my first rifle (AK700 by Echo1) was 3kg accroding to echo1, I found it hard to use because it felt very unbalanced of the weight. The gun is front heavy which really hindered me.
When I moved on to M4 platforms (KA Full metal m4, WE 416 GBBR) I found that no matter how heavy the rifle was, because it was balanced I had no problems using them. The WE 416 is stated to be 3.5kg and I assume the mag (gbbr mags are also heavy) wasn't included in the weight off their website. (I also have attachments that produces extra weight) I had not much trouble handling that, and that is the probably the heaviest weapon I have handled.
However, I actually think your issue is going to be the barrel length. CQB is much easier to move around corners with short barrel weapons. SMG/PDW & pistols are my preferred weapons in CQB fields. Too bad ours is gone...*sob*
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