Some definitions need to be made here:
1. A carbine length gas system is typically in barrels that are 14.5" or shorter.
2. A mid length gas system is typically in 16" barrels.
I have had many more problems with installing the "O" Ring and high powered springs in my mid length MRP Upper (even after trying to break it in) than I ever had with just a Black Insert and a quality 4 coil USGI spring. My LMT MRP ran like a champ until I started reading all of the updates on the Crane "O" Ring, the "Defender", 5 coil springs etc.
And then I was set straight by some members on this forum, and by Lewis Machine and Tool (the manufacturer of the MRP upper). The verdict...the "O" ring is only meant for gas systems that are more aggressive than the mid-length systems. LMY specifically stated t go back to the USGI extractor spring and black insert. (and yes I know they're putting "O" rings in all of their new bolts....go figure. Probably because you can always take the "O" ring out.)
Another point....even AFTER BREAK IN....if a mid-length would run and not "Trap" rounds because of the excessive tension created from the "O" ring and high powered spring, who would want to have a weapon that isn't GTG after a routine maintenance procedure of replcing the spring and insert?
The majority of 16" AR's out there are carbine gas system guns.



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