Rest easy ? your gun?s choking is more than adequate for killing cleanly at 40-45 yards with suitable cartridges.
Tight chokes and heavy payloads of big shot might give you another ten yards over that but few people are accurate enough to shoot consistently enough at extreme ranges.
A 40 yards pheasant is not only a very sporting proposition but it?s also a long way up and a decent enough challenge for even an accomplished shot.
Much better to kill a succession of these birds stone dead than ignore them for higher birds which you will miss more often, and probably prick a higher percentage too.