The 1-7 was designed specifically for the SS109 round by Colt in 1987. It works very well with the heavier bullets. 1-9 twist is a compromise made by Bushmaster and a few other companies to allow reasonable accuracy with both the 62 grain rounds and the light 55 grain rounds that are mainly used in the 1-12 twist barrels.
<font color=red>[**censored**]</font> it DOESN'T, u dumb sob. there's
still BILLIONS of rds of 55 gr ball in National Guard armories. it makes no sense to use a 1 in 7 twist, and RUIN your ability to use that ammo, or all the milsurp stuff, or most of the sp's, etc. The 1 in 7 twist is WAY too fast for the 55 gr and lighter bullets.
The tracer material is a LOT lighter than a lead core, so bullets of the same wt are a LOT longer,and thus, require a much faster spin rate to be adequately stabilized. The tracer must weigh the same, have the same velocity, in order to "stay" with ball ammo. If the tracer's trajectory is not the same as that of the ball ammo, it's pretty worthless, dummy. We HAD a 55 gr tracer rd, the military discovered that it didn't stay with the newer, 62 gr ball rd, and they had to switch to the 1 in 7 twist rate.
I'm going to agree with GK on this one. SS109's work about equally well with a 1-in-9 twist as they do with a 1-in-7 twist. Due to their extra length, the L110 tracer rounds were having a problem with stability from the experimental 1-in-9 barrels of the M16 P.I.P. rifles and that necessitated the 1-in-7 twist.
TODD, the problem with a faster twist, such as the 1-in-7, is the bullet's quality. If the bullet is well made and without flaw, the twist only affects the long range performance (drift, etc.). However, if the bullet has sufficient flaws, the faster twist can cause more wobble than the slower twist.
RIKA, the competition AR's frequently use a 1-in-8 twist. It depends on bullet length and quality. I know folks in the Army that single load 80+ grain bullets in their 1-in7 twist M16A2's for taking unusually long range shots. Since the 1-in-7 twist would be more adaptable to longer bullets, I would recommend it.