Old trap guns, like the Model 1897 Winchester and Model 12 Winchester were built like a tank. Cowboy action shooters sometimes shoot the old Winchester lever action (Model 10 I think) and these guns were also very well made. Shooting modern ammo, assuming that the shooter uses ammo of the proper length, should not be an issue. If in doubt, take the gun to a smith to insure that the gun is in good condition and suitable for use.
While not a trap gun or even a shotgun, I sometimes shoot a 1903 Springfield match rifle. The barrel was made in 1907 and the receiver, a high number over 1,000,000, was made in 1920. I do not know the history of this rifle other than I bought it in 1980 at a fishing tackle and live bait shop, covered with grease and closet dust, for about $100. The original walnut stock has a well made, attached, walnut pistol grip and the sights are a Lyman #48 receiver sight and a Lyman barrel type front sight with inserts. The rifle, even with the added pistol grip, I am told, is eligible to shoot in the 1,000 yard modified military rifle matches at Ft. Knox. While shooting at a local range many years ago, I was offered serious money for the rifle but turned it down.
The rifle, like most Springfield's, is a 30-06. I reload for it and use a load from the Speer # 13 reloading manual to push a 150 grain bullet at 2,500 fps using 45.5 grains IMR 4895, which is similar to the lower powered .300 Savage cartridge. The rifle is a "tack driver". With the peep sights and off of a mechanical rest, I can shoot 3 shot groups of 1" at 100 yards in good sunlight. The rifle weighs about 9 pounds and I can shoot it all day with a pleasant recoil sensation.
If I was to load and shoot an older firearm, I would have a gunsmith first check out the firearm and then stay with lower powered shells. I used to load a lot of IMR PB which generated lower pressures and PB would IMO be a good powder to load for an older shotgun. Alas, PB is no more. Federal used to load a 1 1/8 Extra Lite 7 1/2 shell at around 1100 fps. These were pleasant to shoot and would break 27 yard targets with authority.
I would not use heavy shot weights.
Enjoy shooting the old Ithaca.
