Barrel kit ?

[Southpaw]

What is the barrel kit where you can choose the sized back, the stepped large bore Barrel, and finially the, Tip ?

[gc82000]

can you give a little more detail on the barrel system, for it is hard to name with the details that you just gave.
But I can think of the Evil Pipe system, Kaner kit, Empire edge kit, just to name a few.
The system you are thinking of does not have inserts, correct.

[Southpaw]

I should have been more clear. The way that it differs is that not only can you choose the length of the large bore ported section but it has a separate threaded mussel break in a few shapes/styles. Not just a large bore section front and the sized bore back. This has the third mussel break that you can choose from also.

[FlameboyC11]

I only know of one brand that makes what you describe. http://www.equationusa.com has what you want, I think.