I would convert the weapon's function before converting its numbers. A rifle, pistol, and shotgun should occupy different niches in the fiction and in combat decisions before you decide exact damage: range, concealment, ammunition, reloads, noise, and the ability to keep someone threatened can matter as much as the wound itself.
For a modern weapons conversion, I would start with three or four profiles and test them in actual encounters. Keep numerical differences modest at first and let properties create identity. Noise can attract trouble, reloading can force someone to seek cover, and scarce ammunition can turn a shot into a decision. After several sessions, increase or reduce damage if the weapons feel too weak or dominant. I would also watch protection carefully. If modern weapons make every defensive choice irrelevant, the problem may not be one damage number; the whole subsystem may have changed the relationship between exposure, risk, and survival.
