First, ya gotta understand why the 1-7" twist came about. With the then new M16A2 (1985), the military wanted a heavier bullet that would remain stable farther out than the current M193 (55 gr. FMJ). They settled on the 62 gr. bullet (M855). This bullet only required a 1-10" twist, but the idea was to tighten the twist up in case someone came along with a better or different bullet in the near future. Once the process had moved forward, the government agency overseeing the cartridge mandated it have a matching tracer round. Due to the added distance a M855 would travel over the M193, the tracer bullet had to be long. So long in fact, it took a 1-7" twist to stabilize it.
That is where the 1-7" twist came from. Just to stabilize that tracer
A 1-9" will stabilize anything you can push out of a magazine fed AR-15 in .223/5.56. However, if you want to single feed longer bullets you can make use of the 1-7" twist. Or, shoot tracers, you'll need that 1-7" twist. You will not be able to load anything 80 gr. or even some 75's (Hor A-Max) to mag length. NEVER, when reloading, seat the ogive below the neck!
Fast forward to today's quality capability. In 1985 bullets didn't have the concentric quality that today's bullets have. We're talking copper jacket to lead core. With too tight of a twist, it tends to exagerate the distance a bullet goes off if the harmonic balance of a bullet is off. Meaning, accuracy suffers with a tight twist shooting non-concentric bullets. This is why benchresters shoot the slowest twist they can. It's an old truism that is not as true today as it was thirty years ago. Concentric bullets of today (1/10th the tolerances from 1985) are not nearly as prone to lose accuracy if overtwisted.
The problem with light varmint bullets with light jackets is they come apart much easier if spun too fast. Load them up to a speed they don't come apart and have a blast with them on varmints. But, if you start not seeing any impact, or that dreaded grey cloud 50 yds. out of your barrel, you need to slow them down.
If you want to shoot heavy bullets that need a 1-7" twist from a magazine, get a Valkyrie.