It would be hard to shoot well with a heavy trigger and poor quality optics. If the budget is presenting a handicap you might be best served by putting better optics on your current rifle and seeing if that helps. If you are going to buy optics anyhow this is a logical place to start. The only parts of the optics system that is probably not transferable to a different rifle are the bases. A 243 will take pronghorns easily at 400 yards if the bullet is well placed.
And now the opinion you will not want to hear:
The 400 yard pronghorn must not have been injured badly if it got away, so the shot must have been well off target. It sounds as though you need to lower your expectations in the distance in which you can make a humane shot. Long range wounding is fairly easily accomplished, and nothing in which anyone should take pride. The ability to "hit the steel" at a shooting range is quite different from taking game cleanly using field positions. A very small number of people are capable of making long shots consistently but only with a great deal of practice, the right equipment, and under perfect conditions.