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Posted: 2/12/2002 7:42:15 AM EDT
[Last Edit: 2/12/2002 7:43:53 AM EDT by Woodcarver]
I would like to know if anyone has shot with both a 1 in 7 twist and a 1 in 9 twist and if you had a noticible difference in accuracy between them. Does barrel length matter a 16" or the 20"? I have been told that the 1 in 7 works best with the heavier weight rds but the 1 in 9 works with them all. What have been your experience's shooting 50gr or 55gr in a 1 in 7?

I have a Bushmaster lower with a Colt 20" HBAR 1 in 7 upper. Would it be worth changing it to a 1 in 9? I'm not doing any match shooting but I would like to get good groups out about 200 yds.
Thanks.

Link Posted: 2/12/2002 7:45:42 AM EDT
I have 1/7, 1/9 and 1/12 barrels...and I shoot W3131A...accuracy is good in all 3 ...I seen no reason to swap a 1/7 for a 1/9 unless you have special circumstances...
Link Posted: 2/12/2002 7:53:11 AM EDT
Link Posted: 2/12/2002 8:23:55 AM EDT

Originally Posted By raf:
Your 1-in-7" twist bbl should be ideal with 69 gn BTHP match bullets.

Barrel length affects two things (usually).
Sight radius: Longer is usually more accurate.
Muzzle energy and velocity: Longer is more.




Sight radius is typically the same once you get past 20" due to gas tube length, unless you plan on getting a fancy clamp-on match front sight assembly.

Barrel length also affects harmonic vibration of the barrel. Longer barrels deflect more due to vibration. This is why a lot of barrels past 24" see diminished accuracy.

1/7" twist should be OK for everything down to 55 grains. Lighter, faster bullets, like 45 grain varmint bullets, are likely to have their jacket shredded by a barrel twist that fast.
Link Posted: 2/12/2002 8:38:52 AM EDT
Link Posted: 2/12/2002 9:31:13 AM EDT
Maybe if you are shooting compitition it matters but for the average joe shooter I do not think it makes a diff. I shoot a 9in9 a 1in7 and a 1in12 twist and they all shoot just fine. That is just me though.
Link Posted: 2/12/2002 9:33:22 AM EDT
what is the difference in 1 and 7 or 1 and 9 twist?
Link Posted: 2/12/2002 10:58:37 AM EDT
[Last Edit: 2/15/2002 7:05:01 AM EDT by Boomholzer]

Originally Posted By Citabria7GCBC:
what is the difference in 1 and 7 or 1 and 9 twist?



The skinny:
The twist-rate (# rifling turns per length) determines how much inertial or gyroscopic stability is put on the bullet. There is a sweet spot in relation to the twist-rate and bullet mass when it comes to having the bullet ideally follow the natural arc trajectory or ballistic curve as the bullet travels down range. An aggressive twist rate on a relatively lighter bullet will tend to cause the bullet to remain pointed upward during the negative portion of the ballistic curve. This would add to inaccuracy at longer ranges.

Since I typically use ammunition around 55-68gr, I tend to lean towards the 1:9 twist-rate barrels. I believe that for a 55gr bullet in itself, a 1:12 twist-rate would be optimum. A 1:9 twist rate is most optimal for bullet masses around 68gr. The optimal bullet weight of a 1:7 twist barrel is about 80gr.

Edited to add that these numbers are for 5.56mm, these rules do not apply across the board.
Link Posted: 2/12/2002 11:04:36 AM EDT
oh ok, i understand now. thanks!
Link Posted: 2/12/2002 12:06:19 PM EDT
1 in 7 means its going to rotate the bullet 1 time, in 7 inches. This means that you will have more rotations, for example, compared to 1 in 9 with same length barrel.

This also means that you can shoot any bullet weight since there will be a good spin on it. Kinda like throwing a football, the harder the spin before it leaves your hand, the less it wobbles.

I have heard that occasionally the jacket may shred. If you notice strange holes in your target, or lots of blood coming from the target, then you might switch ammo. So far, I have not had this problem with my 1/7 Colt 20" HBAR.
Link Posted: 2/12/2002 1:27:21 PM EDT
[Last Edit: 2/13/2002 7:31:37 AM EDT by Boomholzer]

Originally Posted By Energizer:

This also means that you can shoot any bullet weight since there will be a good spin on it. Kinda like throwing a football, the harder the spin before it leaves your hand, the less it wobbles.

