If you just want to get in the game as cheap as possible and aren't too fussed with looks.
Then a Tri-shot springer shotgun with an M4 mag adapter and 1 M4 magazine is a cheat code.
Pump action, so no rapid fire and with the shells you'll spend more time reloading than you will be playing. But with the £8 adapter and one M4 magazine, it can be an absolute death machine.
You can use them indoor no problem
It scares the crap out of even experienced players.
With 0.28 BBs or higher, I've been able to send it in the woods, just under normal AEG range, but plenty for what you'll need.
I've had one of these for 2 years, never needed any maintenance, always performs, and I treat it terribly lol.
Something like this is decent for £40: https://www.patrolbase.co.uk/double-eagle-tri-shot-m56b-pump-shotgun?pv=9601
https://www.patrolbase.co.uk/battleaxe-m4-ar15-magazine-adapter-for-m870-shotguns
Then any M4 mag will do, the more BBs the merrier of course.
It isn't too pretty, but it works.
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