AR's seem to come with at least two rates of twist, 1 in 8 and 1 in 12.  Sure there are others but...

I've got a Ruger with a 1 in 8 rate of twist.  Will that gun shoot a 55 grain bullet accurately or do I need a 80 grain bullet.

For 55 and 80 grain bullets, what rates of twist would work best.

1:8 will be fine for most bullets up to and including 79-80 grains (assuming a full length barrel - 14.5 to 18 inch).

General rule of thumb, the faster the twist, the better stabilization of the longer (heavier)
bullets.

The defacto standard for most AR's is a 1:7 twist barrel as this was developed for the military's longer (heavier) penetrator bullets.

SO 1:7 is a fast twist, 1:12 is a slow twist.

1:7 is good for mopst all lengths (weights) while the slower you go with the twist, the less stabilization of longer bullets is had.

Make sense?

Excellent answer.  Thank you!

So if I had rifles with a 1 in 8 and 1 in 12 twists both should work ok with bullets weights of 55 grains, right?

No problem,   You need the faster twist with the heavier (longer) bullets to stabilize properly, but there's no particular disadvantage with using lighter bullet in a fast-twist gun, UNLESS you you use really thin-skinned varmint bullet such a 50 grn  Hornady SX bullet, as these MIGHT suffer seperation in the barrel.

They won't separate in the barrel, they explode from centrifugal force in flight. A 1/12 twist spins a bullet around 180,000 rpm. A 1/7 twist spins the bullet around 320,000 rpm. The thin jacket and high velocity involved when shooting light weight (<50 grain) bullets sometimes causes jacket failure in flight.  Basically the bullet explodes from the G forces. This can sometimes be addressed by lowering the powder charge if you reload.

IMHO, 1/7 and 1/9 are far more common than 1/8 and 1/12.  

This link is full of great stuff:

ammooracle

Twist weight correlates to bullet length,  not weight

80 grain bullets have to be loaded so long that they can not fit or feed from any magazine. Typical overall length is almost 1/4" longer (2.500") than standard magazines (2.260"). Your 1/8 will work great with Hornady 75 grain hpbt match ammo, M262, Federal 77 grain SMK or any handloads using those bullets.

80 grain .224" bullets when used in .223 are intended for the 600 yard stage of NRA/CMP high power rifle competition. Each round is inserted in the chamber by hand turning your semi-auto into a single shot rifle.

Incorrect.  The 1:7 twist was developed for the longer M856 tracer round.  The Mk262 didn't appear until much later.

True.




Understand that I am NOT saying 1:7 is bad.      

The standard for most AR's is 1:9.  Don't take my word for it go look at what the majority of makers list as standard in their catalog.

Go see what twist and what bullet wights the national champions use to take the trophys and medals.

On weight stability and accuracy  Link  and link

 eta

 If your interest is wining fights more than wining matches you might want to start here.

1:7 was developed for tracer rounds. Most quality barrels use 1:7 or 1:8. More manufacturers use 1:9, but that doesnt make it better.

1:7 & 1:9 are much more available than 1:8 & 1:12.

When I build, I never look at 1:9 & 1:12. They just cant stabilize longer bullets.

Define "longer" please.

Bullet length in contact with the riflings.  Which in most cases translate to longer the heavier.  But this is not always a fact, there can be exceptions to the rule.  

 

Indeed.  For instance, monolithic bullets, like the solid copper Barnes TSX and TTSX and even the new bullet used in M855A1 ammo, are much longer for their given weight than lead core jacketed bullets of the same weight.  Copper is lighter than lead.   For instance, I can stabilize relatively heavy 75-77grain lead core bullets in a 1:9 barrel, but the lighter, but longer solid copper Barnes 70 grain TSX bullets keyhole badly in 1:9, hitting the target at 100 yards completely sideways with totally blown patterns.  The ligher Barnes 62 grain TSX is as long as some 75 grain lead core bullets, but shoots right at MOA from the same 1:9 barrel.   It is about the length of the bullet in contact with the rifling and lands of the barrel, and not weight, per se.

I am advocate of 1:9, but it has its limitations with long bullets.

Hollow Point Boat Tails - particularly VLDs, and the monolithic ones mentioned.



My 1:8 seems to shoot 82gr Bergers okay, but the twist calculator said my stability was on the higher side of marginal. I'm switching to 80.5s because they're just a wee bit shorter (I assume they're the same construction) and stabler in more conditions. The 80gr VLD is almost the same length as the 82gr, but barely has a higher BC.





 

I'm not sure I am getting the drift here so let me ask point blank ( pun intended ) from the perspective a newb might have.

gotigers are you saying 9 and 12 twist barrels are poor quality?  What is your definition of "longer" ?

HighpowerRifleBrony are you saying 9 twist will NOT stabilize HPBT and OTM bullets ?  

Remember gents this is from the perspective a newb might have.

Lemme get some links and pitchers.  Even better, I got MS Paint.
While likely not proportional, let's say these all weigh 55gr and are of the same lead-core, hollow point construction. If these were made with lighter materials (copper or brass monolith, steel penetrators, etc), they'd have to be longer to be the same weight.
From top to bottom:


Wadcutter


Flat Base


HPBT


VLD.





This is why length is regarded as more important than weight, though weight is often used as a generalization due to the popular bullets.






1:9 and 1:12 can stabilize HPBT and VLDs, but they have to be lighter ones. When I still had a 1:9, it could stabilize Hornady 75gr HPBT at 100 yards, but not the 75gr AMAX.
Precision depends mostly on the length and concentric tolerance. If the bullet is too long for the twist, it's gonna keyhole randomly. If it's within the maximum length and imperfect (like ball ammo) then the slowest twist possible is preferred. It's hard to "overstabilize" a perfect bullet that's within the max length.
 

 

Why does this place not have a "like" button.