The best replacement for m4 barrel is heavy barrel.

The best alternatives to m4 barrel are rated as following:
★★★★★  heavy barrel
★★★★  hbar
★★★  government profile
★  lightweight barrel

Your alternative for m4 barrel is not listed? Suggest!
Speaking about m4 barrel, one probably shouldn't forget about a3 upper.

Other search terms related to m4 barrel and their ratings are:
★★★★★ a3 upper
You have in mind a companion for m4 barrel and it is not yet here? Propose!



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m4 barrel
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