RE: Modified barrel/shot size
get some 3" turkey loads in 4, 5, and 6 shot...different brands and such...
pattern them all at 20 yards and see what ones pattern well.
pattern those loads at 25, 30, 35 etc till you dont get an effective pattern. many guys go by 8 or so hits in the brain and spine of a turkey targets to be called effective.
a modified choke can and will work fine. just gotta be smart and know your weapon. patterning will be important.
*right around this time Walmart and other chain stores put turkey loads on sale to get them sold....GREAT time to pick up loads at a GOOD price.
IF your gun has removable chokes(i cant say ive seen a mossberg that doesnt) id recommend a turkey choke if your gunna be turkey hunting...will give a much better patter and extend the effective range of your weapon. i had good results with the Undertaker Hevishot choke in my 870...it was about 20$..not bad at all...actually i got it AND hevishot after the season CHEAP...got bot for less then the price of one....
best of luck....be sure to take shots within the effective range of your shotgun setup...trying to shoot farther can and will result into lost, wounded birds...