[Steve_in_29]

The only real common wear item on the bolt is the "extractor spring" as HK didn't limit the travel and a miss feed is most likely going to over extend it and ruin the spring. Always a good idea to keep several spare springs on hand. The spring from the .308 rifle is stronger then the 9mm and is a direct replacement.

 

[Steve_in_29]

Cool, thanks for the advice!
Shouldn't the PTR 91 come with a.308 extractor spring?...
Either way I will source a few and get them just in case I break one.

Yes the PTR will come with the .308 spring, my comment was aimed at your SP5K as they have the same issue with extractor springs and the .308 spring fits it.

The springs don't "break" they just get hyper-extended and thus loose tension on the extractor. It is something you should check after any miss feed. If you can easily wiggle the extractor with your finger the spring is toast.