Its a shame they didnt just stick with the 375 winchester. I have one in the marlin and its a perfect woods gun
there are a few reasons they could not use 375win to do what they were trying to accomplish
most importantly the case length of 375win is longer than 1.8 inches to meet the requirements in some of the strait wall cartridge states max length of case to be legal
any of these strait wall cartridges the manufacture wants to appeal to a wider market than just a few states hunting laws and that means making it fit an AR-15 platform
this is also a good idea because many people have found starting youth with a AR platform reduces the recoil and with the adjustable stock you can fit a range or growth add in that it is affordable often for about 300 dollars you can get a new upper that snaps onto your existing lower if the only thing that needs changing is the barrel then it keeps production costs down on 350 legend uppers
375win is longer than the mag will feed in an AR , it is a rimmed cartridge , and wider than would feed from an AR mag
yes 450 bushmaster and 458 Socom use custom mags however you can see the appeal to keeping to standard mags even if they might work better with a new floor plate , floor plates are cheap compared to an entire magazine redesign.
rimmed cases have always had some complication feeding from mags. even bolt action rimmed cartridges like the 7.62x54R sometimes had rim-lock even with the beveled rim , generally this is user error but in a world of lowest common denominators and rampant social media any thing you can do to avoid user error that your product will be blamed for the better
the 350 legend basically is a 223 rem case expanded strait to the base wider than the rim to get greater case capacity and make a rebated rim then tapered very slightly to the case mouth to hold a .355 bullet.
yes a 200gr .375 cal bullet at 2350fps would have been great
however they got a 180gr .355 bullet at 2100fps or a 150gr bullet at 2400fps which is a lot in a little package because of the powder used in these small cases they build pressure early and by the time the rifle is moving in recoil the recoil impulse is nearly over this is now the rifle of similar weight , velocity and projectile weight produces less felt recoil.
it is my opinion when you can't go fast go heavy as reasonable and the much greater number of positive consumer reviews of the 180gr Winchester load on deer vs 150 reviews appear to show it holds true for the 350 as well.
either with good shot placement clearly makes dead deer and how far they run will be more a matter of shot placement. a heavier bullet carries energy through mass and bone better than light but 30gr isn't a huge difference.
often part of the issue is the manufacture because the round is accurate to 200 yards or better wants a bullet that still expands at 200 yards , the trick is making one that doesn't over expand at 30 yards and fail to give the penetration wanted.
also my opinion many rifles make poor blood trails when compared to shotguns and there certainly seems to be some agreement among others here whom have shot deer with both. a 1oz 72cal or 5/8oz 62 cal projectile moving 1400 to 900 fps makes a different woon channel more blood .
rifles and shotgun can both kill very well within their range and shotgun range is over in most cases by 80-100 yards less for 20ga and more if your running a sabot load through a rifled barrel that is not to say that a foster slug is not lethal beyond 100 yards it very much is you just have little ability to place it on target and it retains less than half of it's original energy.
for any state or local that does allow the use of non strait rifle cartridges of a nominal length the market is loaded with options that deliver greater terminal performance.