You could upgrade your current 12 Ga 870 by purchasing a new barrel that will have the screw in choke tube option.
Take it to a gunsmith and ask if that is possible as I bet it is. Then Google and find a low cost source for a factory made barrel.
That is what I did for my 1187.
Is another option for you.
JW
Thanks for the feedback guys.
A new barrel for my 870 12 ga. will cost $120 - $150 for the Express version and $200+ for the Wingmaster.
I can buy a new 12 ga 870 Express at Cabelas right now for $300. That would be my new gun option. I have the 20 sitting in the closet so it just depends if I want to part with $300 and have another gun around the house.