JP is known for having pretty guns. This comes from attention to the little details like styling, lines and many other cosmetic upgrades. Anyone who owns a Presentation Grade CTR-02 rifle will readily attest to this.
When it comes to barrels—assuming you're not going for paint or another applied coating, the standard practice is a high-grade polish for a nice shiny appearance. Whenever I trained in a new gunsmith at JP, I made sure they understood the importance of this. JP barrels are unmistakable.
The vast majority of JP barrels ship with a JP compensator, whether the large or small version. But some people speculate on just how the comp is machined out of the barrel. Even an existing customer will sometimes ask if the barrel starts at 1.5" OD and then gets cut down to shape the comp.
Nothing near so elaborate, I'm glad to say. It's simply that the intersection between the compensator and the barrel has been polished over to look as if both parts are actually one. Here, I'm going to give you the rundown of how barrels get polished at JP. If you're the ambitious builder, go ahead and try it yourself.
Safety is a major concern in this process. At JP, polishing is done on a lathe. This can be very dangerous and, as a gunsmithing instructor, I don't recommend it for just anyone who wants to try it.
The barrel must be spun at a relatively low speed, and it must be completely secure between the head stock and tail stock. This means that neither end can be allowed to migrate, so the tailstock must also be held rigidly in place. A poorly secured barrel escaping from the spinning lathe can result in serious injury.
Use a lathe only if you are experienced and have safety training.
The safest method that won't hugely increase the time involved is to use a barrel spinner, which is a long bracket with live centers at either end. With the barrel seated in the spinner, you just apply it to a buffing wheel and polish in that fashion. You can purchase one of these from either Brownells or Midway USA.
If you have the time and inclination, you can also perform the work by hand, using the old fashioned "shoe-shine" polishing technique for the entire process. Neither of these is efficient from a production standpoint, which is why we used the lathe at JP. For a one-off build though, either will work fine.
Regardless of the means you choose, what follows is the sequence and the result.
Start by checking for defects either on your barrel's thread shoulder or on the rear face and edge of the compensator. If present, these will spoil the final effect and leave a visible artifact. Both edges must be perfect and clean for the blending to be successful.
Watch for this if you need to remove the comp from the barrel when assembling the rifle. On occasion, this can result in an artifact occurring upon retightening of the compensator. This is the result of a less-than-perfect remarriage of the parts.
In the JP shop, it is not uncommon to make a single small cut to the back of the comp to remove a dent or other imperfection in the rear edge face of the comp. A small strip cut from a Scotchbrite abrasive pad applied shoe-shine style on the intersection will almost always repair such a minor imperfection. Keep in mind that this may affect timing of the comp.