M203 underbarrel Blaster > Shotgun conversion

M203 underbarrel Grenade Launcher style blaster.

After barrel and piston mods, respectable and consistent fps out of both barrels.

Tad long for a short barrel guard. :grimacing:


  • Twin barrel
  • Quick rail mount
  • Pump action can handle stronger spring
  • Solid mechanism will handle upgrades
  • Looks the part
  • Cheap as $20-27

Could be better

  • Low capacity ~50, adequate for a twin barrel shotty
  • Pump too slowly and more gels drop into t-piece. Good spread though. :grin:
  • No seal, 100fps is optimistic
  • It is long (see pic of it under M4SS)
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  • Cap/cover over muzzle
  • Metal barrels with aluminium sleeves to fit in t-piece
  • 1.3 x 16 spring
  • Rubber sheet with smaller holes over t-piece to moderate feed
  • Slotted pistonhead over old pistonhead. End of piston is too weak to just replace.
  • Old spring guide inside piston to reinforce and pre-compress spring
  • Pistol grip for standalone use. BWAHAHA!

240+ fps, more than enough for a CQB shotty.
This will often leave twin impacts 1-2cm apart at 8-10m and without a hop up.
Reason enough to not bother with hop ups or further upsizing the spring…



  • 2 x 13 silicone o-rings to t-piece. Why not? :sunglasses:
  • Meranti grip joining piece glued and pinned. Grip screws in from below with a pin to lock
  • spring to assist trigger return
  • aluminium spacer to sear for crisper piston release
    570gm empty

Just a touch of paint, pic rail and laser and it can go on the wall as a standalone blaster. :grin:

A most satisfying mod.

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Cool idea i had one of these too used it alot but had problems with the 9mm gels they said that these are supposes to fire. The 7 to 8mm didnt fire so well. Also it broke after maybe 100 gels so its not very durable

Could be a Friday build you got there.
I converted it to 7-8mm gels in the first week and I’ve tortured and put thousands of gels through it.

Lining up the laser from 5m.
I’m done. :grin:

Very cool! Tempted to try this myself.

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Had it in mind for over a year and glad I finally got around to it.
It’s cheaper, shorter, has more consistent fps and a higher gel capacity than both the 97 or 870 and in a pinch can still be attached to a bottom rail. :wink:

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Was thinking of adding 1 or 2 sling mounts to this today.
Then I remembered my archery gear.
images (68)
Neat way to carry a shotty.

Sweet geezus, your mods and detail have given me goosebumps.
I LOVE my M203 double-shotty. All I’ve done to mine is a bigger spring and cleaned out the internals for smoother action and dremelled out some bits for more gel capacity.
I rock it underslung on my ACR and while it still only has about 10m range, I like to pump and fire, lob-style, to get gels in behind cover. Sometimes it even works!

If I may ask soemthing you’ve probably covered… but what would you recommend to get that seal tighter? New nozzles? Oring?

Thanks for the tingles.

No worries. Glad to inspire anyone.
I tried o-rings. Not a regular size and thickness and gap is still huge.

So I placed a standard pistonhead over the old one. Easy to do with a manual as there are no gear engagement constraints.

Barrels are tight 7.0

Where did you get one for that cheap? I’ve been thinking about one of these for ages

Anywhere, although some charge over $34.

I haven’t really seen them in stores for a long while. Might have to look again. But getting the Thor is also really tempting (or one that uses green gas. The co2 ones are expensive)

Looks like I’m making another purchase this week :joy:

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