I have heard that occasionally the jacket may shred. If you notice strange holes in your target, or lots of blood coming from the target, then you might switch ammo. So far, I have not had this problem with my 1/7 Colt 20" HBAR.



Imparting more spin on the bullet does not provide best case stability for all bullet weights. If that was the case, all barrels would simply be rifled with 1:12. Again, an aggressive twist rate (1:12) on a relatively heavier bullet (80gr) will tend to cause the bullet to remain pointed upward during the negative portion of the ballistic curve. This would add to inaccuracy at longer ranges.

I've never heard of the spin imparted on a bullet causing the jacket to separate before striking an object.


Edited to add that I F'ed up the relation in my haste (thx GG) change the bullet mass from (80gr) to (40gr) and 1:12 to 1:7.

In the original test, the example I have given would be of a "understabilized bullet". I go on to descibe the characteristics of a over-stabilized bullet.

My apologies.

Link Posted: 2/12/2002 1:27:51 PM EDT
I have had 55gr varmint bullets come apart in flight when fired from a 20" 1:7 barrel. It seems to happen only rarely with a 16" tube. I have never had a problem with FMJ ammo in a 1:7. I prefer the 1:9. Watch-Six
Link Posted: 2/12/2002 1:39:36 PM EDT
[Last Edit: 2/12/2002 1:41:34 PM EDT by raf]
Link Posted: 2/12/2002 2:11:32 PM EDT
Interesting…..I feel nerdy today so here goes:

The copper jacket leaving the bullet would be a function of the jacket’s mass, diameter, (i.e. bullet caliber and jacket thickness) and the rotational velocity of the bullet.
This centrifugal force vector, which is tangent to the length axis of the bullet can be defined as: (I think)
((jacket mass*(rotational velocity^2))/(radius of jacket from axis of bullet))

Since the rifling twist rate is per length, the faster the bullet travels down the barrel, the faster it’s imparted rotational velocity.
Therefore, if a there is indeed a occurrence of the bullet’s jacket separating during flight, it is due to BOTH the rifling twist rate and the velocity of the bullet. In other words, if you are having these problems, reducing the MV only may solve the problem.


Link Posted: 2/12/2002 2:52:54 PM EDT
Link Posted: 2/12/2002 2:56:58 PM EDT

Originally Posted By Boomholzer:

Originally Posted By Energizer:

This also means that you can shoot any bullet weight since there will be a good spin on it. Kinda like throwing a football, the harder the spin before it leaves your hand, the less it wobbles.

I have heard that occasionally the jacket may shred. If you notice strange holes in your target, or lots of blood coming from the target, then you might switch ammo. So far, I have not had this problem with my 1/7 Colt 20" HBAR.



Imparting more spin on the bullet does not provide best case stability for all bullet weights. If that was the case, all barrels would simply be rifled with 1:12. Again, an aggressive twist rate (1:12) on a relatively heavier bullet (80gr) will tend to cause the bullet to remain pointed upward during the negative portion of the ballistic curve. This would add to inaccuracy at longer ranges.

I've never heard of the spin imparted on a bullet causing the jacket to separate before striking an object.




I wouldnt consider 1/12 an agressive twist.
GG
Link Posted: 2/12/2002 3:00:23 PM EDT
Link Posted: 2/13/2002 6:49:22 AM EDT

Originally Posted By Gun Guru:

Originally Posted By Boomholzer:

Originally Posted By Energizer:

This also means that you can shoot any bullet weight since there will be a good spin on it. Kinda like throwing a football, the harder the spin before it leaves your hand, the less it wobbles.

I have heard that occasionally the jacket may shred. If you notice strange holes in your target, or lots of blood coming from the target, then you might switch ammo. So far, I have not had this problem with my 1/7 Colt 20" HBAR.



Imparting more spin on the bullet does not provide best case stability for all bullet weights. If that was the case, all barrels would simply be rifled with 1:12. Again, an aggressive twist rate (1:12) on a relatively heavier bullet (80gr) will tend to cause the bullet to remain pointed upward during the negative portion of the ballistic curve. This would add to inaccuracy at longer ranges.

I've never heard of the spin imparted on a bullet causing the jacket to separate before striking an object.




I wouldnt consider 1/12 an agressive twist.

