55gr .223 with 1 in 12" barrel, OK?

55s will work and won't hurt a thing. I can't imagine an unsuitable bullet weight actually harming a barrel, but maybe I just haven't heard of EVERYTHING. Before settling on 55-grainers, though, I would try 50-grain bullets as well.

Actually, weight is not really the parameter of a bullet that dictates a certain rate of twist, but length of the bullet is. It just so happens that heavier bullets tend to be longer, requiring faster rotation for stability, but the overall shape of a bullet is a factor also. A blunter design can be heavier without necessarily being longer.
 
I have never heard of anyone having any problems with a 55 grain bullet, in terms of their twist rate. Some have said that a 62 grain is too heavy for a fast twist, but 55 grain should play just about anywhere. It also seems to be the most common.
 
RiverRider is right, it's length not weight. The 1/7 M16 barrel is needed to stabilize the 62 gr tracer M856 projectile which is waaaay longer than either the 55 gr FMJ or the 62 gr "penetrator" FMJ (which is itself somewhat longer than an equivalent all-lead FMJ would be). M856 is so long because it has to have room for the payload (which is much, much less dense than lead or any other metal).
 
One of my uppers is 1-12". 55gr works fine in it. That is, when it doesn't have a rimfire conversion in it. :)
 
My Ruger #1-V is 1-12" twist and eats 55gr bullets fine, but prefers 50's. My best groups to date were punched by 50gr V-max.
 
My Daewoo DR-200 has a 1:12 twist barrel on it. It shoots fine with 55 grain bullets It does not like the 62 grain penetrator at all at 50 yds the 62gr is keyholeing. Stay at 55 grains and under, you should be fine.
 
I think it's important to remember that the length of the projectile determines the rotational velocity requred to stabilize it. What that means is that a bullet one inch long must spin at a certain speed (or rpm) to keep it stabilized. So, muzzle velocity becomes a factor.

In other words, a .223 rifle that spits out a 68 grain bullet at 2800 fps may stabilize that bullet using a 1:9 rate of twist. Fine. But that does not mean a .223 pistol with a much shorter barrel, but the same 1:9 rate of twist will work as well. That is because the rotational velocity of the bullet will not be the same as with the longer barrel.
 
As a rule of thumb:

1 in 12" twist - 45 to 55 grain bullets.
1 in 10" twist - (most Ruger Mini-14s) - 52 to 62 grain bullets.
1 in 9" twist - 55 to 69 grain bullets
1 in 7" twist - 55 grains to 70+ grain bullets.

These are approximate. Some bullet designs are longer than others and may require very specific twist rates in your gun. Most early guns have 1 in 12" twist (older AR15s, Daewoo DR-200s, older MAKs in .223); Mini-14s are usually 1 in 10"; newer AR-15s are normally 1 in 9" so they can shoot the non-tracer 62 grain bullets. The military uses 1 in 7" so the tracer 62-grain bullet will stabilize.


Hope this helps.
 
The 55 grain projectile falls within acceptable length range for nearly all common twist rates in .223. If you are in doubt and need ammo *right now* grab 55 grain. It is as close to universal in .223 as one can find.
 
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