:14" and 50 grain bullets. With longer bullets, even 55 grains it was not. The 55 grain was marginal with 1:12", thus its tendency to loose stability at impact into "denser media" of blood, guts and rice. We now use 1:7", or 1:8" to stabilize longer bullets for longer ranges, loosing some of the effectiveness of the round against human targets. Loss of stability at impact cause a bullet yaw, veer off course, turning over, end for end and doing more damage than what a straight through shot would do. (This all primarily applies to just FMJ's, non expanding spitzer bullets.)

    I would have responded sooner but the bar was high with the smart people limit. I hope this helps a little.

    The 5.56 ball M193 ammo has a velocity of 3100 fps not 31000 fps. oops.
    Last edited by Murphy; 09-26-2012 at 22:27. Reason: too many zeros.
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    Moderator Daveinthebush's Avatar
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    Quote Originally Posted by Murphy View Post
    thus its tendency to loose stability at impact into "denser media" of blood, guts and rice.
    Humm. I never though to see if my targets were holding their rice. I might have violated the Geneva Convention with my actions. Oops :-(

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    Also colder temps usually means lower muzzle velocity which means less rpm and less stability.

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    Thank y'all for the explanations! I had never thought about it before watching the show. BTW Murphy- all kidding aside, one of the reasons I am on these forums is the fact there are a bunch of smart people here willing to share knowledge and I appreciate it! Thanks for taking the time to post.
    BEE

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    Murphy,

    I'm curious about your comment on standard atmospheric. Since you mentioned 29.92 and sea level I'll assume you meant 15 C, if not please explain the 27 C number, that's a little warm.

    I've seen very low pressure numbers on a few ballistic programs and wondered why they use those. 29.53 shows up frequently but standard is 29.92 at sea level on a cool 15C day. Is standard for ballistics different than standard for say aviation?

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    Quote Originally Posted by marshall View Post
    Murphy,

    I'm curious about your comment on standard atmospheric. Since you mentioned 29.92 and sea level I'll assume you meant 15 C, if not please explain the 27 C number, that's a little warm.

    I've seen very low pressure numbers on a few ballistic programs and wondered why they use those. 29.53 shows up frequently but standard is 29.92 at sea level on a cool 15C day. Is standard for ballistics different than standard for say aviation?
    No standards are the same I just don't know what they're supposed to be too many computers. I thought 22 C but maybe that's wrong too. I didn't mean 27 but didn't think anyone would notice. Point being temperature and barometric pressure effect things that fly like bullets and airplanes. The difference in temperature and pressure causing a bullet to become stable (actually never stabilize) pretty much means the twist wasn't fast enough in the first place. We're splitting hairs here. But temperature, barometric pressure and humidity effect a bullet in flight.

    My main ballistics program has 29.92 as "standard" and 72 degrees F. and altitude above or below sea level. So I think those are the important factors. I have my local altitude in it and put in the corrected barometer and temp. before I run any of my calculations or bullet drops.
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    OK, just wondering since I see so many programs out there using different pressure numbers.

    Just for the record, standard is S/L 29.92 and 15C /59F. The guys up in Alaska are closer to those numbers than I down here in hells gate.

    I've read each 1000 foot difference in elevation and or 10 degrees in temp will affect trajectory 1 MOA. My testing area is 2100 AMSL and generally 85+ degrees F. I call and get the local airport altimeter setting for record. There is a difference of 1000 feet density altitude between 29.92 and 28.92. Combustion engines will make more power when the pressure is higher than 29.92.

    My hunting unit last year was 6800 AMSL and 30F. My point of impact was identical at 200 yards confirmed be testing. 5000 feet higher, less dense and 50 degrees colder, more dense appeared to cancel each other out in my example. Can it really be that simple?

    I haven't used a Kestrel yet. I'm sure you have hands on time with one of those. When you take a pressure reading I'm wondering if just plugging that actual number in the ballistic program compensates for elevation values compared to S/L. I'm guessing the pressure reading drops as you climb to higher elevations, that would be standard. I'm assuming that's how these things work in the ballistic program. I just haven't had the need to drop the bucks to acquire one of this instruments for extended range shooting yet.

    Thanks in advance,

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    You guys are killing me with all these numbers. Simple, when in doubt use a faster twist.

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    Quote Originally Posted by rbuck351 View Post
    You guys are killing me with all these numbers. Simple, when in doubt use a faster twist.
    Easy for you to say. Try this on for size. A 375 H&H or Ruger with a 300gr bullet only needs a 1:16 twist by accepted formula practices. However, you will only find them sold in 1:12 twist.

    When you step down to the small burners there are several options and picking the correct twist for your desired bullet length / weight can be important.

    I once saw a guy that was loaded so hot in a over twisted barrel that he was spraying chunks of bullet and jacket at the 25 yard line. Why so close? Because he couldn't hit anything he shot at so I asked him to move in so we could adjust the scope. Turns out he was red lined and spinning the jacket off his bullets.

    Just remember, never shoot a strangers reloads.

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    I'm thinking he wasn't having any doubts about his twist being fast enough. When in doubt (ie marginal stability ) go a step faster twist. Had a buddy that was shooting 110gr Speer plinkers in his 300WM with a case full of powder. It looked like he was shooting a shotgun at about 20yds. I don't think he was worried about lack of stability from lack of rpm either.

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