Welcome to TFL!
I don't mean to rain on your parade,but I'm giving you honest feedback.
The youtube post said he necked it down. What I think I saw was a shorter neck and a shoulder blown forward. Am I wrong? Or is it 222Magnum with the neck trimmed short?
And the author saw it as a way to use 90 gr bullets while magazine loading.
Well.... I'm not crazy about the short neck. It will make the neck tension less effective. Folks worry about bullet setback upping pressures. There are other cartridge "straightness" concerns.
Given the 5.56 brass (I assume) is the same length overall, I just do not see where any advantage is gained for mag loading the 90 gr bullet. Isn't the case mouth in the same location/length? I'm puzzled.
The only reason I know of to move the shoulder forward would be a small gain in powder capacity. The intent might be to make up for powder capacity lost to deep seating the bullet.
Something to be aware of is how that works with the length of the bullet ogive. For example,a 75 gr Hornady A-Max cannot be loaded to mag feed with 5.56. All of the cylindrical portion of the bullet seated into the neck is not enough. The cartridge is still too long for the mag. Deeper seating pushes the ogive down into the neck. Thats no good.
If I understand correctly(and I might not. The vid is not 100% clear) you will have a special deep chamber for a very niche loading.
Blowing the shoulder forward is extra work for brass you MIGHT load 4 times.
And you can't use 5.56/223 factory loads.
If you had a clear and specific need for 90 gr mag fed ammo....maybe...but I personally can't see how moving the shoulder forward gains real advantage.
You might consider 6.5 Grendel , etc But Have fun your way!