Colonial Marine Conversions #4: Autoloading Mortar


This conversion actually has nothing to do with Aliens, but it uses several leftover bits from the Aliens-themed conversions, and follows the “waste as little as possible” philosophy. We’re going to use up most of the leftover autocannon bits to make a pair of autoloading mortars.

Anatomy of a Thumper

The mortar we’re going to make is called a “Thumper” because that’s the name that immediately popped into my head when I saw the finished product. Because the rear half of an Imperial Guard autocannon (left over from the last few conversions) looks too cool to waste, I decided to see what I could turn it into. Initially, I was simply going to do a snub-nosed autocannon, but because I don’t have anything to mount a snub-nosed autocannon on, that wasn’t an useful option. When I looked at the heavy weapon sprues again, I noticed the mortar parts and everything clicked.

Tube mortars seem so ghetto compared to the conversion work we’ve already done so far, so an autoloading mortar is exactly the kind of thing that goes well with the nature of the other conversions, and it’s also a nice companion to the sentry gun and missile launchers.

The two smartgun conversions use up one and a half of the heavy weapon bipods–one bipod yields two barrels and two waldo attachment points, so the only thing that comes off the second bipod is the cradle that you glue the targeter to. That leaves two usable sets of bipod legs, albeit without the cradle/yoke doohickey on top. Those cut-down bipod legs happen to be exactly the right height for this application. The grenade launcher barrels left over from the sentry gun conversion are also used in this conversion.

Required Parts

The photo below illustrates the parts needed for this conversion. When only a small portion of a part is required, the important piece is highlighted in red. (Save the leftover pieces! They’ll come in handy later.)


The photos below show how all the parts listed above go together to assemble a Thumper.


  1. Slice the baseplate off the mortar half where the ball socket meets the tube. Follow the angle of the ball-and-socket connection when cutting, as you will need that angle to properly seat the new mortar.
  2. Slice off the little nub near the business end of the mortar. Make sure it’s nice and even at the back.
  3. The upper back of the autocannon body has a box-shaped overhang–cut that off so that the back of the autocannon is completely flat.
  4. Slice off the little pin on the yoke that normally fits into the hole on the tripod.
  5. Glue the ammo drum to the autocannon body.
  6. Glue the grenade launcher barrel to the front of the autocannon body.
  7. After the glue sets on the parts you just attached in Steps 4-5, glue the mortar baseplate to the back of the autocannon. You want to position the baseplate so that the bottom rear of the autocannon body is sitting directly on the front “petal” of the baseplate, about half a millimeter in.
  8. After the glue on the baseplate sets, glue the whole thing to a 25mm slottabase, positioning the baseplate so that it hangs over the edge of the slottabase by a millimeter or so.
  9. Immediately apply some glue to the feet of the bipod and position it so that the bottom rear of the yoke is resting on the “pins” of the bipod legs, and the top of the bipod is aligned neatly with the yoke.
  10. Run a bead of glue across the joint where the top of the bipod and the yoke meet.
  11. Glue the mortar bipod nub to the top of the bipod, upside down, so that the 45 degree slope on the nub is as close to the autocannon yoke as possible.

You’re done! Repeat this one more time, and you’ll have a pair of Thumpers.

In the next article, we’ll build a pair of piggy-wiggies.

2 thoughts on “Colonial Marine Conversions #4: Autoloading Mortar

  1. Joe

    Just for fun you could add the "crew shield" to this weapon and mount it on a vehicle, ie Rhino and use it as a pintel mounted grenade launcher…

  2. Sean

    I just stumbled upon your Colonial (IG) Marines conversions, and you can bet your bitz I'm going to use them as inspiration for a planned IG city-fighting, compact-and-advanced weaponry army (similar to the Elysians).

    Thank you for your blog and showing off your marvelous work!

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