According to SR3 every active skill has a B/R version. Obviously this was a time-saver typo as I can't really see some skills, like Etiquette B/R or Divining B/R.

However when I was thinking that the idea of a Martial Arts B/R skill came into my head. Is such a skill imbalancing? Absurd? Or an interesting way to affect martial arts.

As I see it the skill could work in the following ways:

Option 1: A Martial Artist with B/R (Let's say an Aikido user) can use his B/R skill to develope new techniques, allowing him to learn maneuvers not associated with that skill (Say Ground fighting in this example). There would have to be a test to see if his B/R skill is high enough to learn the maneuver and possibly more karma to get a nonstandard maneuver, I'd say 50% more so that Ground Fighting (aikido) would cost 3 Karma after you pass the test to learn it.

Option 2: It simply eliminates the requirement in Cannon Companion for you to have a martial arts instructor to learn a skill. I don't really care for this one but it does make sense in a way. You could have a B/R test against the level of skill you want to learn to see if your character can develope it (This could also be a sneaky way to circumvent the disadvantages of using Ninjitsu).

Any other ideas?

B/R is a Build/Repair skill. There is nothing to build or repair with regards to martial arts, except certain weapons which are covered under other skills. If you want to develop a new maneuver, I'd rather suggest a knowledge skill based on the martial art in question.

...Ninjitsu has disadvantages?

Sorry. I don't own CC.

B/r in martial arts could be skill to make that styles weapons.
so i have got it.

Um...yeah.

Sorry, no dice. It would be like B/R Ballet or B/R Athletics -- it doesn't really work. If you want to "invent" a technique, put some numbers down and run it past the GM -- you'd be surprised how many "martial arts" developed much the same way.

Although if you want to B/R that style's weapons, get the appropriate weapon B/R skill.

-Siege

Katana ain't no a Ninjutsu weapon man ... it's kendo (way of the sword...).
Tanto and all the peasant weapons are...

SR3 does not say every Active Skill has a Build/Repair Skill. Page 85 clearly states "Many Active Skills have a corresponding Build/Repair (B/R) Skill." Unarmed Combat and Martial Arts would be one of those that don't.

Katana? B/R edged weapons.

-Siege