This summer I will be taking a rather inherently dangerous multi-day hike by myself. I was considering buying a flare gun since I will be out of cellular range unless there is something more modern and equally reliable. Any suggestions?
In increasing order of complexity and price you have,
Personal Locator Beacons
Personal locator beacons are high-powered (typically, 5 watts) devices designed primarily to send out a personalized emergency distress signal. They generally require an open view of the sky to transmit successfully.
Satellite Messengers (Like a Garmin inReach)
Much like PLBs, satellite messengers are handheld transmitting devices that are useful in backcountry areas far from reliable cell phone coverage. These user-friendly devices allow you to communicate short text messages and/or your location coordinates with friends or family back home so you can report on your trip’s status or, in an emergency, send calls for help.
A satellite telephone, satellite phone, or satphone is a type of mobile phone that connects to orbiting satellites instead of terrestrial cell sites. They provide similar functionality to terrestrial mobile telephones; voice, short messaging service and low-bandwidth internet access are supported through most systems.
None of these are a substitute for good judgement and the electronics can fail or run out of batteries, but they would be the modern equivalent of a flare gun.
While Charlie Brumbaugh's answer is good, there is a cheaper and simpler alternative: a LED marine distress flare. These are handheld LED lights, as bright as a distress flare. Many jurisdictions certify them as legal substitutes for distress flares.
However these are substitutes for handheld flares, not flare guns, and are designed for marine environments where hills, trees etc. can't block them, and where a bright coloured light is recognized as a distress/location signal.
Here is a link to an example, without recommending this brand in particular.
Maybe you should explain what you need a flare gun equivalent for... A flare gun has a very specific purpose which generally is to alert somebody who is already looking for you to your immediate location. On top of that you must know that the people looking for you are actually within a certain range, otherwise the flare will be wasted.
A flare gun might have a certain appeal to the romantic adventurer, but is not a useful tool to call for help when alone in a remote wilderness area!
--> The flare will only be visible within a certain range for a rather short amount of time. Anybody not currently paying attention to the surroundings will likely miss it, also everybody without direct line of sight won't have any chance to spot it in the first place.
If your intended purpose is to have a tool that allows you to direct the attention of search-and-rescue personnel to you there are several tools, all with their specific advantages and disadvantages:
- Mirror to reflect sunlight (optimally with a hole in the center, through which you can 'aim' the mirror at whomever you're trying to signal)
- Smoke petard
- Flashlight/LED flare
- Signal rockets/bangers
- Rescue whistle
- Last but not least: flare guns still exist and are used in emergency scenarios.
If your intended purpose is to call for help and alert rescue personnel in the first place then go with the options mentioned by @Charlie Brumbaugh:
- Personal Locator Beacon
- Satellite Messenger
- Satellite Phone
A safety whistle is cheap and reliable. Also, it won't run out of flares or batteries. You can hang it from your backpack shoulder strap for easy access. (Imagine trying to take off and dig through your backpack with a broken arm.) Three chirps is the distress signal.
Similarly, a signal mirror is an inexpensive visual indicator, like a flare.
A laser can be used. There are some lasers with SOS function but it is not necessary. The beam is visible from large area and they can locate you easily.
It should be so powerful to be seen but not so much powerful to endanger airplanes above you.