The hop up is suppose to make the bbs curve up. Have you adjusted it?
The CA hop up is probably metal and the JG is probably a two piece plastic like a TM. Then they are not compatible with out a lot of modification. Plastic bodies are designed for plastic hop ups and metal bodies are made for one piece metal hop ups.
The JG is a TM clone with a 2 piece plastic hop up, but CAs have a metal body and use a one piece metal hop up so it will not work without a lot of modification.
Here is your problem: JG M4's arent the best typr of M4s around but really the only thing a new tight bore barrel will do for you is give you slight better accuracy rating. The best way to stop the bb's from curving up is to 1) get heavier bb's (.23g recommended) and 2) adjust you hop-up accordingly. This will improve your accuracy rating greatly and give you that much of a better chance of hitting your target accurately 100-150 ft. away. That help at all?