you should probably read the HMRC website. It’s all laid out there.
In short, there’s no such customs code as “CO2 magazine” - they might have put the item description as CO2 mag, but that’s largely irrelevant.
Just because it’s listed as a gift, doesn’t mean it’s immune from customs charges, since we left the EU, everything is subject to customs charges. Even if it’s listed as a gift between two private individuals.
Finally - border force have a massive backlog at the moment of about 2 weeks, so if it’s stopped for inspection or customs charges, it might take as long as two weeks. My last order sat at customs goods in for 2 weeks but was processed within 30 mins of being scanned in at customs.