As I said, I will happily purchase any twist barrel that comes out and works for .177 slugs. If that barrel happens to be a 10" or 12" twist, that's just fine by me. I don't want to sound repetitive, but all my testing with every weight slug I could get my hands on, points to a little slower twist than what you're suggesting. Of course there are sizing variables and bullet shape and rifling type etc. etc. that must also be considered. We also can't compare .172 ammo/barrels to .177. That is physics, and those physics are quite different between the two. The diameter of the .172 is smaller, the bullet is longer, and the speed is faster. The guys shooting .172 in air guns are usually using a 9" twist I believe, with 29 grain bullets in most cases and sometimes 32 grain. That is a whole different projectile, that most air guns(short of an Airforce) have a hard time dealing with. Yes, there are exceptions, but generally speaking there aren't. My replies are directed at the FX platforms and magazines. I don't foresee anything much heavier than 24 grain fitting the FX magazines.
The calculators are usually on point, assuming the right info is entered. As mentioned, there are variables the calculators can't account for. Some of the problem happens when we try to do it for .177 slugs, in which there is no standardized sizing, or bullet shape/design. One maker sells .178, another .177, and another still .1775. We can make good guesses, but it isn't 100%, because we aren't. That's why I said, I've looked at the calculators, a lot. They all told me the same thing you're suggesting, give or take. However, I haven't been able to duplicate that in my testing. The results from my testing are different, and I can't change that. It is what it is. The slugs that I have had good results with are not "good at certain distances" either. If I say it is good, I mean it stabilizes and stays that way from 30-400 yards. I can't get the 16.5" twist barrel to shoot well with anything under 15 grain, but it is good from 15-22 grain. I can't get the 14.6" twist barrel to shoot well with anything under 20 grains, but it does well from 20-24 grains. I tried every speed from 800 to supersonic, and I tried sizing from .177, 1775, and .178. I tried changing the leade in, seating depth, porting type, still the same results. I could only get the LW 17.7" twist barrels to do well with the lighter 12.5 grain slugs. I tried polygon and grooved, choked and unchoked. Heck, the same LW polygon that shot like a laser with the 15.5 grain .177 slugs, wouldn't shoot the .178 15 grain slugs to save it's life. That's a sizing issue though, obviously.
I'm definitely not saying the calculators are wrong a lot or anything like that. I am saying that there is more to it than just that calculator. Ask any of the air gun slug gurus on here or any other air gun forum. They will also tell you the calculator is not gospel. It's a great place to start, but it isn't always perfect, because we and our data aren't always perfect. The calculator is the best place to start most of the time. I'm also not willing to say my data or testing is gospel. Even with all of my testing, there are things I surely missed. I do trust what I've seen with my own eyes more than a calculator. At least, until I see those calculations reproduced on target.. I'm more than willing to admit I could be wrong, and I know those calculators have been proven on more bullets and platforms than I can even imagine. I'm just not willing to bet the bank on the calculators is all
I completely agree with one thing. We can make 1000 yard powder burners all day long without much effort, so why on earth can't we do the same with a .177 slug at 100 yards, LOL! There are a couple of guys that have figured that out, and shooting sub MOA with .177 slugs well past 100 yards, but you might be disappointed to find out the twist of their barrels, haha. I have done it, but not documented it like they have. Flintsack has been doing amazing things with his Red Wolf and .177 slugs to name one. He's got some pretty amazing groups and photos on here too. Honestly, I'm happy to be wrong, especially if that means we get a laser of an FX .177 slug liner as a result! I welcome whatever they come up with. If it happens to be a 12" twist, awesome. I'm betting it will closer to the 15" range though. Either way, I want the same thing the other folks in this thread want, and I don't mind one bit if I have to be wrong for it come to fruition