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#5485397 - 12/18/14 09:08 AM Accuracy at distance
dee Offline
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Registered: 04/10/10
Posts: 4665
Loc: Red River way
There are a lot of people who claim that some bullets aren't as accurate at 100 as they are at distance, well Bryan Litz wants you to prove it. He is providing use of his range and equipment to you but be aware he has yet to produce results that agree with this theory. Here is a link to one of the forums where he opens the challenge.

http://www.longrangehunting.com/forums/f19/applied-ballistics-shoot-thru-target-challenge-144359/
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#5485413 - 12/18/14 09:20 AM Re: Accuracy at distance [Re: dee]
redchevy Offline
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Registered: 10/25/04
Posts: 23444
Loc: Texas
popcorn
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#5485452 - 12/18/14 09:34 AM Re: Accuracy at distance [Re: dee]
jeffbird Online   content


Registered: 03/09/09
Posts: 1720
Parallax issues probably account for a lot of these reports.

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#5485486 - 12/18/14 09:48 AM Re: Accuracy at distance [Re: dee]
ChadTRG42 Online   happy
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Registered: 09/16/09
Posts: 9425
Loc: Lewisville, TX
Very nice!! I would like to make this road trip. I have seen this issue with the 208 A-max in a 308 Win, 115 DTAC in a 243 Win, and 180 VLD in a 7mm-08. We have shot some 208 A-max in a 1:12" and 1:11" twist out of a 308 Win and seen this problem. This is why I will NOT test long skinny bullets at 100 yards. I will test them at 200 or 300 yards for accuracy. I have had 2 customers that I have done custom load development with a 208 in a 308 Win short barrel (18" and 20" barrels) with 1:10" twist. The accuracy was great from 300 out to 700 yards. At 100 yards, the groups would be about 3/4". At 300, the groups would be less than an inch. The main problem we had though, was at 800 and out, the groups would open way up and not have any kind of grouping. It would be all around the target within several feet, and this was the case at 900 and 1K. Technically, the bullet is stable at these distances in a 1:10", and shot nice and tight from 300 to 700 yards. At 800 things opened up, and 900 to 1K was all over the place. I still do not know why these acted like this. And it was 2 separate customers with 2 different 308 Win rifles both 1:10" twist.
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#5485698 - 12/18/14 11:10 AM Re: Accuracy at distance [Re: dee]
catslayer Offline
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Registered: 12/11/12
Posts: 971
Loc: Straight out of Johnson County
Sounds like you can get a free trip to play at somebodies range chad?! let us know lol

ok for real I read some of this and have a question. Chad you will probably be best to answer this.

Ok so assuming that you ARE having this issue, not parallax not aiming... nothing like that.

It seems these issues happen with more heavyish for size calibers from the posts. sooo HERE IS MY THOUGHT

so your shooting this heavy slug down the barrel, and you've probably got it loaded a on a little bit of a hot load. So the pressure is a high, not crazy high but could the bullet "jump" some of the rifling's? so that is starts spinning but doesn't get to the rpm that is SHOULD. because of this is has a bit of a knuckle ball thing going on early because of the slow spin, but as the bullet moves down range the spin corrects this and it gets its act back together.

So the issue is the combination of the lands, rifling, bullet, pressure combo. The only way I can figure to check this is figure out how to measure the spin on a bullet as is leaves the barrel and compare it to what it "should" be, OOR to capture the bullet downrange with NO damage and look at the cuts from the rifling's...

ok there is my W.A.G.

tell me where I messed up now
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#5485720 - 12/18/14 11:20 AM Re: Accuracy at distance [Re: catslayer]
FiremanJG Offline
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Registered: 12/16/08
Posts: 17893
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I can't see it gaining RPMs down range. Its as fast horizontally, and spinning as hard as it ever will at the muzzle.
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#5485728 - 12/18/14 11:26 AM Re: Accuracy at distance [Re: dee]
catslayer Offline
Tracker

Registered: 12/11/12
Posts: 971
Loc: Straight out of Johnson County
Not gaining rpm. It's spinning slower, so it takes more time to stabilize that big bullet. It's impossible for it to gain energy I get that but a gyro has to spin up to stabilize. In this case time equals yardage cause it's moving pretty dang fast
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#5485782 - 12/18/14 11:56 AM Re: Accuracy at distance [Re: dee]
ChadTRG42 Online   happy
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Registered: 09/16/09
Posts: 9425
Loc: Lewisville, TX
First, bullet stability is based on bullet length, not weight. So the longer the bullet, the more twist needed to stabilize it. Generally, your heavy for caliber bullets are longer, like the 208 A-max.

When I load the 208 in a 308 Win, I have the bullet seated touching the lands, not jammed, but just touching. This does 2 things. It helps with the accuracy, and allows the most room for the powder. So, there is no jump to the rifling. The bullet will be spinning at whatever twist rate the barrel is (generally). I've seen some issues with the ratchet style rifling firing subs and such. That's another topic, though.

