The Mk18 uses a 9.55" rail. The 12" rail you're talking about (actually 12.25") is for the older 14.5" M4A1 Block II carbine before the CQB-R was adopted. Ebay actively takes down listings for handguards/rails now, so your only option will likely be a firearms-specific P2P hub like Armslist or similar.
You will probably need a die in order to rethread the LM4 receiver, 1-1/4x18tpi for AR-15 spec barrel nuts.
the rail does fit lm4s. i had a friend put one on his lm4. i dont remember him complaining about fitment. only thing he complained about are the mags of the lm4.
he said he doesnt remember. If i remember correctly he sold it to a guy with an aeg, so it was probably and madbull one. i will try to fish more info out of him
EDIT: he said he got it off another gun he bought.
Madbull's DD RIS II rail was originally made to metric specs to fit the receivers threads cut on TM-style M4 receivers. At some point, Madbull did make a version that was designed to fit both imperial (G&P, ICS) and metric (TM, CA) receivers. I only learned about this because I PMed them on FB trying to get some answers about fitting a Madbull RIS II. I have personally put a Madbull RIS II on both a G&P as well as a Tippmann M4 with no issues. Even if you buy a Madbull Mk18 or RIS II rail and the barrel nut won't thread on, you can use the nut as a die to re-cut the threads on your receiver. You just need to go slowly and tighten the nut on and then back it off as you go. Otherwise, you could try find the die that Madbull made to re-thread receivers to imperial spec, but good luck with that if you can't even find the Madbull rail itself.
It's also worth noting that a bunch of other companies now make both RIS II and Mk18 rails.
I mean; if you're really hard up you can always have someone 3D print you one in PLA (even include custom markings/rails/etc) and do a "lost wax cast" style pour with melted aluminum cans, or I've had excellent success with melting spent Co2 capsules. You've got to clear off some muck from either option before you cast, but I've made some functional cosplay items that way with great results, and it's a lot easier than it sounds. (Great YouTube tutorials & how I learned)
Obviously easier to just find/buy it...but remember there are options to acquiring exactly what you want beyond "retail". I have a couple blaster builds this winter, one is the DL-44 (Han Solo's) into a functioning airsoft pistol. I'll journal it just in case you still haven't found your DD by then. I have to cast some parts that aren't machinable.
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