Initial impressions are prefaced by TT record of quality. That being said the quality difference in stock springs and TT springs is definitely noticeable especially during install. those tenderfoot fingers will feel it. Gauge of the spring feels denser than OEM, same dimensions but you can feel a difference. More compact coiling of the spring are obvious standouts. They line up easily in the magazine and merry up to the follower without issue. True to design they hold the claimed extended amount, essentially creating the capacity of four magazines in three. Additional weight is not a flaw because you know what you're getting into here. Practice reloading and shooting with these as they will change your timing, accuracy and efficiency until you are proficient. I found that the follower & spring had no issues bringing each round up and the heavier gauge allowed the action to cycle properly retaining the bolt to the rear once the mag was empty. My only gripe is the retention pin in the extender. I see this slowly working its way out over time and mostly likely during the worst possible time. Granted the extender slides along the magazine rail and offers a snug hold. Overall I found these to be a great addition to my loadout and look forward to updating this review as I push more rounds through them. Top quality!!

Very good

The spring with the extension work great. The only problem is that the extension is very difficult to remove so the magazine can be cleaned. I had to drill a hole in the bottom of it so the spring could be pushed into the magazine allowing the extension to be removed without bending the spring. The only malfunction I have had with my MPX was due to dirty magazines. They need to be dissembled and cleaned.