The best replacement for carbine gas system is mid length.

The best alternatives to carbine gas system are rated as following:
★★★★★  mid length
★  midlength
★  mid-length

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Speaking about carbine gas system, one probably shouldn't forget about rifle buffer.

Other search terms related to carbine gas system and their ratings are:
★★★★★ rifle buffer
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carbine gas system
mid length
midlength