I am having crazy issues with pressure on ammo I have loaded, none with factory ammo.  I am well below starting change weights on both H110 and LilGun.  I have seen the crimp-sensitivity.  However, no-crimp has pressure signs as well.  I have also seen overall length growth when hand-feeding, ~0.015" over 4 hand feedings on one round, this drove me to experimenting with more crimp as I suspected the bullet was approaching the lands.
I am loading the Hornady 170 interlok, same as I have shot factory ammo.  My muzzle velocity is below factory ammo, I am at the same seating depth, shooting virgin starline brass.

Things I have not tried: different primer, seating deeper.

Gun is a 16" BCA barrel, standard bolt and carrier, carbine gas, suppressor.  I have tried both standard and H3 buffers.

I already did

Haven't used Lil Gun but H110 is best used with heavier charges in most rounds it is suited for. Too low a charge weight will show pressure that isn't there. I believe I read this is because the flame from the primer skips over the powder surface before it properly ignites. If you are using data from a reliable source that specifies charge weights DO NOT go below the minimum recommended.

I bought an inertia puller and some factory ammo with the same bullet.
The factory ammo pulls significantly easier than anything that goes through my resizing die. I'm trying to figure out how to reduce that neck tension. Some of them I can't even hammer out

The 350 Legend is only slightly behind the 357 Remington Maximum in potential velocity. Dave White and Mike Bellem (https://www.bellmtcs.com/ ) have done extensive work the Max and routinely push it up to 223 Remington pressure levels (55,000 C.U.P.) in single shot actions like my CVA Apex, and both the T/C Contender and Encore.

As the previous poster suggested, and is written as a warning on every container of H110, Lil-Gun, and WW296, do not go below minimum listed loads or detonation is possible.

My brother shoots a 180gr Hawk bullet over a charge of 22gr of WW 296. We are getting right at 2300fps with one of Mike's 20" barrels on my brothers Contender. It holds MOA for three shots every time we take it to the range.

My CVA Apex was rebarreled by Dave White in the Max. Two very accurate loads that I have developed for it both use WW 296 with Remington 7 1/2 Primers. This rifle was built just for Iowa's straight walled pistol cartridge deer season to be shot from my farms blinds.

140gr Sierra JHP Power Sport:  w/27gr of WW 296 give me 2550fps. The deer I have shot with it are dead right there, but those bullets pretty much loose 2/3 of there weight on impacts inside of 80 yards. Clearly it's too fast for those bullets at impact speeds over 1800fps.

150gr Cutting Edge ER Raptors: w/25gr of WW 296 give me 2450fps. No deer yet this year but the bobcat I shot dropped in her tracks and the bullet performed as advertised by shedding it's petals inside the animal and .357" entry and exit holes.

All of the 140-160gr bullets I have tried in my Apex will touch three shots at 100 yards with standard loads and proper crimps. I would think that the 350 Legend is more than capable of producing similar performance. Had it been introduced two years ago, I would not even have a 357 Max.

None of my loads show any pressure signs, but you should work up to any reloads in your rifle carefully.

we've seen some pretty disturbing stuff with a lot of bear creek arsenal barrels.  lots of chambers that aren't even remotely within specification.  what you describe could easily be a chamber issue.

Add this to the sloppy throats...one barrel will allow loading a. 358" bullet and the next won't-no two makers agree on how to make it. It's a handloader's nightmare.

sorry dude...  just calling them how i see them.  i wish that weren't the case, but the quality control isn't appearing to be as tight at bca as others.  i don't mean to suggest value can't be found in lower cost items, but when people sell you barrels for the price of chicken feed, you should be questioning the quality of what you're getting.  there are specific things that lend themselves to tight tolerances, many of which take time, multiple steps and/or tremendous machinery to overcome, both of which have a hard cost associated with them.  when you can buy a stick for $60, and it doesn't perform, it should be on the list of things to consider as a problem.

as far as the 350L goes, i wish these guys would make up their minds and stick with it.  if i was going to default to something, i'd default to whatever winchester says to do.  they developed it, so i tend to like to let them lead on what they've created.

i'm also hearing some real horror stories with consistency in terms of feeding and cycling in gas guns.  keep in mind why the 45 works so well in a 1911 and why the 38super takes so much more work...  it's hard making a long skinny and straight cartridge round the corner into a bore.  the don't like that shit.  lol.  so it takes subtle modifications.

I agree with you there. When they can't even drill the right size gas port in a .450 barrel when their own website says what size they use(mine was just over half THEIR published dimension) theres no way they're going to get it right with a caliber less than two years old.

I reload a few calibers, but have no knowledge of 350 Legend other than one can kill a deer with it in Iowa.

That said, I am curious what your pressure signs are. It appears you are using a Labradar or chrony and aren't seeing decreasing returns as you add powder there, so what are you keying on other than case lengthening? Brass splitting and flat primers are two entirely different ballparks.

Cerrosafe is cheap and it seems that you may benefit from it. Be aware that the best readings are taken about an hour after casting. If you wait until the day after casting to measure, it will run .002 larger than the actual chamber size.

ETA: Knowing nothing else, if I were to guess, if pressure is actually an issue, the problem is due to bullet to throat fit. Once upon a time I split some 45-70 brass while at the minimum load, entirely because my bullet was sized incorrectly for the firearm I was pushing it through.

ETA2: Consider a collet puller. For what you have wrapped up in loading your own, it is well worth the money and hassle.  I use Hornady's and can't imagine a scenario when I will use my inertial puller again.