I would like to get an older Parker for duck hunting. It would be a working gun so I am not at all concerned with ruining any collectors value. It seems that the general view is steel shot should not be used in these guns. Is it a matter of wall thickness at the choke or softness of the barrel steel?

If a choke of no tighter than modified is used do you think that it would be a problem?

What about getting the inside of the barrels chrome plated?

I know I could use Bismuth or Tungsten-Matrix but it is very expensive and chrome plating would be cheaper after the first year of use.

Generally they say don't use steel shot in older guns due to the fact the metallurgy of the barrels back in the day wasn't up to par as what we use today, and the barrels are a tad softer. I imagine chrome lining should work out for a spell, it's a rather hard coating, plus it would be more economical than going the bismuth/tungsten matrix route. The choke shouldn't matter too much I don't think, correct me if I'm wrong. Best of luck with your new gun!

It is a combination of "problems" that make steel shot less than desireable in older quality doubles. Nice thin lightweight barrels, low temp solder joining them together, steel shots resistance to being compressed, and more than likely at least one barrel is a full choke.
You can open up the chokes, smaller steel causes less problems than the larger sizes, BUT if the barrels decide to seperate you will be left with the cost of re-joining or retireing the shotgun. I would just play it safe and use bismuth- it will be less costly in the end.