20 gauge full choke and steel shot?

My buddy got a Harrington and Richardson 20 ga break action shotgun recently with the aim of chasing grouse and bunnies.  The only thing he didn't think about was the choke.  It is a full choke.  It's also from the 1970s but in damn near new condition.  He'd like to use steel shot.  

As I understand it, a full choke shrinks the barrel diameter a bit at the end.  I think that's fine for lead but maybe a problem for steel.

Questions:  
1.  If he shoots steel will it be unsafe?
2.  Will steel shot damage the gun?
3.  Will steel shot damage just the choke part of the barrel?
4.  If the last X inches of the barrel were sawed off, would that change it to a different type of choke and then steel would be fine?  What would X" be?

Thanks!

It doesn’t seem like the best of ideas to use the steel shot. I seem to remember someone telling me smaller shot would be Ok.  I an thinking #5 or so. But while the person I spoke to knew his guns, this conversation was a long time ago.  I would call a gun shop and ask.

I think when shooting 3-4 shot the consensus is that it ok to shoot steel shot. Even though the H&R is a relatively moderately priced shotgun I don't know that cutting of the choke section is something I would do. Shooting the 3-4 shot shouldn't erode the barrel or cause any belling of the muzzle.  Should you want to shoot birds at some time the full choke would still be there. H&R/NEF has toll free customer service lines for inquiries and should be able to set your mind at ease. Bismuth is an alternative.

I think he was going to use #6 or 7 for those critters if that changes anything (or if he's wrong).  Ok, I'll have him check with a gun shop and call the company.  Did some internet searching and the answer from some reputable sources varied from yes to no so apparently it's not clear cut.

It's hard enough to find steel around here, not to mention bismuth...

Thanks!  

Most of the time the answer is no its not advised but I know from experience when it became illegal to use lead shot for waterfowl that all we had were fixed full choke guns (12 gauge). We shot thousands of rounds of steel through these guns a lot of #4 and #2 even some BB without any issues. From my research later the biggest negative effect it has is the shot pattern may be deformed from the shot squeezing through the choke. This has a bigger impact with larger shot size, it doesn't mess it up to bad because we still harvested many ducks and geese with that combo.

Full choke is going to be good for rabbit and grouse in a 20 gauge because this will result in a nice pattern at longer distances, similar to a 12 gauge with modified choke. Although I wouldn't recommend steel shot for this gun and certainly don't saw off the barrel.

I think though if you tried a decent 12 gauge pump shotgun with a vent rib and screw in choke tubes, you probably would never go back to the single shot 20 gauge. The screw in choke tubes will produce the desired pattern for any situation and any type of pellets.

Regarding the steel shot, you are forced to use larger pellets to achieve the same velocity because of the weight difference. This also reduces the number of pellets, which is already reduced with the 20 gauge.