I think your estimate of 4100-4200fps for a .223 with ANY bullet is really high. If you'd have said .22-250 or 220 Swift, I might have believed it, but not the .223. My 22-250 shoots the factory 45gr Winchester JHP's (which are advertised at 4000fps) at an average velocity (by chronograph) of 4010fps, and the rounds are loaded hot enough to flatten the primers every shot.

I don't have my manuals handy (still packed from our recent move), but according to Hodgdon's data on their website, a MAX load of 28 grains of H335 should propel a 40gr Nosler BT at about 3572fps (and though the details of the barrel used aren't given other than length, I'd quess it was a pressure barrel mad to min. SAAMI specs that would give higher than normal velocities and pressures). You say you're using 30 grains of H335 (a 2 grain overcharge according to this data) to achieve 4100fps+, which indicates to me that you are either severely overestimating the velocity of the bullet, or you are getting pronounced, and potentially dangerous, pressure excursions due to the overcharge that are causing the velocity to be abnormally high.

Be careful with those loads, a couple hundred fps are NOT worth losing your hands, eyes, and/or life. If you really crave velocity that much, you need to step up to a cartridge that's designed to do it safely.

Mike