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The length of the standard carrier allows the recoil spring to coil up inside of it as it comes rearward. On a shortened carrier the recoil rod must be collapsable or you will not be able to bring the carrier rearward far enough to cock and chamber a round. While the carrier rides rearward allowing the rod to pass through it the cocking support does not. On my 51k length I modified a HK33 cocking lever support (cut it down and rewelded) to allow the carrier to be brought far enough rearward. I also had to convert the vector pistol backplate I bought off a member here to a collapsable style. Had to cut the channel and lockup back into the reciever about center above the mag well, and made a buffer spacer moving my buffer forward so the recoilling carrier did not bottom out on the recoil rod. One thing other members pointed out (they used slightly longer carriers with standard length cocking lever supports, was the tendency of the support to slip off the carrier when cocking, their fix was to make a sleeve for the support so it slid into the carrier to keep that from happening. With mine I see it does try to slip but the longer support minimizes it with mine. Part of the issue of slip is with the shorter carriers is the cocking lever support actually comes rearward out of the cocking tube and is unsupported, on the shorter length the support can slip off or angle and cause some issues. I had the additional problem that while I had reamed the cocking tube before assembly after I welded up the custom tube I had a freind make I found that there was just enough of a lip to sometimes catch the support and hang it up as it was returning to the tube. I had to modify a hand reamer to fit into the reciever and ream the tube to remove the lip. I also adjusted the leading edge of the support (broke the corner so to speak) so the support could guide itself back into the tube. This adjustment also changed my cocking tube gap so keep that in mind before choosing to make an adjustment like that.

Hope this was helpful and I did not over explain. In short the shorter the carrier causes more things that have to be adjusted. Good luck with this

Whatever length your 51P carrier is should be what you have your training bolt modified to. Each part is designed to work with the others, if you change carrier length, then other stuff needs adjusting.

As for a collapsable recoil rod, it will work fine in any of the G3 variants. So you can use one with a standard stock if your papered to do so. The collapsable one I built I used the correct diameter music wire, annealed it to remove its hardness. Used roll pins to secure it in the rear section of rod attached to the backplate, cut the appropriate amount of recoil tubing from the rod that I needed it to be able to collapse properly, attached the front portion to the music wire with another roll pin. Cut slots in the recoil rod to allow it to slide past the roll pin as it collapses and springs back full length, then I removed a couple inches of spring coils (after much measuring and testing). Builders like PCS have custom springs made for theirs).
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