Not absolutely sure I am clear on the first question but I'll try anyway.
The belt on the 60 loads with the metal links up only.
Is that what you mean?
The cartridge is stripped from underneath. The gun could not feed them with the links down.
As to use, as long as you don't step on them
they are going to last a long time.
Eventually, a few can get a little weak on the extraction groove tab.
When that happens, the cartridge will slide forward or back in the links slightly and just stop the gun by hitting on the feedtray.
Even that usually only happens when the belt is bouncing and swinging around and has no support into the gun.
You will feel the weak links (no pun intended) when assembling the belts.
I have had that happen with a few brittle war2 8mm belts, so now I mostly use the NATO DM-1 belts made for the MG3/MAG.
Shrike owners are looking for the opposite. A way to loosen the links slightly in an effort to get them running consistently.
It is pretty hard to polish and buff the interior of the disintegrating links! LOL!
If anything, I found the M13 links a little stiff.
I bought and use the German improved NATO DM-6 links when shooting .308 with the proper top cover on my 42.
Here is a link for them you might check out.
http://www.robertrtg.com/store/pc/DM-6-LINKS-100-PACK-WITH-STARTER-17p2573.htm
Robert and his wife only sell the best.
They do link up much more smoothly and the starter tabs are handy.
JT