Re: J-lock Rem 700 Question?
Changed out a J-lock last spring on my 6.5-08. Put in a Tubb SpeedLock wanted to go w/ a new shroud. Despite what Brownell's told me, I needed an *old* length cocking piece, and an *old* size bolt shroud. The new J-lock cocking piece is slightly longer to accomodate the lock mechanism in the shroud.
Not a terribly difficult job, w/ the right tools (voice of experience, after being impatient and doing it w/o the right tools first). Get a bolt assembly changing tool, such as the Kleindorst tool from Sinclair's, it's handy for taking the whole assembly (shroud, pin, spring) out of the bolt body for cleaning anyway, and then get their Possum Hollow mainspring changing tool.
Basically all you do is remove the major assembly from the bolt body, put it in the mainspring changing tool, and crank on the knob until it takes the tension off. Unscrew the shroud, slip out the pin retaining the cocking piece to the firing pin, change the parts, and reassemble. It's been a while so bear w/ me if the description isn't exact or misses something.