But one supporter of Greg Brophy, a state senator who ran for governor in Colorado, got something else: a Smith & Wesson M&P15 rifle.

It was the top prize in “Greg Brophy’s Gun Club Giveaway,” an online contest last month in which people handed over personal information that is the currency of modern political campaigns — first and last names, email addresses and phone numbers — and in exchange, one lucky winner would get the gun.

“I tricked this baby out,” said Mr. Brophy, a Republican, boasting about how he had added all manner of accessories — extra grips, a backup sight and a strap so it could be slung effortlessly over the shoulder.

Online gun sweepstakes have become one of the most useful tools for campaign outreach in the 2014 Republican primaries. Across the country, from a race for sheriff in California to the United States Senate primary in South Carolina, candidates are using high-powered pistols and rifles as a lure to build up their donor lists and expand their base of support.