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We welcome all to post product questions, technical questions, and advice, in the PSA section of AR15.com; it is why we pay to have it here.
The industry adopted standard for a mid-length upper is a triangle hand guard cap. I do not know why Magpul decided to design the SL mid-length for a round hand guard cap, it makes no sense.
If you want advice on how to change the endcap we will gladly help you out.
However, the profanity is a TOS violation, and belongs in GD.