I think you're saying horizontal and bore-line, as opposed to horizontal and perpendicular to the bore-line. I often considered that question, but I can't see it happening without partitioning in the feed tube and some sort of loader mechanism. The rounds stack inside each other so well that I don't think you can hope to get them to separate reasonably by themselves.
Something like an ATS chain on a torsion spring would get the rounds to the chamber, but you may need a pair of tippmann F/A-ish fingers to get it set right.
Some 3/4" square tubing would make the mag tube with room for some tension wire or guides. The problem is re-orienting the round (imo).
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