For us it is in no way reinactment. There are several types of airsoft out there, and though most revolve around shooting each other, that's where the similarities end. Anything involving airsoft guns is called airsoft. The majority of airsoft is generally what is referred to as "mil-sim". Like 80% or more of the players, games, teams, fields, events, etc, are based on a modern tactical, combat sport. That looks really cool and all, and I myself might make a lancer from GOW, or a pulse rifle, etc, it would end up on my wall, not in game. To make a gun more bulky, awkward, noticeable, etc, etc, etc, etc, and unable to be compatible with real world optics or accessories is tactically a very far step in the wrong direction to any groups like that. That's why you'll find a home with either a war-hammer group (good luck finding one in your area), a sci-fi group (again good luck), or an "anything-goes" type group or field. That last one is fairly likely.
It's not about being strict, it just isn't the same game. It's like showing up to a hockey game with a pool cue, that's the reaction you'll likely get. Hopefully this is a differing perspective for you. There are events I've seen where people show up dressed like Indiana Jones, carrying a large broad sword, in futuristic/post-apocalyptic garb, and everything else under the sun. Most places though, you'd be the only one with something like your "bolter", and they won't allow it because it defeats the purpose of items that actually make guns more ergonomic, your ability to conceal yourself, etc. Which is the whole point of the main/progressive style of airsoft.
It does look cool and all, and this is just a warning on what to expect. When you're in someone else's house it's their rules, their way. If you look around and your in the middle of a bunch of modern warfare gear up competitive players and you're standing their with that, you'll feel like a goof.
As for making it, I'd grab a gun that is nearly the same size, and work off of it. You could make it out of a smaller gun such as your M4 CQB, but then you wouldn't need to modify it at all, just build over it. You could then remove the gun at any time and it's still an M4. This method may take some more engineering though.
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