It makes a guy's life a lot easier to simply pick up a 1:7" if you have a mind to shoot heavy bullets in the AR-15. The consequence of not enough twist is greater than having a little too much twist. I've never been able to blow up a 50grn vmax with too much twist in an AR length barrel (including a 29" match rifle), but I HAVE had trouble stabilizing 75-77's with shorter barrels in 1:8", let alone the 70grn TSX.
For heavy bullets, I certainly wouldn't ever recommend a 1:9", and I'm reticent to even recommend a 1:8". A 1:7" will always work, a 1:8" usually works, the 1:9" might work... Would you buy a car thinking it might work? Thinking they usually work? Or would you rather buy a car knowing it WILL ALWAYS work?
When you step outside of that simple paradigm, you run the risk of stepping on different landmines. For example:
- Barrel length matters. Spin rate depends upon velocity AND twist, and velocity depends upon barrel length. a 1:9" 26" bolt action rifle will stabilize a 75grn A-max or 77grn TMK, whereas a 1:8" 16" might not, simply because you're talking about a 350-500fps difference in velocity. Running 3000fps in a 1:9" is 240,000rpm, whereas 2450fps in a 1:7" is 252,000rpm... So a guy can sometimes get away with less twist in a longer barrel. I can comfortably shoot the 75 Amax in a Savage 12 BVSS with a 1:9" to 600-800yrds, whereas the wheels fall off using the same bullet in a 16" Bushmaster M4 Hbar with a 1:9"
- Bullet LENGTH matters, not just weight. Not all bullets are created equal, so it's not really fair to say a guy can stabilize 50-77grn bullets in a 1:8" without talking about specific barrel lengths (above) or specific bullets. Mono-metal, tipped, long boattail, and ELD/VLD type long ogive bullets are very long for their weights, so they need MORE spin to stabilize than other bullet types or profiles. For example, the Barnes 70grn TSX is 1.037" long, the 75grn A-max is 1.105" long, and the 75grn VLD Target is 1.063" - whereas the 77grn SMK, heavier than any of these others, is only 0.994" long... The 75 A-max is so long it can't even be loaded to AR mag length, because the ogive will be below the case mouth. These longer bullets require faster twists than their shorter profiled or denser metal cousins. The 16" Hbar mentioned above will shoot the 77SMK to 600yrds, but again, it won't shoot the 75grn A-max worth a dang at that range.
- How far you're shooting matters. If I stay consistent and keep picking on my 16" Hbar - it does acceptably well with the 75grn A-max's at 100yrds, but they're tumbling by 600. If all I need to do is kill paper at 100yrds, they're a fine bullet choice, but I want to do more, so they're not. Worst case is a bullet starting out unstable, but more common is the case where a bullet starts out stable enough, then destabilizes in flight. For a 6.5 creedmoor shooting a 140grn bullet through a 1:8" barrel will destabilize out there past 1200yrds, whereas an AR-15 shooting a 75 A-max through a 1:12" barrel might be unstable right out of the muzzle.
- Source for recommendations and the basis of experiences matter. There are a lot of internet experts who give advice based on absolutely nothing more than what they have read online. There are also a lot of "experienced" folks online giving poor advice because their real experience doesn't match your application: Like when guys say: "you'll be fine with a 1:9," because they've only ever shot the 77SMK (easy to stabilize) and not the long Berger VLD's or 75 Amax's mentioned above, or because they got 3/4" groups at 100yrds, and have never fired a shot at 600yrds in their life. If I pretended to be an internet expert - WITH EXPERIENCE - because I'd shot the 75 A-max out of my 1:9 Hbar and got sub-moa groups, but didn't admit I'd only ever fired it at 100yrds, I wouldn't be a good source (that's REAL for that Hbar). However, I HAVE fired the A-max at 600yrds with that rifle, and punched profile shaped keyholes... It's not that the experience of the guy shooting 100yrds or shooting the 77SMK aren't true, but they don't tell the whole story if you're shooting 600yrds or wanting to shoot a Berger 75grn - lighter than the 77 SMK. It's kinda like saying a Ford Mustang is a fast car, in a conversation about Top Fuel drag cars... It IS a fast car, but not when you talk about drag racing... No joke, one of my first conversations on this forum a few years ago was poster asking about the 75 A-max, I pointed out it couldn't be used in an AR except single loaded. Another user jumped my case saying he shoots the 77SMK in AR's all of the time, and saying I was full of it... The truth came out that he didn't know the two bullets were VERY different and he'd never seen a 75grn Amax, but by God, he was experienced with heavier bullets than 75grn and I was full of **** because I said something with which he didn't agree...
For my coyote rifles, I know I'll shoot a lot of 50 V-max's, and maybe 10-20% 77SMK's, so a 1:8" works ok. Alternatively, for my match rifles, I know I'll be shooting 77SMK's or 80/90 Bergers, so I get a 1:6.5" or 1:7" barrel.
