Keep it short, friendly, and articulate. A very long letter is (in my opinion) less effective than a short and concise one.
Something like the following (reworded as appropriate) might work. The idea is to keep it short, don't get bogged down in details, and be polite and intelligent.
Maybe something like the following:
Good afternoon,
I would like to express my opposition to any new restrictions on lawful gun ownership this year, such as magazine bans or restrictions on rifle aesthetics. There are far more important issues on the table than harassing lawful gun owners, and I respectfully ask that you please oppose such restrictions.
Thank you very much for your time.
Respectfully,
xxxxxxxx
but make sure it is in your own voice and style, not mine. Resist the temptation to pile too many details in an initial letter of this nature, particularly if it's unrelated to a specific bill; if you get a reply, you could follow up with relevant details then, if needed. And of course don't stereotype or insult, don't make it a conservative/liberal issue, don't bring in other issues unrelated to gun politics, and proofread to make sure it doesn't come across as incoherent, uneducated, or agitated.
Also, it's probably a good idea to write it, save it, re-read it the next day and edit as needed,
then send it. Take the time to craft it carefully.
