The midlength gas system being longer lowers the amount of gas pressure at the gas port thus heavier buffers aren't required. CAR would work fine, H will work but with weaker ammo or a dirtier gun it will likely jam (short stroke, running to slowing to feel as good as a faster cycling gun).
H, H2, H3 and buffers were designed for CAR and shorter gas systems. Do they sometimes work with longer than CAR gas systems? Of course.
Think about the design of the AR15, it was designed as a rifle. It had a 5.2oz rifle length buffer and a reduced power buffer spring. Then along came the CAR length rifles XM177 it with short barrels because the gas port was much closer to the muzzle the gas port needed to be make larger. The larger gas port and shorter gas tube sends more gas volume and higher pressure through the gas tube, this results is higher bolt velocity and more wear on parts etc.
To ensure that the shorter gun has enough has volume and enough time to completely cycle the gas ports were left large, but to slow the violent bolt velocity heavier buffers were designed.
CAR buffers springs are shorter and stiffer than rifle length buffer springs.
CAR buffers = 3.0oz
H buffers = 3.8oz
H2 buffers = 4.6oz
H3 buffers = 5.5oz
Rifle buffers = 5.2oz
Gas port (that hole in the barrel) pressures:
Rifle = 13.5K
Midlength = 19K
CAR = 26K
Be care running heavier buffers is some barrels like BCM, LMT, Colt and some others which run mil-spec sized gas ports designed for 5.56mm NATO pressure ammo. I've seen many people use a LMT 16" gun and a H2/H3 buffers and/or a LMT enhanced bolt carrier and Crane O-rings on the extractors and then only ever shoot .223 Rem pressure ammo (because it's cheaper) thinking their being wiser then the next guy and then wonder why their rifle keeps short stroking or failing to feed. Sometimes a little bit knowledge is dangerous.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
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