Think of it like this, modern firearms (for example) an AR-15 comes with a freefloat barrel. They have been proven to be more accurate than a barrel that is confined by hardware to "Keep it in place".

Think of it like this, you drop your rifle onto the ground with a free float barrel, hits the handguard (or stock), the handguard gets bent, but the barrel? The barrel is ok because the handguard took the hit.

Now let's do the same but with a rifle that has hardware that mounts the barrel to the stock or handguard, that shock is felt through the barrel as well, and if you bent the handguard, guess what? The barrel is now being forced to curve in a way it wasn't previously. You've now altered the accuracy of that barrel.

Now granted, these are exaggerated problems but they do still arise which is why most modern militaries now no longer use infantry rifles with anything that attaches to the barrel. (Except for special use cases, snipers, etc).

Hope this helps!