Joh Blissett was a gunmaker in London England from 1840 until 1877.
The address on the barrel is probably 321 High Holborn which with 322 was his shop from 1850 only until 1856 so the gun is a good deal older than you think.
It is a cap lock - percussion, not a "flintlock of some sort."
It is what the British call a stalking rifle and was used hunting non-dangerous game; not an "old elephant gun," in spite of its big bore. The British did not load them real hot in those days, if they wanted more power they went to a bigger bore, LOTS bigger.
It looks in nice condition and everything I could find about Blissett said he was a maker of good quality guns, even if he is not as well known as other British gunmakers of the day.
A careful shooter could use it a reasonable amount without hurting it but you should do a lot of study before trying it. I sure can't tell you the caliber over the internet, for example.