In a 20" barrel with a 1:7 twist the bullet will spin 2.8 times before leaving the barrel.
Using the same length barrel with a 1:12 twist the bullet will spin 1.6 times before leaving the barrel. Therefore the 1:7 is aggressive not the 1:12 as you have stated.
GG

Link Posted: 2/13/2002 7:22:15 AM EDT
[Last Edit: 2/13/2002 7:28:00 AM EDT by Boomholzer]

Originally Posted By Boomholzer:

Again, an aggressive twist rate (1:12) on a relatively heavier bullet (80gr) will tend to cause the bullet to remain pointed upward during the negative portion of the ballistic curve. This would add to inaccuracy at longer ranges.






I wouldnt consider 1/12 an agressive twist.

In a 20" barrel with a 1:7 twist the bullet will spin 2.8 times before leaving the barrel.
Using the same length barrel with a 1:12 twist the bullet will spin 1.6 times before leaving the barrel. Therefore the 1:7 is aggressive not the 1:12 as you have stated.
GG



Yes GG, "agressive" was a very poor choice in wording and the 1:12 in my text should be replaced with 1:7 (I was in the higher # = more twist mode). In that reference, the statement wasn't even correct, my bad. My intention was to point the effects of under and over-stabilization on bullets with twist-rates in a moderate spectrum (1:12-1:7, which are all common).

1:7 40gr bullet = overstabilized
1:12 80gr bullet = understabilized
Link Posted: 2/13/2002 8:21:24 AM EDT
Yeah Boom, your nerdy analysis got it right. Thats why I have had less trouble with jacket shedding from a 16" tube than a 20". The velocity is lower, so the rotational velocity is also lower. Watch-Six
Link Posted: 2/14/2002 5:46:10 AM EDT
Need a different option. My match rifles all have Kreiger 1:7.75".

A good comprimise I feel.
Link Posted: 2/14/2002 6:23:02 AM EDT
A few years ago I bought a colt tactical elite linited edition. I didn't know any better, and started right off shooting the standard military 55 gr ammo. I was so disappointed, as I expected one inch groups @ 100 yards. Then I got educated to the heavier bullet. I contacted serria, and tried their 77gr match king (new at the time) and what a difference. day and night. My opinion if you don't want to reload for accuracy, and just like to shoot military ammo, then the 9 to 1 is the way to go.
Link Posted: 2/14/2002 9:26:39 AM EDT
Jacket shedding
I'm a Colt HBAR 1x7" twist shooter. I've shot my uncles preban 1x9" and several Bushy 1x9's.
As a reloader I keep track of what bullets will perform in my barrel, and at what velocities. So far, Sierra GameKing 55gr HP and SP both fail at velocities over 2800fps. Hornady 60gr V-Max work quite well, even up to 3000fps. The V-max seem to be really quality bullets. Winchester FMJ's sometimes fail at about 3100fps, giving really bad "patterns" at 100yds. Other times they work fine, and group like factory ammo. I think pressure has something to do with this discrepancy. Achieving the speed of Q3131A is difficult to do with the relatively low prssure that ammo shows in my rifle. My recommendation is to stick to soft points above 55grs, or limit yourself to .222 velocities with lightweight softpoints.
Bottomline, get a 1x7 if you want to shoot highpower, otherwise it's worthless. Shooters will tell you how wonderful the twist is. Personally, I think that the concept of a semiauto rifle with a twist designed for bullets that are so long that they have to be single loaded, is quite silly.
Link Posted: 2/14/2002 6:20:11 PM EDT
Actually the Sierra 77 Match is an LTB (Length Tolerant Bullet) and fits in the magazine. With these heavier weight bullets, at over 2800 f/s they out perform, both near and long, anything under 60 grns., at least for punching paper. The 62 grn. don't perform that bad out of a 1:7 either for that matter.
Link Posted: 2/14/2002 6:35:43 PM EDT
Just dug out a little info from 8/99 tests.

The theme was "How well does 55 gr ammo perform in 1:7 and 1:9 twist barrels ??"

Some broad conclusions:

1) 62 gr and 64 gr bullets do not properly stabilize in a 1:12 barrel.
2) 62 and 64 gr bullets provide good accuracy when used with 1:7 and 1:9 twist barrels.
3) 55 gr bullets do stabilize properly and provide good accuracy in 1:12 barrels.
------------------------------------------------

4) 55gr projectiles lose about 20%-30% of accuracy when fired through 1:7 and 1:9 twist barrels.
------------------------------------------------

(Source: Small Arms Review Vol 2 No 11 - August 1999. Author: Chad Haire)
Link Posted: 2/14/2002 7:54:19 PM EDT
[Last Edit: 2/14/2002 8:10:30 PM EDT by Boomholzer]

Originally Posted By 5subslr5:

4) 55gr projectiles lose about 20%-30% of accuracy when fired through 1:7 and 1:9 twist barrels.

