AR Twist Rate/Bullet Weight

[camper4lyfe]

Since the news is coming out that Gander Mountain is about to go belly up, and I'm sitting on a gift card, I figured I should get something to feed my AR.

I'm running an 18" 1:8 twist barrel in 5.56.

I just want to make sure I've been reading things right and that 1:8 means I should be shooting 62-77gr and up, correct?

I'm probably just going to grab some of the cheap American Eagle stuff, assuming it's the right weight, just to get as much as I can out of the $40 I have to spend.

Thoughts?

[big flash]

really only depends on how much accuracy you want. you'll be able to throw 55 gr cheapo ammo well enough for 100 yard plinking with 1:8 barrel imho
now if you want to shoot sub-moa groups at hundreds of yards then you'll want/need to be more persnikety with the bullet weight.
but i think 1:8 is a great twist for good accuracy out of the widest range of weights.

[camper4lyfe]

The flexibility is one thing that lead me to the 1:8 barrel. That and it became available first. It was just so long ago that I bought it, I couldn't remember the specifics of bullet weight with it.

[big flash]

yeah, someone more knowledgeable will have more specifics than i, but basically the higher weight projectiles will shoot more accurately out of the faster 1:7 twist rate. lower weight projectiles will be more stable and accurate out of the 1:9 barrels.

[WARFAB]

My understanding is that a twist rate will stabilize bullets up to a given weight. Barrels and guns can all have their idiosyncracies when it comes to what bullet weight shoots the tightest groups, but anything under the max weight per twist will stabilize out to a given distance. You have problems when you're shooting a really slow twist and a heavy bullet. I saw a guy keyhole a target inside 25 yards with factory ammo. Relatively heavy bullets and my guess based on looking at his old upper was a 1:12 twist.

Shilen twist rate chart based on bullet weights:

http://www.shilen.com/calibersAndTwists.html

Technically, the twist needed to stabilize a bullet is based upon the bullet length, not the bullet weight. The bullet weight/twist charts assume that for any given caliber, a certain bullet weight will equal an approximate bullet length. These charts become less dependable if you're dealing with VLD bullets or monolithic bullets that have a very long length for their respective weights.

Berger offers a calculator that will tell you if your bullet will stabilize based on it's length:

http://www.bergerbullets.com/twist-rate-calculator/

For most people buying factory ammo, the Shilen chart is sufficient.

[camper4lyfe]

The interesting part is that chart, for 1:7 and 1:8 (.224 CF) it says "heavier than", but everything else says "up to". I actually saw that chart before I posted, which is partially why I got all confused.

[Meketrefe]

The necessary twist is not a function of the weight but:

- Bullet length
- Speed
- Rifling
- bullet shape and construction/design

A 70gr barnes TSX needs a 1:8 twist minimum.
The reason is because they are longer and even with the long bearing surface they need more twist than other bullets some much heavier.
The best is to follow the manufacturer recommendations.
Some of the best bullets the 223 has to offer need a 1:7 twist and this includes some of the 77gr hpbt like berger VLDs.

[dwa12479]

[dwa12479]

General guideline for factory ammo.

[Meketrefe]

General guideline for factory ammo.

I think this is not totally accurate.
The 9 twist will stabilize the 75gr hornday hpbt like the one used in the hornady TAP ammunition.
and the 1:8 twist will not stabilize the berger 77gr hpbt.
Both cases function of the shape, bearing surface and weight distribution.
So again, better follow manufactures recommendations and/or ask around when having doubts.

[camper4lyfe]

So after all my questioning and fumbling, and pretty damn good answers, I ended up with 55gr FMJ American Eagle cheepy stuff.

We'll see how it fairs at some point in the, hopefully, not too distant future.

[WARFAB]

What the heck is a 'eurodriver'?

[camper4lyfe]

What the heck is a 'eurodriver'?

I don't know. What?

[WARFAB]

Don't ask me. It's in DWA's diagram.

[Liberty's Teeth]

What the heck is a 'eurodriver'?

Pretty much anything made by Anschutz