Re: .223 remy
As the distance increases the effects of wind and MV consistency are going to become more apparent. Those are what will determine your max effective range with that ammo on paper since they determine your ability to hait a certain sized target. The difference in twist rate between your rifle and your mate's rifle isn't going to make a difference with 55gn bullets. The difference is that your rifle probably will not be able to stabilize bullets much heavier than that. Heavier bullets can have a better ballistic coefficient and thus resist wind better, increasing the effective range of a particular cartridge.
Your rifle *should* be able to stabilize the 53gn Hornady V-Max. If so, that bullet will give you the longest effective range. It not only has the highest ballistic coefficient of any .224 varmint bullet, it is also significantly better than any of the target bullets in that weight range. In addition, it's relatively light weight means that you can get better velocity out of it as well. The double whammy when it comes to bucking the wind and increasing range.