Title I firearms are ordinary rifles, pistols, revolvers & shotguns.
Title I firearms can be owned and built by citizens as long as they are not a “prohibited person” (defined below) per the ATF. The ATF 5300.9 – 4473 Firearms Transaction Record is required if you are purchasing your Title I firearm from a FFL Dealer.

Handguns
Semi-automatic pistols, single or double action revolvers or braced AR-style pistols

Rifles
Semi-automatic, bolt or lever action rifles typically with barrels longer than 16″

shotguns
Pump action or semi-auto weapons firing shotshells through a smooth bore

receivers
Serialized component that is designed to house firing components (i.e., bolt)
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The ATF defines a prohibited person as anyone:
convicted in any court of a crime punishable by imprisonment for a term exceeding one year;
who is a fugitive from justice;
who is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act, codified at 21 U.S.C. § 802);
who has been adjudicated as a mental defective or has been committed to any mental institution;
who is an illegal alien;
who has been discharged from the Armed Forces under dishonorable conditions;
who has renounced his or her United States citizenship;
who is subject to a court order restraining the person from harassing, stalking, or threatening an intimate partner or child of the intimate partner; or
who has been convicted of a misdemeanor crime of domestic violence.