Not compatible. Each RT Bullet fired passes through a logic filter that checks the target's stance, lean, tilt and facing. It then checks to see if the bullet hit any one of over 15+ hit zones. It then passes any hit information to the damage calculator which adjusts damage based on bullet energy remaining and location. All that, per bullet, per split second. That's just for the player hits. There's other stuff in RT for visual effects, dangerous richochets, broken texture based realistic penetration, UN Matrix Mode, tracer modifications, etc.
IBT is the replacement mutator to end all mutators (kind of like the One ring). It isn't compatible with anything it replaces, because it REPLACES everything. So if I were to release UN Handgun Fix II tommorow, it would not work correctly with whatever IBT does until it is recoded into IBT.
But, if IBT can do everything, there is no need for the other stuff
Edit: I just looked up two posts about the .50AE and penetration. I'm 100% certain the .50AE won't penetrate the front armor of the Lvl3A vest used in INF because INF's version is equipped with a special hard plate insert. This detail was made apparent to me a long time ago (UNHGFix is like a year old.) Now, whether or not the .50AE can tear a hole through 32 layers of Kevlar is a very interesting question. My guess would be no. But have you seen what a 300 grain bullet travelling at 1550 ft does to ballistic clay wearing 32 layers of Kevlar? It's not 1 damage, it's closer to 30
(Remember, death in INF isn't just lethal death, it's your ribs are shattered, you can't breathe and you don't want to fight anymore, so just lay down and cry, death.)