The .45 Colt is one of my favorite calibers, but honestly it isn't a 100 yd or better cartridge. At those ranges you'll have to really pump it up, and cast lead bullets will really start leading up the bore. If you want to load jacketed you'll do better, but still the trajectory at over 100 yds will be quite a "rainbow" arc to get there.
I think I'd look at one of the Browning or Winchester High Wall copies in a larger caliber, if you want to shoot with your Dad's Browning in .45-70.
Regards!

I shoot a 44 Mag NEF rifle and have found it to be very accurate with both jacketed and cast bullets although the cast bullets cannot be driven very fast (1250 fps is about he limit) as it seems to be rifled using the Marlin Micro Groove rifling. over that velocity and groups open up, sometimes wildly. However, it will put 250 grain cast bullets into 1.25-1.5" all day at 75 yards. It shoots Speers 270 grain Gold dot into 5 shot cloverleafs just over an inch outside to outside of widest holes. I see no reason that a 45 Colt shouldn't be capable of the same knd of shooting, I have seen revolvers that would do this and better. It may take some load work though.