What year is your 1897? Mine I believe is 1904 and although it was not printed on it I took it to someone who said it was 2 3/4. Not to say yours is. I went skeet shooting the other day and decided on trying it out after spending a week getting it to work as it was siezed solid from sitting in an attic. My best advice is to find some old paper/cardboard loads as the plastic loads I will not fire in this thing as even when they are 2 3/4 or 2 7/8 they are meant for those in older paper crimped loads and I believe the plastic will only open up a small way in these chambers thus increasing the pressure tremendously.
I will let you know however I shot 25 rounds though it and my dad 10 and it functioned flawlessly however the recoil on an old steel plate instead of a 1/2" rubber pad is quite considerable even though it is a heavy sucker. Just get it checked out again as I am not sure unless it is really, really old pre 1900 that it would be 2.5". If it isn't 2 3/4 then its probably 2 7/8" which means yeah 2.5 is a safe bet or if you can handload or find someone online I remember seeing both of them off a shotgun ammo site.
Good luck, be safe, have fun, they are amazing shotguns and the ext. hammer is a great option for when it was initially designed to be able to hold the trigger down and just pump all the shells out of it but if you are going to do that make sure you are using low pressure rounds. That action is like a swiss watch and if its 100 years old your may end up hurting it. Not saying you can't do it once or so (I tried it out and it worked find but just with 3 rounds)
