Limit rotation within a volume

I'm working on a turret controller. The turret has a certain range of motion (it can't tip down past a certain point and it can't fire behind itself at a certain point).

I'm using a joystick for input and the joystick axes are mapped to different axes for Quaternion.AngleAxis rotations. The rotations work great, but the way I'm limiting the rotation doesn't. It works in the sense that it limits the rotation correctly, but I think because I'm totally halting the rotation calculation, it gets stuck and/or stops - it feels "sticky" at the edges. Pulling the joystick away from the limit-zone get things going again, but I'm looking for the turret to glide across the boundary I'm establishing.

I can't just test for Euler values, because many of the x,y,z Euler values are blended throughout the range of motion, so limiting it "in space" seems like the right starting point.

Thoughts?

 using UnityEngine;
 using System.Collections;
 
 public class TargetInput : MonoBehaviour
 {
         
         public float deadzone = 0.25f;
         public float angularVelocity = 0.5f;
         public GameObject target;
         private Vector2 stickInput;
         private Quaternion lastRotation;
 
         void Update ()
         {
                 stickInput = new Vector2 (Input.GetAxis ("Horizontal"), Input.GetAxis ("Vertical"));
                 if (stickInput.magnitude < deadzone) {
                         stickInput = Vector2.zero;
                 } else {
                         
                         Vector3 axisX = transform.forward;
                         Vector3 axisY = transform.right;
 
                         transform.rotation = Quaternion.AngleAxis (stickInput.x * angularVelocity, axisX) * transform.rotation;
                         transform.rotation = Quaternion.AngleAxis (stickInput.y * angularVelocity, axisY) * transform.rotation;
 
                         if (target.transform.position.z < 0.1f || target.transform.position.y > 3.8f) {
                                 transform.rotation = lastRotation;
                         }
 
                         lastRotation = transform.rotation;
                 }
         }
 }