1:7 or 1:9 twist

[Bad Dog]

OK...What are the advantages of one over the other?

[esaabye]

OK...What are the advantages of one over the other?

Depends on the bullet weight you want to shoot. 1/9 max generally is considered 69gr, 1/8 or 1/7 can do the 77 and 80gr bullets. Only people who shoot those that I know of are high power shooters at 600 and 1000 yds. My new highpower upper is a 1/7.5 and I will shoot 69gr to 300yds and 77grs at 600.

[Bad Dog]

I got it, but I have a barrel with a 1:7 twist do I have to shoot heavier bullets or can I stick with the 55 grain bullets I have been shooting? Will this affect my accuracy if I'm shooting 300 yrds max?

[hurley842002]

I got it, but I have a barrel with a 1:7 twist do I have to shoot heavier bullets or can I stick with the 55 grain bullets I have been shooting? Will this affect my accuracy if I'm shooting 300 yrds max?

You can shoot 55 grains just fine

[Hoopty]

What about the heavier bullets in 1/9?

[gcrookston]

You can shoot 55 grains just fine

+1.

Generally speaking, the 1:9 is optimal for 55 gr., the 1:7 for the 62 gr. but both barrel twist rates should present them selves well with a spectrum of bullet weights. The tighter the twist, the heavier the bullet.

[esaabye]

What about the heavier bullets in 1/9?

As I understand it you need the higher twist rate for the 70+ to stabilize the round. Less stable = big group. Not sure if damage can be done to the barrell but Sierra actually puts a warning on the heavys, "For use in 1/7 and 1/8 only.

[hurley842002]

What about the heavier bullets in 1/9?

You can shoot the heavier bullets in a 1/9 barrel, but rather or not a particular 1/9 barrel will stabilize them is a hit or miss. Some 1/9 barrels will stabilize heavier bullets no problem, some will not. As far as damage, you won't do any damage to the barrel by shooting the heavier bullets. I had a 1/9 Bushmaster upper that shot 75gr Hornady's no problem, but i've since gone with a 1/7 BCM for a few different reasons.

[hip55]

http://www.ar15.com/forums/topic.html?b=12&f=6&t=102

more-

http://www.snipercountry.com/Articles/AR15_part1.asp

[Colorado Osprey]

Some strange answers here.
Historically, the 223 was stabilized in a 1-12 or a 1-14 .
That is the reason that the M16 was originally made that way.

Bullet stabilization per twist is a function of length and velocity; not weight of the bullet.
That is the reason that you get varying answers as to what will work and what doesn't.
This is why some guns will stabilize a 62 grain bullet with a 1-9 and some won't.
The Military tracer 62 grain rounds are long for weight and needed a faster twist to stabilize than the FMJ training rounds like the M193. That was the primary reason the military went to the 1-7.

A 1-9 will stabilize all other 62 grain bullets that are not a tracer.

Bullet manufacturer's will state specific twists required for large for caliber bullets like the 75 grain and up bullets in .224 cal. Heed their advice and you will be just fine.

If high power long range using these large for caliber bullets you will want a 1-7 to 1-8 twist. Otherwise a 1-9 will work just fine.

The downside to using a twist too fast is loss in velocity and premature barrel wear.

There have been reports of using 1-7 or 1-8 and shooting light jacketed bullets like v-max and having them explode upon exit of the barrel from over spinning.

My point is determine what you plan to use the barrel for and it will guide you to the twist you need. There is a reason why most commercial 223/5.56 barrels are 1-9 twist even though the "chart" crowd has to have a 1-7 twist.

Here is a link for determining barrel twist for different bullets: (Greenhill Formula)
http://kwk.us/twist.html

To prove this point those 77 grain Match Kings that many consider a 1-7 required shoot in a 223 will shoot just fine from a 1-9 223WSSM at a velocity 800ft/sec faster. In fact the Greenhill Formula says that it will take just a 1-13 at that velocity to get it done. The lower velocity in the 223 is a contributing factory to the faster twist needed.
The is almost the exact opposite issue they are having with the 6.8SPC. They went to a slower twist to gain a little more velocity because the 6.8SPC shoots light for caliber bullets.

[trlcavscout]

All I have been shooting is 50 gr HP walmart stuff out of the Bushy (1/9) I had, the Colt and my now BCM both 1/7, I cant tell a difference? They all shoot about the same 100yd groups, probably the best that ammo will do.

[Bad Dog]

My AR has a 20" barrel with a 1/7 twist rate, if I am understanding all of the information posted mine was designed to shoot a heavier bullet.
My question is if I shoot a lighter bullet (55 Gr.) will that lead to poor accuracy or will it have no affect at all on accuracy?

[Colorado Osprey]

My AR has a 20" barrel with a 1/7 twist rate, if I am understanding all of the information posted mine was designed to shoot a heavier bullet.
My question is if I shoot a lighter bullet (55 Gr.) will that lead to poor accuracy or will it have no affect at all on accuracy?

Generally speaking there is no such thing as over stabilization so you can shoot light bullets just fine it will not effect accuracy.

Like I stated before.. the only possible problem shooting light jacketed varmint bullets; there is a possibility with a 1-7 that you could have a bullet come apart upon exiting the barrel. There have been no definitive studies to determine if the type of barrel rifling vs twist is more of the culprit to this phenomenon in the fast twist barrels.

Shoot away... light, heavy You'll be fine
BTW a 55grain bullet is not light, it is the most common bullet size in .224
Light bullets in .224 are 36-40 grains

[Bad Dog]

Thanks for breaking it down and making it easy to understand

[2ndChildhood]

My accuracy in that barrel was poor with Hornady 55 gr fmjbt's and with wolf 55 and 62 gr ammo.
When I went to the hornady 75 gr fmjbt's it shot like a champ.

Here's another guy with a different 1:7 gun that had the same experience:

AR15 had a chrome-lined 1 in 7″ twist barrel. It was freakin’ picky on ammo and preferred match stuff in the 68-75 grain range (Ultramax, Prvi, etc). With those heavier bullets I could turn out some 1-2 moa groups. After break-in I was quite pleased this rifle with the M4 profile 16″ barrel turned out a 3″ group with Ultramax 68 gr at 300 yards. However, with various types of 55 gr ammo (Federal, PMC, Wolf) my groups ran anywhere from 5″ to 7″. This is like a car that prefers premium gasoline to “run right”. I’ve shot many other AR15s with 1 in 9″ twist barrels which have grouped everything in the 55-75 gr range far better. http://gardenserf.wordpress.com/2011/02/15/5-56-east-vs-5-56-west/

[GandNuateta]

Buh Bye

[Akyla]

One of my planned builds is a heavy 24-26" varmint barrel AR. Lots of varmint loads are the lighter faster bullets, so do I use a 1:9 or go even a bit slower? What's best for a varmint barrel like I mentioned? Thanks!