Starter pistols in/out of Chicago Midway I'd be leary of, for instance, but a flare gun? Done it 2-3 times. Ditto PIT, SFO, SJC, and LAX. Since they count as a boating safety device nobody has a law against possession/ownership/transport for those 21 or older (below that, check local laws for destination/source/etc).
So what a lot of folks do is get a 24-gallon action packer (ends up ~61.5 "linear" inches, less than the 62 "oversized baggage" by a smidge) and a cheap Orion flare-gun from Wal-Mart w/ some long-shank padlocks.
Just make sure you don't bring any flares! FLARES CAN'T TRAVEL AT ALL! Not checked, not carry-on, verboten.