I know this is old, but for anybody wondering in the future:
JT TAC-5:- Yes, you can get barrels.
- Yes, you can get bipods.
- Yes, you can get adjustable stocks.
Yes, you can get all this stuff, but it will be hard. They seem to be quite rare. I however, got a set of this with a field kit, a 12" barrel, an 8" barrel, a bipod, an ADJUSTABLE stock, a 200 rd. hopper, a 9 oz. CO2 tank, and a mask for $150. It is definately a good deal, and from playing with just those basics (I didn't even have the bipod or the stock on yet, it was a good gun. It was easily comparible with the 98 Customs everybody else was using, and is probably a better basic gun. That however, is diluted by the fact that it is VERY hard to get mods for it.