Well...version 2 and version 3 mechboxes are very common and upgradeable..
Of all the guns I have had...the G3 has been the simplest and most reliable. Fits a big guy too...and you don't see many of them around these days...
But as for mechboxes themselves....V2 boxes need to be of the "re-inforced" variety if you are doing any upgrades as the front end of the box is very weak and problematic. On the other hand...V3 boxes are very tough and handle upgrades well..
So...currently I personally have an assortment of G3's and G36's as well as 3 M4 variants (as the team I am on does the USMC thing) ut again a V2 gun.
But when it is all said and done...my CA based fully upgraded G3SG1 is my favourite and most reliable..
my 2 cents...and then some...good luck with your choice
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