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Get your Bitz box, and get literally ANYTHING that looks like it could fire something, then slap that on the end of a normal Shoota. That way it's easy for the boy to actually hold it (it's just like a normal shoota) and looks like it'll blow up in the Orks face.
The best way I've found is to put a Mortar on the end, and put some of the Spikey shoulder plates on top of it. I'll get a pic up if I can.
One (rather silly) idea I've toyed with in the past is making a crossbow-type thing that launches bombs, such as the IG mortar bomb from the heavy weapons sprue. I know it sounds daft, but it would fit quite fluffily into a snakebites army (as they mistrust technology). There are plenty of crossbow pieces made by GW, or you could convert one from a shoota without too much fuss.
If you don't like this, then Robizzle's idea is a good one. Grab some dreadnought rokkit launchas from GW, cut the rokkits off and stick them on the end of some shootas - instant rokkit launcha. Sticking this onto the end of a shoota (after cutting off the square bottom bit) might make a crazy looking but cool rokkit launcha also.
Hope this helps
Make friends with IG players. Scavange their spare mortar tubes and missile launcher missiles (if you have to talk up the benefits of hvy bolters and autocannons against orks then so be it, but to be fair they should already know :p) assemble mortar tube and shove IG missile down said tube. Jobz a good un.
Does it Hurt? How about now?
all right, I'll try again later with a bigger hammer.
One fun conversion to do is to take BloodBowl models, and find ones that look like they are throwing the ball. Replace ball with a bomb of some sort. Simple a humorous.
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Ork. Rokkit. Launcha.
Take a piece of plastic tubing - any will do, as long as it's big. Stick something that looks like an Orkish missile inside.
And that's it, an Ork Rokkit Launcha.
a friend of mine made some cool looking rokkits by taking fantasy ork spears, and adding a missile (he pillaged my IG bitz box in the process) onto the top to make it look like a bottle rocket or something equally dumb. But it looks pretty cool, definitely orky.
"And any man who comes through this fight mostly unharmed will be my sister! It'll be free frocks and jollies for ever, you'll see!"
- Rome: Total War
9th Black Watch (Imperial Guard) 4000 points
Red Corsairs of Khorne 2000 points
Space Wolves 13th Company 2000 points
If you can get access to them, the Chaos havoc launcher will give you 6 rokkits to play with, just separate them then stick to shoota! My rokkits mainly use tau missile pods, but this is only because I have a ready supply of them (brother). Otherwise, tank kits usually have some form of rocket/missile.
Actualy for a feral ork army, Ive seen some fantasy orc bowmen shooting rockets with their bows.