You are correct about the "theory" of epicyclic swerve, which is what Bryan is trying to reproduce in his challenge. Think of it like a spinning top that you played with when you were a kid. As soon as you spin the top and release it from your fingers, it has some wobble to it when it hits the table. After a split second of spinning, the top smooths out, and is perfectly balanced. This is the "theory" of epicyclic swerve with a bullet. The moment it leaves the barrel and at 100 yards, it is not perfectly on track in it's flight path and has some yaw to it. You see this in the accuracy of the bullet at 100 yards and compare it to the results you get at 300 yards. At 300 yards, the bullet has traveled far enough to be fully on track of it's flight path and have no yaw, or epicyclic swerve.


Bullet RPM formula: Bullet RPM = MV X 720/Twist Rate (in inches)

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#5485803 - 12/18/14 12:09 PM Re: Accuracy at distance [Re: dee]
redchevy Offline
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Registered: 10/25/04
Posts: 23444
Loc: Texas
In theory if you weren't spinning the top by hand "read human error" would it take time for it to stabilize? or would it be stable from the get go?

Could it be cases by imperfections in the crown/barrel throwing it off and making the bullet have to re-stabilize.
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#5485874 - 12/18/14 12:53 PM Re: Accuracy at distance [Re: redchevy]
catslayer Offline
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Registered: 12/11/12
Posts: 971
Loc: Straight out of Johnson County
Originally Posted By: redchevy
In theory if you weren't spinning the top by hand "read human error" would it take time for it to stabilize? or would it be stable from the get go?

Could it be cases by imperfections in the crown/barrel throwing it off and making the bullet have to re-stabilize.


chevy, as for the human error thing I think it would. You still have lateral energy to get the top going and at "pull" would have an equal and opposite reaction, which in turn would be stabilized by the gyroscopic force.(unless I missed something)

They talk about the crown being an issue latter on in that forum post the op put up. and they make some pretty good arguments against it. But the guy says if your rifle shows it bring it on.

CHAD... Question, ok so stability is a result of bullet LENGTH? Ok yes bullet weight would directly correlate to the length... but if I put a hollow bullet with the same profile in the rifle. wouldn't the bullet balance differently with the spin? I mean we talk about some rifling's stabilizing bullets better I feel like this is just an extension of this?

these are questions... I LOVE this stuff... The fact that chad makes a living with this kinda stuff makes me very very jealous lol

Chad since you've experienced it what do you think it is?


Edited by catslayer (12/18/14 12:55 PM)
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#5485876 - 12/18/14 12:54 PM Re: Accuracy at distance [Re: dee]
dee Offline
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Registered: 04/10/10
Posts: 4665
Loc: Red River way
Elliptical dispersion or yaw is said to be minimal. Usually 1/10th of one caliber is the referenced number but that's not enough to cause the difference.
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#5486055 - 12/18/14 02:42 PM Re: Accuracy at distance [Re: dee]
ChadTRG42 Online   happy
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Registered: 09/16/09
Posts: 9425
Loc: Lewisville, TX
cat,

What you are referring to is the center of gravity (COG) vs. the center of pressure (COP) for the bullet. Take a shorter, flat base, spire pointed or round nose hunting bullet vs a bthp bullet. The distance between the COG and COP will be greater on the bthp compared to the SP or RN bullet, which will require more spin to stabilize the longer bullet. Think of a .308 Sierra 220 RN vs a 220 bthp. The bthp is longer and will require more twist to stabilize. Also, the greater the distance between the COP and COG makes the bullet less stable. This is why the heavy solid copper bullets need more twist than the lead bullets of same weight. The copper bullets are longer than the same weight lead bullets, and need more twist. Copper is less dense than lead.

220 RN
https://www.sierrabullets.com/store/product.cfm/sn/2180/308-dia-220-gr-RN

220 BTHP
https://www.sierrabullets.com/store/product.cfm/sn/2240/308-dia-220-gr-HPBT-MatchKing


Originally Posted By: catslayer
bullet weight would directly correlate to the length.

Not always, but generally yes. See the 220 RN vs 220 bthp links above.
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#5486065 - 12/18/14 02:48 PM Re: Accuracy at distance [Re: dee]
ChadTRG42 Online   happy
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Registered: 09/16/09
Posts: 9425
Loc: Lewisville, TX
I am fairly certain I can show this yaw issue between 100 and 300 yards. I don't think 200 yards is far enough distance to get all the yaw out completely. But 100 to 300 I'm pretty confident. I've already been emailing Bryan and talking to him about this. He said he has not tested the 208 A-max in a 308 Win like this. In my personal 308 Win with 1:11" twist, the 208 A-max is just barely stable enough to work, and it would be a great example for this project. I need to push it around 2400-2450 fps and I think it will work.
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#5486124 - 12/18/14 03:17 PM Re: Accuracy at distance [Re: dee]
dee Offline
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Registered: 04/10/10
Posts: 4665
Loc: Red River way
Chad is your typical experience with long bullets that are marginally stabilized or have some had more than a high enough sg?
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#5486146 - 12/18/14 03:34 PM Re: Accuracy at distance [Re: dee]
jeffbird Online   content


Registered: 03/09/09
Posts: 1720
Chad,
are you interested in trying Bryan's challenge?

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