This is a common problem with .223 Rem, and probably a lot of other commonly-used cartridges as well. The longer, heavier, higher BC bullets cannot be seated optimally in the case and still feed properly from a magazine. I think I'll let someone else debate whether that is an issue of mags and/or actions being made too short, factory rifles not being throated properly, or something else entirely. Nonetheless, it is commonplace to find factory rifles that won't allow you use the high BC bullets you'd really like because they will be too long to feed from a mag when seated optimally.
A reloader in this situation has a few different choices:
1) Use a shorter bullet that can be loaded to mag length and still be seated optimally in the case neck.
2) Load the longer bullet to mag length, accepting that the bullet shank will be seated well below the case neck/shoulder [Note: pull apart a factory 77 gr .223 round sometime; this is exactly what has been done]
3) Purchase a magazine that can accommodate longer loaded rounds, or modify an existing magazine.
4) single feed
There are probably a few other approaches. Any or all can work, it's up to the individual to decide what's best for them. Chaser - if you decide to go with route #2, simply determine the longest loaded round your magazine will feed reliably. Many use 2.260" as "mag length", but all magazines are not created equally. Set that specific COAL as your "max" or "baseline" value, then move the bullets incrementally farther off the lands during your seating depth testing. Yes - you'll be moving them even further into the case neck, but they'll still run reliably from a mag (i.e. they only get shorter, not longer). Seating depth is cyclic, so you're really just looking to optimize seating depth in the region where the bullets are seated pretty far down in the neck. Obviously, it will be neither the same seating depth, nor COAL, as for an optimized load you might generate if you were to decide on single feed as in option 4. Nonetheless, you can still find a seating depth that works by moving the bullet only in the direction farther away from the lands.