reasonable percent of my paycheck I should spend...
Are you married (or previously married)?
Do you have a mortgage? Child Support? Car payments?
Are you currently saving for retirement? Are you saving for your children's college?
What's your gross monthly income?
What is your disposable monthly income?
What is your discretionary income?
HINT: If you don't know what your disposable or discretionary income is, then the answer is zero percent.
At present for pretty much anyone living in the United States, ammunition is a luxury (professional and subsistence hunters excepted) and until you're living comfortably within your means and have all of your necessities covered, you don't need to be spending anything on it. Once you are living comfortably within your means then you should be able to easily determine how much you can spend without cutting into otherwise prudent financial behavior.