Every gun/choke/load is its own law and what works for one person may be the worst option when tried in a different combination of the same components. I would try a more open choke as suggested by another. As pellet size and shot charge increases, the less choke one often needs to get a desired result. The same can often be said when using harder shot. #5 shot seems to fall in the grey area between large and small shot and can go either or even both ways.
Another potential issue is the barrel length. 18.5" is cutting it pretty close to where the gases propelling the shot charge can have an effect on patterns. 20"-22" is generally accepted as a minimum length with standard powders and shot charges. Even with the extra length of a turkey tube you may be a bit short for at least that ammunition choice. With the slower burning propellants used in heavy loads this effect is more likely to happen. A box of Hevi-shot Classic Doubles is about the price of an inexpensive choke tube and might produce the desired patterns due to lower velocity and resulting muzzle pressures/gas speeds.
Something else to consider: the closer you bring the bird, the "better" the pattern becomes.
