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I can move my carnitine from another brand subq with 30g fine, but for IM I don’t go smaller than 28g for water-based like most of that, and 27g for oil (like the Trest & Yk11).
Carnitine at 200mg at least 4x a week is always a great dose. IM immediately preWO or intraworkout assuming you have eaten carbs or will be drinking them (insulin presence improves shuttling) for more noticeable acute performance enhancement (lactic-acid buffering, glycogen-sparing, enhanced pumps), or subq on off-days for background effects (driving fat-burning, improving androgen receptor affinity/lowering binding constant, mitochondrial health).
If you have 31g pins, they are probably too short to even do shallow IM effectively. But you could do all of these subq if timing is less important to you and volume isn’t exceeding 0.5ml at a time.