I own a CVA Staghorn in .50 caliber and find it a very good shooting rifle. In fact I was shooting it today, but due to the number of people around I could only get back to the 35 yard line for safety reasons.
I shoot with open sights so I would limit my shots to 100 yards, but with a scope I see no reason with the right load this would not be a 150 yard rifle. I have shot some excellent groups with light sabots and even the 348 grain powerbelts.
I was shooting 320 grain R.E.A.L. conicals out of it today and using 90 grains of Goex FFFg. This is a real powerful load and has great accuracy. I personally would go for the .50 caliber instead of the .45 caliber. I have heard and read on different posts that the .45 caliber can be hard to find a load that shoots good in it. That does not mean there are not excellent shooting .45 caliber rifles out there. I just think the .50 caliber can do the same as the .45 caliber and the bullet selection is much better.
You are still going to shoot sabots for that long range shooting your talking about. You probably will only be shooting 100 grains of powder. I found when you push the projectiles too much, the accuracy goes out the window. Also you will notice that with the 24" barrel, it can not burn all of the 150 grains up, it will only manage to get about 120 grains of it used up before it blows the rest out the barrel, from my observations.
I never shoot pellets, only loose powder. I have been shooting Goex and really like it in the rifle. Today I shot this group.
My understanding is the Staghorn Magnum is discontinued. You should look at the Buck Horn which is their entry level rifle. They are on sale at Natchez for $89.95 right now. Another good rifle that is on sale right now is the Knight Wolverine II LK-93. This has a 22" barrel and is #11 cap ignition. I have one of them also and it is an excellent shooter.