the rock river arms detachable carry handle on my FN upper keeps sliding backward on the rails (toward the charging handle) per 40 rounds discharged, about 1mm at a time. Any of you have some non-permanent solutions that won't mess up my finish? Thanks.

This could have something to do with your problem: www.ar15.com/forums/topic.html?id=87543

Their handle was probably machined to fit their uppers.

Mike

My Model 1 Sales "FN" ( FN= made by whoever) had the same problem. My swann sleeve, #38, would not slide on because the receiver rail was to large. When I did get the sleeve to fit with a hair bit of sanding the receiver rail shoulders (/ \). I found that the receiver was too tall to zero the weapon. 8" high at 50 yards with maximum front sight elevation. Same problem with the carry handle.It was sent back for replacement Tuesday. Hopefully I will receive a upper that is in spec.