Barrel has to be removed, cocking tube cut off, cocking tube insert bored out. THEN, either your original barrel reprofiled to a G3K length (rifling would be incorrect but it would be the HK made barrel) or use an RCM made barrel which are excellent. Install new G3K length cocking tube, refinish rifle. You might also get the paddle mag release installed since it is getting refinished. Also new triple tree with the grenade launching rings.
https://www.hkparts.net/shop/pc/G3K-...K-125p1462.htm $80 for cocking tube
RCM G3K barrel $235
Looks like RTG is carrying these same ones for $178
Rtg parts, new eye bolt $8
Rtg parts new triple tree $20
Then the paddle mag release parts will run about $45 for the spring kind, and the undersized pin and bushing as well as the contact peice you are looking close to $500 in parts.
Then the labor of pushing out pin, barrel, removing cocking tube, resetting barrel, rewelding cocking tube, installing paddle, refinishing, reparkerizing, repainting.