I have a lyman flint GPR (1 in 60 I think)and I'm shooting .495 patched balls. I have tried 75-90 grains of 3f and my groups stay about the same. I have a tang sight which I like, but my groups are usually about 12" at 100yds. Should I be doing better than that? Is it worth trying some of the short lead bullets?(ball-etts?)
I have tried cleaning between shots (which helps), trying to just seat the ball consistently,(tried .490 balls) anything I can think off, but I'm stil averaging about 12". Any thoughts?

Adjust the thickness of your patching next. I found that a thicker patch liberally soaked in 1000+ wonderlube worked wonders for my acuracy in my older Hawken with .490 balls. Thin patches didn't hardly work at all with the smaller ball, but again made a large difference withy the .495s. My 1-48 TC New Englander will shoot to 6" all day with either load.

I am assuming that you are shooting nice round swaged round balls. If they are cast, they may have a flat casting mark on them. Make sure you load with the sprue in the same direction each time (typically up).

Thanks,
The balls are'nt cast, so no sprue. I have a couple of thicknesses of patches, I need to sort them and try a few groups with each.

Keep trying diff. patch/bullet componations and also a wad over the powder. The GPR 50 I had shot very well with 110 gr. of FFG with a .15 patch, .495 ball and a fiber wad over the powder. Barrel needed to be clean to seat.

First,since you asked for advice...I'd suggest you quit shooting 100 yd. targets. Shorten up your range to 50 yds. MAX for now. As the range you shoot increases, more factors come into play to affect your group size. At this point, you just want to find the best combination of ball/patch/lube. It will most likely be a tight fitting ball/patch. I find the best groups in my Green Mtn. roundball bbl come with a ball/patch combo I have to start with a hammer, and sturdy short starter. At 50 yds...you oughta be able to find a combo that will consistently shoot an inch or two before you even think about going back to 100. 12" at 100 yds is pretty worthless. Not trying to be insulting...but you asked. A big part of the fun of shooting traditional muzzleloaders is finding the right "combo". Enjoy the experience..!!!

I think it is very likely that the OP identified the issue in his initial post.

The rifle shoots the same or nearly so regardless of powder charge or ball/patch size and thickness. That tells me that the rifle is exhibiting consistency and it is likely not a load issue.

I think the issue causing the 12 inch group at 100 yards is the fact that the rifle is a flinter, and he is unable to shoot it. I am a decent shot. A damned good shot, in fact. I have a 58 caliber Hawken that is a PRB gun and will hit a poker chip as far as you can get a good sight picture on it. Cuts playing cards offhand, etc.

Years ago I had the late Ron Ehlert build me a left handed flinter. I couldn't hit jack diddley with it, couldn't hit a one pound powder can at 50 yards or a 55 gallon drum at 100 yards, both from a rest. Yet I had friends that were good shots with their flinters take my rifle and cut playing cards offhand with it, despite the fact that they were righthanded shooting a left handed flinter. The rifle WOULD shoot, I just couldn't shoot it. Not only that, it took me about 18 months to get my former profociency back when I went back to my percussion Hawken.

I suspect the OP is dealing with the same issue I had years ago. I had to accept the fact that while I can shoot percussion, the rock lock is beyond my pay grade. I would bet that if he were to convert the rifle to percussion, it would shoot for him.

I'm shooting seated at a bench, off sandbags. I'm "following through" with my hold(if I get a flash in the pan, I'm still holding on the target) and keeping repeatable cheekweld, sight picture etc, using the set trigger.
I'm using a 15 thou patch and a .495 ball. I have to start it with the short starter, don't know if I could load much tighter.(maybe if I clean after each shot)
I have'nt tried anything over 90 grains, I thought lighter loads might shoot better, rather than going over 90.

Brass Thief, I have limited experience with blackpowder but have been told not to test for accuracy using sandbags for a rest. I was told why but cant remember Also a round ball needs speed. Try increase to 100 grains or even 110gr.
Good Luck

Flinter's are definitely a challenge. Properly set-up...properly built and using the correct type and amount of components they will shoot as well, AND as fast as a caplock. I do have a lot of blackpowder experience both in the field, and on the firing line @ matches. For the best precision,lighter powder loads are universally preferred 60-75 gns. Using 100 gns or more in a .50 MIGHT be preferable for hunting...(personally my RB load is 90 gns.in a Hawken. I'm getting through-body penetration on many animals..don't see the need for more. They ain't gonna be no deader. The problem most people have with their first flintlock is that they just added several new variables which may affect their shot. That said....I'll repeat UNTIL one is shooting ragged 1 hole groups off a rest @ 50 yds with boring consistency, I would'nt even think about going further back. If THAT goal is not yet reachable...move in to 25 yds. There's a reason most blackpowder rifle matches have a 25 yd relay. Once you're making those 1 hole groups reliably off the rest...stand up. DON'T move back...yet. Learn to shoot that rifle WELL @ that range, even if it's only 25 yds. When you have BOTH precision, AND accuracy dialed in, standing on your feet...you're ready to mave back a bit and begin the process again. Eventually, you'll find a small sight adjustment might be needed, or perhaps a small increase in powder charge. ONE CHANGE AT A TIME.
Good Luck.

I found some 18 thou patches, hope to get around to shooting them sometime soon. Also found a smaller aperture for my lyman sight.

Try lapping the barrel with a couple hundred strokes of JB bore cleaner.. get the sharp edges off.. I shoot a green mountain 42" round ball barrel flinter and get 2 to 3 inch groups at 100. 15thousandths patches and 490 rb. 90 gr 3F.. does the trick.. Les

Having my fair share of experience with flinters I agree with Don Edwards on this matter.