I load a lot of AR rounds, in a few calibers, and one boomer (50 Beowulf). Played with 450 BM as well. With 5.56, and .308, some guns have issues with standard dies, hence the small base die sets you see out there. Plenty of guys run standard dies with no problems.......and some guys just can't get their guns to run without it. 350 Legend will be very similar to 50 BW, you're gonna basically treat that has a handgun cartridge for all intents and purposes. Minimize your belling, and taper crimp just enough to take the bell out. If you use cast, coated cast, or plated, use enough bell enough not to score the bullet......if you use boat tail type...you can get away with no flare. The boomers are particularly sensitive to OAL. The traditional mag length is not necessarily going to apply. The 350 legend is supposed to be much more forgiving, but be prepared to face the possibility that you may have to change your bolt load to run in the AR....or have two different loads. I'm not sure what the common primer for 350 Legend is, or what it was designed for. In 50 BW, I run large pistol magnum primers, in 300 BLK and 5.56 I run small rifle magnum. I've shot literally thousands of rounds through AR's with standard small rifle primers, but years ago I switched to magnum just because then I could store just one type of small rifle primer, and I run that in everything that uses small rifle. Case length and trimming...think pistol. My Beowulf cases ***shrink***. The webbing above the rim collapses over time, causing them to shrink and become shorter, they never get longer. After about 10 firings, the primer pocket starts to get sloppy too...they never get trimmed, they never crack, they never separate, no other signs of wear....when the primer go/no go gauge fails for stretched pocket, they go in the recycle barrel. Don't know if 350 Legend will behave the same, but something to keep in mind. With the BW case, because it's rebated to the 7.62x39 rim, there is a lot of stress at that point, the legend looks a lot more robust in that area. Even so, I suspect it won't grow much. 5.56 on the other hand....you'll need to keep an eye on those, and trim when needed. If you're converting mil brass, you'll need a swager/cutter, and you really should get a primer pocket go/nogo gauge, saves a lot of pain over the years.