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M4 profile in the upper was created to solve the rapid fire feeding problem for the GI. It is not something you must have to shoot the gun but definitely nice to have for a little extra or even the same cost. There are a few combinations that you will see if you build your own upper.
If you have M4 feed ramp barrel extension, you should get a M4 profile feed ramp upper. So they match. If the upper doesn't have the M4 feed ramp, you can file the two grooves yourself an touch up black "alodine" or cold blue, whatever chem film solution.
If you have a non M4 barrel extension, do not pick an upper with M4 feed ramp. It will cause the tip of round to get caught at the gap.
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发表于 2013-12-13 16:37