Tooling is expensive - a reamer/tap set with a guide bushing for your barrell can be $250 to $300. Your barrell may also require a standard reamer to remove the old choke and reset the barrell diameter to cylinder bore. You then use the stepped reamer to open up the bore for the new choke threads and a thread die to cut the new choke threads for your choke mfg (Remington & Winchester have choke tube threads on the opposite end of the choke tube). The barrell wall thickness after all this machining is critical. The tooling mfg will specify the minimum wall thickness for the choke tubes you are installing. A barrell that is too thin can "shoot the choke" off the barrell. The good news is new tube installation can usually be done without a lathe, depending on your barrell. Also, tooling can be rented for less than half of the cost of buying new set. Good luck.