unityでrayをcollider裏面に当てる方法

Physics.queriesHitBackfaces = true;
↑このAPIを使えばいい！
5.5からのAPIです。

テスト結果：

Shooter.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Shooter : MonoBehaviour
{
    public GameObject testBox;
    // Start is called before the first frame update
    void Start()
    {
        
    }

    // Update is called once per frame
    void Update()
    {
        Physics.queriesHitBackfaces = true;
        RaycastHit hit;
        if(Physics.Raycast(transform.position,transform.forward,out hit,1000)){
            Instantiate(testBox,hit.transform.position,Quaternion.identity);
        }
    }
}