Your original over-all-length hasn't changed, so who cares what you stuff down the barrel in regards to inserts.
No, the original barrel would not act like a silencer. First of all, there are no wipes or baffles or anything- right? Right.
You will still get no reduction of sound, perhaps directed a little more forward but no reduction at all. A .45 will still sound like a .45 and a 12 gauge will still sound like a 12 gauge.
These types of inserts have been around for a gazillion years, and would have been banned generations ago if they were contrary to law.