The only real deifferences between a SIM-15 and SIM-4 are a composite feed neck, double trig, and no muzzle break. Internals are the same but a lower grade alloy.
Any and all mods for a SIM-4 can be done for a SIM-15.
This would include barrels, rails, barrel extenders, and triggers. All you'd need to make it a SIM-4 (aside from feed neck) are the single trig, muzzlebreak/flash suppressor, and the delux parts kit (bolt mainly).
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