Can't really help with your actual build question, but just in case you haven't already found it,
here's a link to the rules. M-16 rifle requirements start at the bottom of page 20.
Have fun.
Hoss
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I ain't as good as I once was, but I'm as good once as I always was
Kahr PM-9 / Sig P-245 / Para NiteHawg / Walther PPK / Beretta Tomcat / Ruger LCP
BDA 380 / Taurus 85 / Kel-Tec PF-9 / Am. Derringer 357 / Sig Sauer 239 SAS / Walther PPS .40
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