On and off, I've been working on an all-metal (and hence non-shattering) version of the Jam Jar Jet. My latest version was supposed to be adjustable, but it turned out to be ballistic.

Step 1: Do It Outdoors!

When I first made this, I made it inside my shed, expecting a vertical jet of flame at most.

Instead, I had a can of burning meths ricocheting around my ears.

Oops.

(Note to self - buy fire extinguisher!)

Step 2: Materials and Tools.

I used five things:

An empty tin of tomatoes (ring-pull kind).
An empty can of coke. I only ever drink the full-sugar caffein-riddled version, but I don't think it's vital to the efficiency of the projectile...
My Instructables Leatherman. It's a well-used tool already. I even use it to open the shed since the door handle disintegrated (I really must get round to fixing that).
Matches. Long ones. You'll see why.

Step 3: Make It.

Use one of the pointy bits to poke a hole in the bottom of the coke can, then enlarge it to around a quarter of an inch across.

Use the pliers to rip the top out of the can. No need to be pretty, but try and avoid too many sharp ragged edges, and don't bend the can, otherwise it won't fit smoothly into the tin.

Step 4: Fuelling and Firing

Put a splash of meths in the bottom of the tin. Just a splash, it's the fumes we're after.

Drop the can into the tin - if the fit is right, the can should take a moment to slide down as air forces its way around the sides and through the small hole.

Put a finger over the hole and give it a shake to generate plenty of vapour. Watch for spills from between can and tin.

Stand the can somewhere clear and firm.

Touch a flame to the hole.

Duck.

Step 5: The Aftermath

Be careful!

It will be almost invisible in daylight, but the meths will still be burning in the tin. Look for heat-haze over it, and hold your hand well above the tin to feel for rising heat. Extinguish any flame by placing any handy flat thing over the tin to exclude oxygen.

If you used too much meths, there will be some left in the coke can as well. If it landed somewhere non-flammable, use the remains of the match to stand it up and extinguish it like the tin.

If it landed somewhere flammable, pick it up by poking the match into the small hole and lifting at an angle (see, I told you to use long matches), but watch for more flames as leftover meths continues to burn.

If the can is dented, it won't fit in the can well enough to fire again, but dents can be pushed out fairly easily by hand, just watch the jagged edges.

Repeat until bored.