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I'm heading to AI500 this month so hopefully be able to give you a better review after a good few days skirmishing. Initial impressions are good - a few minor production marks visible on the separate mount but not much of an issue
I'm 6'2" and I used this for well over a year. Fits most types of mags, space for spare ammo in the pockets. Decent quality and well worth a look in! Bonus is that you don't cook like you do with a plate carrier
http://www.flecktarn.co.uk/sdtmb6nx.html
If its not feeding like that it might be the mags not being seated high enough to reach the short feed tube. Has he had the spring chopped down at all?
I've used MAG 120rds plastic midcaps currently using Socom Tactical Lancer magazines.
Try either changing ammo or the stock hop up unit as the feeding tube is slightly shorter the normal.
I've got one for sale,
The only thing I've noticed is you need good magazines that feed well, the usual stock red cylinder chronos about 400 fps with good airseal and you need to keep an eye on the piston. Aside from that its snappy as hell and and a good ROF