UL (a bit more accurate, but no adjustable bore sizes, you'd have to buy multiple backs)
Oddssey kit(freaks are way more accurate and look much better)
Assassins (freak more accurate/adjustable)
Tear Drop (freak way more accurate/adjustable)
Any kind you want, as long as you have multiple bore sizes.
I've owned a couple different freak systems now. I've had an older style one and the stainless steel version. Both were very accurate and quiet. I personally prefer the all-american barrel tip with the aluminum back and aluminum inserts.
-They're a tad heavy for the price, but much lighter than many other barrels that you'd normally get on your marker stock.
-You have options from multiple fronts, backs, bore sizes and even different types of metal.
Noise level: 9
-I didn't notice any problems in the amount of noise from the barrel
-Not the most accurate, but definately up there. The adjustable bore sizes enable you to match your paint with the specific size, creating a good solid straight shot.
I bought a full kit for $55, it was a heck of a deal. I wouldn't reccomend paying anything over $65 for this kit though, there are nicer barrels to be had.