You want to shoot long range, and you already have a Triumph. I would have suggested a Savage smokeless gun, but you would not have gotten such a deal.

If I were you, I would start with Black Horn 209 for powder. I can't recommend a specific bullet, but for the distance you want to shoot I'd bein the 200-250 gr range.

Then, it boilsdown to range time. You may select a great bullet/sabot combination right awayor it may take time to find something your gun shoots well.

A 250 bullet at 2100 fps (about what 120 gr of BH 209 will give) can be about 3" high at 75 and 3" low at 200. Zero is around 175, so you can see it may be tough to get 250 out of it unless you are willing to be fairly high at 75 and 100.