Fixed or detachable. I say both...and I'll do so with the same rifle.
The rifle in question is the Blaser R8. The magazine is part of what Blaser calls the trigger group. A button located just in front of the magazine insert will allow/restrict the trigger group (magazine) removal.
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If it's in the forward position as the picture above shows, the trigger group can be removed for magazine loading or a complete change of it.
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Sliding the button to the rear reveals a "S" and this means the trigger group can not be removed. In order to reload the rounds need to come from the top of the loading port.
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Also on the Blaser R8 rifles, to remove the trigger group, there are tabs on both the left and right side of the trigger need to be depressed at the same time while pulling the trigger group out. Pushing just one of these will not result in it coming out, making this system even more foolproof.
So yes, I will definitely take the option of having both.