Yes, the extended feed ramps on the upper will cause an issue if your barrel doesn't have them as well. The bullet will get caught at the juncture of the chamber and receiver.
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Yes, the extended feed ramps on the upper will cause an issue if your barrel doesn't have them as well. The bullet will get caught at the juncture of the chamber and receiver.
According to the BCM USA website, all BCM barrels, including 20" barrels have M4 ramps. I know it's not a Colt barrel and if you use it, I won't tell anyone :)
Since most people will not part out a Colt, good luck in finding one. I went to Specialized Armament and they only had M4 style upper receivers- 225.00-275.00 dollars.
I'm in no hurry, if I do find one then cool...if not, my A2 works just fine....I could always build a complete A3 clone from BCM parts.