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Of course the cartridges have to fit in your magazines with a little clearance to function reliably. But what's important for accuracy is how well the loaded bullet will fit the throat of your barrel. Generally best accuracy in a subsonic is found with the bullet just touching the lands to give a gas seal without pushing the bullet further into the case. Crimping is usually not good for subsonics. How your particular barrel throat was reamed will determine what bullets work best. There is no standard which is right for all bulllets and there are lots of 30 caliber bullets with various ogive shapes.
There are three choices.
1. Find a bullet to match the throat of your barrel.
2. Have the chamber cut to match a partiuclar bullet during it's manufacture.
3. Live with whatever accuracy you get. The limiting factor in the range of subsonics usually comes from vertical stringing caused by velocity dispersion. High drag (low BCs) results in more wind deflection but that's not usually the limiting factor on useful range of a subsonic.
I personally don't like to have the shank (bearing surface) of a bullet extend inside the neck of a cartridge. I believe that causes inconsistnant bullet release. It's ok for a boattil to extened beyond the neck. But worse for accuracy is a bullet which allows blowby befre the bullet engraves. That can be the result of the seating depth or the angle of the ogive where it meets the lands. Don't assume a particular barrel manufactuer cut the thoat of your barrel to match the bullet you want to shoot unless you had the throat matched to a specific bullet and seating depth. Otherwise your task is to find a bullet which works.
I start by measuring how far a bullet will drop into the throat and see if it seal on the lands just by blowing air into the breach. VLDs with long ogives generally produce longer OALs before sealing regardless of bullet weight. Long angent ogive bullets may not seal even though They are touching the lands. Feeding from a limited lengh magazine is just one more parameter which needs to be satisfied for a semi auto or a bolt action.
The additional requirement for a semi-auto over a bolt action repeater is that the load cycles the action. That's true whether it's a gas operated, blow-back, or recoil operated action. Certainly there are barrels for AR-s on the market which make meeting all of the above requirements very difficult or impossible. That's mostly becuase there is no >standard< 300 whisper cartridge, load, and bullet combination. Probalby the 220 grain Sierra Matchking loaded to 2.26" is the closest to a "standard" as its one of the very few commercially available subsonic loads. (made by Corbon). That's not to say that's what you want to use if you're building a custom rifle or that all commercial barrels will work with that cartridge.
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