I've never shot the 180 speer's in my 357 maximum, but I do shoot alot of 180 grain XTP's. I can get about 2100-2200 or so seated long in a 13" barrel. I would expect similar results from the legend given the length and extra capacity. Unfortunately the XTP's pretty much pancake at that velocity so I download them to about 1900-2000 fps.
There’s WAY more bore resistance on the 180 XTP’s than the 180 Speer HotCor. Hell, I get greater pressure signs with 158 XTP’s than the 180 Hotcor.
The “rifle type” bullets I expect to be coming out for the .350 Legend would likely be in that same class as the 180 Hotcor and 180 SSP-SP we’ve been shooting - long ogives with shorter bearing surfaces, so higher speeds with lower bore resistance, getting more out of the case.
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I get flat and cratered primers with 23.0grn H110 under the 158XTP, bad case thrust, hanging cylinder, and sticky extraction at 24.0, and only reach ~1830fps. 23.0 also gets sticky at 1780fps with a 180XTP (honestly can’t recall which cannelure was used for this data, I’d assume the longer of the two - COAL was 1.700”). Alternatively, I have no issues at all with 23.0grn H110 under the 180 HotCor at 1760fps - round primers, no cratering, no cylinder drag, no stick. I haven’t touched one off that hot, but I can get 27.5 grn under the HotCor without compressing, but they start cratering over 25.5grn and uncomfortable flat and sticky by 26.0 which is where I’m running in that upper 1900-2000fps ballpark.
As you mention - the 180’s really aren’t made to run into stuff at high speeds. The 158-Flat Point is designed for higher impact velocities than the standard 158 and 180, but still, not for the speeds we’re talking about in a long .357max, B&D, Legend.
I made a spreadsheet cataloging the available bullets at the time when I built my first B&D Mag. I called manufacturers for expansion specifications for over 2 dozen bullets, culling down to about 20 I thought might make sense. My specifications at the time were muzzle velocity 2000-2100fps and impact velocity 1500-2000fps. The Swift 158 A-Frame and Barnes 140 XPB had the same expectation - worst case scenario, the tip sheds and the shank penetrates like an FMJ, which I do consider acceptable. Speer claimed ONLY the Bonded Deep Curl 158 and 170 would be suitable in their revolver bullets (none of the JHP’s) and the 180 HotCor would be suitable. Hornady wasn’t laying many bets, said the 125 and 158 XTP-FP and the (discontinued) 180 SSP-SP were the only ones they thought would suit for deer, the 158 & 180 probably surviving for coyotes. Sierra backed their 125 and 158 SportMaster JSP and JHC for coyotes, but not for deer. Nosler said they didn’t have anything suitable, and the Sporting Handgun 158 would over-expand at anything over 1500fps impact velocity.
In the end, I settled on the 180 HotCor as meeting my needs, with options for about 10 other pistol bullets which could be usable. The Legend will likely be a BIT too hot for most of these bullets still, since I’d guess it’ll get a 158 WAY up over 2200fps. But it’s getting into the lower speed end of rifle bullets, where a few more options open up.