common failure point on these is in the optic itself on the gun's board. after repeated drops eventually one or both of the legs will break (really only 1 break will kill the gun in all fairness

) and under some circumstances you can desolder, cut the legs even and resolder them, unless it's a very high break towards the optic, then you'll have to replace that part. the gun boards themselves can get somewhat pricey... I've had luck finding them relatively cheap on ebay years ago. I don't know anything about finding the replacement part for the optic itself... maybe someone else knows.
other failure points will be wire breakages, which I've replaced entire wires in these guns before, so it's entirely doable. the other will be that the connector that goes to the gun board popped out... this was so bad on one particular gun on our House of the Dead that I said fuck it one day and just hardwired all the wires to the board by soldering them to the joints of each pin. I don't encourage doing that however... although it's been successful, I can foresee it becoming nasty one day, and that was during my early days of doing tech work lol
nothing a little silicon could've done the same friggin thing for.
also if you have motion in the gun in the gun test, but the trigger won't work, you're probably looking at either a bad switch or something mechanical came undone in the trigger. either way, under any of the above circumstances, you'll have to open the gun up to see what's going on. I STRONGLY encourage that if you don't already own a hot glue gun, to purchase one, if only for the purpose of hot gluing the nuts to the bolts that hold the gun shells together, because if they're the acorn nut variety, you're going to be doing a lot of tossing them into the hole and hoping they line up right... and then holding them on to make sure they don't fall out when you put the bolt back in. lol
last but not least, if you're huge into cosmetics, if your gun shells look like they've been through several wars or they're the wrong colors or something (player 1 is blue and player 2 is red) you can always buy replacements from Happ.
I think that's everything.
otherwise the more longterm alternative would be to replace the damn things with Happ .45s, but that's not "authentic" and under some circumstances the holsters on these Sega shooters don't necessarily allot for those.