I think letters are easy to ignore / route to someone else. Besides, it's weird to say, but when I get letters anymore (other than something truly official), I immediately start to think you are a nut case with an IBM Selectric in your basement.
I still think an insistent phone call or series of phone calls to the corporate offices would be more likely to get a response. However, those among us who need warranty service have the strongest claim to more immediately attention.
Daniel Cox is listed on LinkedIn as the Handguns Product Manager at Remington Arms. They list their phone number as: 1-800-243-9700.