Hello,
this week I saw a video on Youtube where a guy playing Mame shooting games with Wiimotes instead guns:
https://www.youtube.com/watch?v=VqA78ssTlFY
So today I would like to do the same...but on Linux.
After hours of tests, searches, read, tested decades posts/tutorials on internet, I finally found an easy and satisfaying soluce.

Requirements:
Mayflash Dolphinbar (20 € on Amazon)
one or more motion plus Wiimotes with nunchunk (nunchunk is essential here)

1) install your Dolphin Bar. Don't forget to specify bottom or top button (depend if you put the DB on the top or below your screen). Then set the mode 3.
There are 4 Dolphin Bar modes:
mode 1 and 2: the Wiimote act like a mouse, you can connect only one Wiimote
mode 3: the Wiimote act like a gamepad and you can connect 4 Wiimotes
mode 4: for Dolphin Emulator, you can connect 4 Wiimotes to play WII games
The mode that interests us is the mode 3, Mame will see our Wiimotes like gamepad.
2) sync your Wiimotes
push the sync button on the Dolphin Bar and push buttons "1-2" on the Wiimote. The Wiimote leds will flash then only one will stay on, it will indicate you the player number place.
3) Your Wiimotes are sync, now open the Mame configuration file, mame.ini, find and change this lines:

#
# CORE INPUT OPTIONS
#
joystick 1
joystick_deadzone 0

#
# CORE INPUT AUTOMATIC ENABLE OPTIONS
#
lightgun_device joystick

4) now you can launch Mame. I think it's better to turn on and sync your Wiimotes before to launch Mame.
5) Mame see our Wiimotes as guns, but it's not yet finished, we have now to configure the game and calibrate our Wiimotes, you will have to do that for each game as each game uses different gun parameters
6) choose a shooting game and launch it
7) as soon the game launched, press "tab" key and select "input (this machine)"
8) select "play 1 button 1" and press "del" key. Now personalize this input, push enter key and choose the Wiimote button for fire, generally button (B)
Do the same for "play 1 button 2"(A) "1 player start"(HOME) and "coins 1"(+), I put them all in my Wiimote, as this I do not need my keyboard when I want to continue the game.
Now do the same for player 2 if you have a second Wiimote. During the second Wiimote configuration, hide the first one to avoid bad settings.
9) Now the direction: select "lightgun X analog", press "del". Now press enter and in the same time direct slowly your Wiimote horizontally to the right. Then select "lightgun Y analog", press "del". Now press enter and in the same time direct slowly your Wiimote vertically upwards.
Now do the same for player 2 if you have a second Wiimote. During the second Wiimote configuration, hide the first one to avoid bad and conflictuals settings.
Press "tab" to exit the menu and return to game.
10) I's not finish. Now you have calibrate your "Wiigun" exclusively for the game you choose: press "F2" and select the "calibration" menu. Follow the indications to calibrate your "Wiigun".
11) you can now test your "Wiigun" with your Mame game, have a good game.