I am a relic from the previous century, as are my old shotguns, side by sides from an even earlier century. So, yesterday, in a fit of generosity I purchased a minty Browning Miroku A-5, 3" 20 ga. with 2 star (modified?) fixed choke barrel....for a grandson. The big question is how compatible is this barrel and choke with non lead shot types, I have only used lead and bismuth in my own guns so have no experience with the new shot offerings. Please comment.

Should be fine. Go get some ammo you want to hunt with and pattern it.

You may have problems cycling the lighter (<1 oz) target loads as the A-5 Magnum was designed to work with the recoil of magnum shells and usually the high velocity loads. If the gun demonstrates problems with cycling less than magnum shells, a very clean and well lubed magazine tube can alleviate some of these problems.

You should be OK with the Modified barrel when using steel or tungsten/iron shot though I am not positive. the warnings against using steel shot in Improved Mod and Full chokes were for the 12 ga models with #2 shot and larger so how that correlates to the smaller bore I don't know. The lower velocity of most 20 ga loads would also be a help in preventing barrel damage. Opening the choke to Improved Cylinder or likely Light Modified should fix potential problems.

The use of bismuth or tungsten/polymer shot should not be an issue at all.

Had cycling issues with a magnum 12 Miroku A5 when fed low base shells. A call to Browning and a spring for a light 12 was secured at a modest price. May want to check around to see if a lighter spring is still available and swap them out depending on the load being used.

Ran a lot of #2 steel 3" magnums with the modified choke back then with no ill effects but as mentioned this was with a 12 not a 20.