outer barrel compatibly

[2nd battalion 2LT]

okay ..I am at The end of a rather pain in the ass build ..I am building an 16a2 sorta c7 ..Have furniture ..have gear box and all that jazz ...down to lat bit of fitting ..

So I put on the upper to the lower .. put the front pin in ..and with the hop up and inner barrel there .. the mag release now does not hold the mag in ..

After changing the hop up.three times even tried a two piece hop up ..as well as tried two different uppers I have narrowed it down to the outer barrel.. switched the outer barrel to an m4 length . And every thing fit's

My question is ..like the uppers and lowers .do g and p barrels only fit g and p bodies ..or am I missing something .. the body ..upper and lower are both jg and seem to work well with ca and g&g parts ..my first build went with no hitch .. just this one's been a pain in the rear ..Any help would be great ..even better if you have an a2 compatible barrel or g&p lower ..sitting around ..lol I am at my wits end