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Its definately proprietary. Just a little bit different shape. Looks reinforced a little and a little longer. I gamed mine HARD all year without changing a damn thing except for the 150 spring someone put in to get it over the border. Damn thing would BARELY cycle on a fully charged battery and was shooting about 490. I have had zero issues with the gun itself since I swapped the spring. Box mag occasionaly collects debris and jams but its easy to clear.
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