I am old , but new to this forum. I don't see much about Swift Sirocco bullets. Limited experience is that they make elk dead at 400:yds with fairly generic 30-06
The Swifts are one of the best bullets out there upon impact on game.
I can only speculate as to why they aren't used more, but here are some possible reasons, especially when talking long range.
The b c is pretty good but quite a bit less than some popular long range choices.
The bearing surface is long which makes them pressure up earlier reducing velocity potential also important for longer ranges.
The solid copper jackets tend to copper foul easier.
The cost is double some more popular long range choices and most long range hunters tend to practice a lot.
They CAN be as accurate but not as consistently as some others used by long range guys.
The best thing about them is they work the same every time from high velocity impacts down to velocities under 2000' fps. (I think around 1600 in my testing)
IMO, impact performance is about as good as it gets out to 700-800 yards.
They expand fairly easily but hold together VERY well at high impact.
IMO they are a good choice for large animals out to moderately long ranges.
They are used in some higher priced shelf ammo for good reason.