Rifling

I'm going to wade into a controversial topic.

Rate of twist.

The AR-15 today is commonly found in 1:9 rifling.  What that means is the bullet will spin 360˚ every 9".

Originally the AR-15 was equipped with a 1:14 twist.

Lemme back up a bit more.

The entire point of the rifling is to spin the bullet, this spin is to stabilize it in flight so that it goes where you pointed it.

The 1:14 twist in the original R601 AR15 did stabilize the 55gr bullet developed for it.  The thing is the rate of spin required changes with the density of the air and the length of the bullet.  Longer bullets and thicker air need more spin.

1:14 did not stabilize the bullet in temperate even though it worked fine in tropical places like Vietnam or hot summer demonstrations for Air Force generals.

Hot, humid air is less dense than cold dry air.

For the AR-15 R602 the rifling was changed to 1:12; a change that held all the way through to the M16A1.

1:12 works great for the M193 and M196 rounds in all the conditions we'd send troops, from tropics to arctic.

This was all fine and dandy until NATO standardized on a different round altogether.  SS109 is a composite construction with a steel core surrounded by lead with a copper jacket instead of the normal all lead core with copper.  It weighs 62 grains, but is longer than an all-lead round would be.  As long as a 69 grain round.  Fabrique National of Belgium developed this round and determined that a twist of 1:9 was enough.  But then they tried the L110 tracer round under arctic conditions...  That round required a 1:7 rate to stabilize, and didn't have a too adverse an effect on the SS109.

The plan, at the time, was to issue the light machine gun with a 1:7 barrel and the rifle with 1:9 because the rifle would not normally use tracer ammunition.  However, it was decided that it was desirable for the rifle to be able to use any ammo issued, so the rifle got 1:7 as well.

That covers the military...

What about you?

Older .223 rifles come with 1:12.  Some older Ruger Mini-14s even come 1:10.  Since the most common loading is 55gr, this is fine.  1:12 does not stabilize NATO ammo at all, or other longer, heavier ammo.  The unstabilized flight-path is erratic to say the least; you can miss a man sized target at ranges as short as 20 yards!

Newer guns come with (mostly) 1:9 tubes.  This is fine for the common 55gr ammo and, as I mentioned earlier, fine for NATO spec ball ammo.  Depending on the manufacturing tolerances to the twist, 1:9 should be good for rounds up to 69 gr.  What you lose is the lighter 40-50 grain varmint rounds.  They become over-stabilized.

What's that?  Bullets travel in an arc.  To hit farther away you have to elevate the barrel.  A properly stabilized bullet will point along this arc.  An over-stabilized round will retain the orientation of the barrel, so it will be traveling at an angle to its flight path.  This is bad for accuracy and can cause unpredictable behavior in the target.

1:7 does this with 55gr ammo.  What you gain with it, in addition to being able to shoot tracers in northern Alaska in the winter, is the ability to accurately fire really long rounds like 77gr.

All other things being equal, heavier rounds do more to the target than lighter.  With varmint rounds, it's too much (unless you want a vaporized red cloud of rodent).  Heavier rounds also reach out farther and are less affected by wind.  All things are not always equal.

Heavier rounds normally are going slower, they take up more space in the case displacing powder capacity.  Even if they didn't keeping the velocity the same would lead to higher, perhaps dangerous, pressure levels.

Again, what about you?

You have to honestly assess what you are going to use the gun for.  What ammunition are you going to buy for it?  Committing to a twist commits you to a specific range of bullet weights.  Likewise, committing to a bullet commits you to a twist.

If you don't plan on arctic hunting trips or plan on using heavy rounds, there's nothing wrong with using 1:9 rifling, don't let anyone tell you that you MUST have 1:7 simply because that's mil-spec.  Unless you have plans that specifically require 1:7, don't pay extra to get it.  Getting a 1:7 barrel sets the most commonly available ammunition off your list.

Likewise, don't expect a 1:12 barrel to handle heavyweight ammunition!