(Source: Small Arms Review Vol 2 No 11 - August 1999. Author: Chad Haire)



I was curious in how the author defined the 20%-30% loss of accuracy. Does the group spread widen by 20%-30% at 100yds? I think that we would agree that a distance must be defined.
From what I have read on bullet stability is that a 55gr bullet fired through 1:7 twist will show decreased accuracy as it travels downrange. This is common sense, like your sights being off zero...... However, what is interesting is that in the case if a overstabilized bullet, there may be zero or little loss of accuracy untill after bullet hits apogee. Afterwhich, accuracy (consistancy) will get increasingly worse as the bullet drops downrange.
Link Posted: 2/14/2002 8:50:19 PM EDT

Originally Posted By Boomholzer:

Originally Posted By 5subslr5:

4) 55gr projectiles lose about 20%-30% of accuracy when fired through 1:7 and 1:9 twist barrels.

(Source: Small Arms Review Vol 2 No 11 - August 1999. Author: Chad Haire)



I was curious in how the author defined the 20%-30% loss of accuracy. Does the group spread widen by 20%-30% at 100yds? I think that we would agree that a distance must be defined.
From what I have read on bullet stability is that a 55gr bullet fired through 1:7 twist will show decreased accuracy as it travels downrange. This is common sense, like your sights being off zero...... However, what is interesting is that in the case if a overstabilized bullet, there may be zero or little loss of accuracy untill after bullet hits apogee. Afterwhich, accuracy (consistancy) will get increasingly worse as the bullet drops downrange.



Looks like 250 meters and he used 'MOA' to define accuracy.

Folks, I'm not holding out this article to be much but thought it worth posting.

1:12 barrels stabilize 55gr bullets very well and perform accurately.

1:7 barrels are not very accurate when 55gr bullets are used.

1:9 barrels like 62gr a little better than 55gr.
etc., etc.

(All comments are meant to be very general - Troy and Forest could be bringing bullet 'length' and not just bullet weight into the discussion - exceptions; most always there are exceptions.)

There's not much new ground here but if nothing else there is some comfort in seeing reconfirmation of the old.
Link Posted: 2/14/2002 9:55:38 PM EDT
Is it possible to over stabilize a bullet? Is there any harm in firing light (<55gr.) bullets from a 1:7 barrel?
Link Posted: 2/14/2002 10:43:10 PM EDT

Originally Posted By slt223:
Is it possible to over stabilize a bullet? Is there any harm in firing light (<55gr.) bullets from a 1:7 barrel?



No - the only problem would be with light varmit bullets who have thin jackets - they can be pushed past their tolerances in a quick twist at higfht velo's - you can even see them explode down rnage some times.

If you shoot M856 Tracer get a 1:7, if you just said WTF - then don't worry you are likely a match shooter and don't care, but the 1:9

BTW Troy's FAQ covers all of this.
Link Posted: 2/15/2002 3:23:19 AM EDT
[Last Edit: 2/15/2002 3:24:18 AM EDT by Energizer]

Originally Posted By Boland:
you can even see them explode down rnage some times.



COOL!
Link Posted: 2/15/2002 7:02:21 AM EDT

Originally Posted By Boland:

Originally Posted By slt223:
Is it possible to over stabilize a bullet? Is there any harm in firing light (<55gr.) bullets from a 1:7 barrel?



No - the only problem would be with light varmit bullets who have thin jackets - they can be pushed past their tolerances in a quick twist at higfht velo's - you can even see them explode down rnage some times.

If you shoot M856 Tracer get a 1:7, if you just said WTF - then don't worry you are likely a match shooter and don't care, but the 1:9

BTW Troy's FAQ covers all of this.



I agree that there is probably no prob or harm in firing a 55gr down a 1:7. I have never had a problem. However, in the case of over stabilization; match shooters will avoid this condition and the effects it has on accuracy.

www.povn.com/~4n6/stab.htm
www.fulton-armory.com/fly/fig15.htm
www.armalite.com/library/techNotes/tnote50.htm
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