Biggest problem with the J-lock system is the firing pin spring Remington used in these rifles. Everyone I have seen is an extremly poor fit and as mentioned coils up and rubs very hard on the inside of the bolt body, not really the firing pin.
It will bind on the pin when the striker assembly is removed from the bolt but when installed into the bolt body, the spring bares very hard against the inside of the bolt body.
This creates inconsistant bolt vibrations when the striker is released and can change the position of the bolt in the receiver from shot to shot.
Best fix I have found, do not just replace the bolt shroud which has the J-Lock on it, replace the entire striker assembly with a quality made, match striker assembly.
The best I have found and the only one I use in my shop is the Gre Tan striker assembly. They are not the cheapest but they are the best I have used.
You can also just replace the firing pin spring with a quality match grade firing pin spring and solve 80% of the problems associated with the J-lock system. But I still recommend going with the entire assembly as they are built to work as one match system and work extremely well.
Go to www.brownells.com
and search under part number 100-000-446 and they will come up. They list them at $65.00 which is well worth it.
You get much more consistant trigger pulls using these assemblies as well because of their tight fir to the receiver compared to the factory assemblies.