So I've put 25 rounds downrange with my 336 in 30-30, yet I still can't keep 75% of my shots on a paper plate at 50 yards shooting off hand.
It's the first time I've used semi buckhorn sights, is this part of learning or am I just a bad shot ?
1. Is the gun zeroed, preferably from the bench? If not, even Slamfire might have a hard time, at least for the first couple shots til he figured out the Kentucky Windage.
2. As others have said, buckhorns aren't the most accurate sighting setup. that said, if this is to be one of your hog guns, buckhorns are adequate for that, and might be very handy when a pissed-off boar busts out of the mesquite at 10 yards. I'd leave them on and learn to shoot them for accuracy, or replace them with a Williams aperture sight, as Doc Rock suggests.
3. If the gun is zeroed, as Slamfire says, offhand is the hardest position to shoot from and requires the most practice. Some safe dry fire training might help. The dime drill is a good training tool; You'll need a helper; when you have the (unloaded) gun mounted, pointed out level, have them lay a dime on the top of the barrel. Remembering the breath control training you should have, try to gently pull the trigger without disturbing the dime. When you have this down, you have developed the skill that will keep you from pulling the round off your Point Of Aim when you fire.
4. Correct, consistent shouldering of the gun is also important in accuracy from any position, especially offhand. You should be putting the buttstock in the 'pocket'; This is the area about 2-3" in from the actual shoulder joint. The top of the buttstock should be even with the top of the shoulder, no higher. A little lower may be acceptable, depending on stock style. The pectoral muscle creates the protection that will keep you from getting the 'bad form' badge, a bruise on the shoulder joint from holding the stock too far out. Good, solid rearward pressure is essential for good offhand field shooting. (there are two major offhand positions. We are just concerned with the 'field' offhand position here.)
Take time to concentrate on the basics, even if you already 'know' them. When my shooting starts to go awry, in any discipline, I revert to the basics, and suddenly, I am back on track again.