I had a somewhat simlar experience 3 weeks ago. I am shooting a T/C Triumph, 120 gr. Blackhorn 209, Federal 209 primer with a 300 gr. Scorpion Gold from Harvester Muzzleloading. The load chronographs a consistent 2050 fps. My 100 yard targets were good but the 200 yard groups were were 5 inches high. After much thought I wondered about the Bushnell DOA reticles. Back to the range this week and used the 150 yard reticle at 200 yards. Bingo!! The group moved down to the bullseye. I have used Barnes bullets in my center fire rifles for many years and they produce outstanding accuracy. However all attempts with Barnes in the muzzleloader were futile even at 100 yards so I gave up and called Harvester Muzzleloading. They graciously sent me a small supply of several bullets and their crush rib sabots. This proved to be the answer for my T/C.
Don't know if this helps.
I am NOT a fan of the BDC or any other multi reticle for a muzzleloader. However the maufacturers realized they could market them and it worked well for them.
That said, there may be a possible or multipal possible reasons why the rifle shoots like its doing. Yes, it very well could be the bullet/sabot combination or the charge but, there are also other things which may contribute.
One very over looked, yet can be an issue that can drive a shooter nuts, is what's called scope "springboarding". The springboarding affect is caused by the scope raising at recoil and can be related to either over lapping the taper of the barrel or, its not secured properly. I'm not saying that this is your issue but, you should be aware of it. T/C Encore platform rifles and the CVA Apex can have this issue with the taper the manufacturers do with their barrels. Some shooters drill and tap another screw in the mount, or have it done. Others have tried shimming it, while others have added an additional ring for stability.
Another issue that has surfaced, causing vertical stringing at long distance is related to "break open" rifles. Most noticable with centerfire break opens but, its starting to show up with break open muzzleloaders when shooting long distance. Its related to the locking bolt on break open rifles. If there is excessive slop in the locking bolt, upon recoil it will actually raise the barrel just enough to cause vertical strings, noticable at long range. This concern can be corrected by a VERY GOOD gunsmith.
Getting tuned up for a hunt next weekend. My setup: Thompson Omega, Nikon Omega BDC 300, BH 209, CCI Mag Primers, Barnex 250 Spitfire T-EZ.
All shots off bi-pod on a cement bench, rear bag.
Three shot group using 100 grains of BH are pictured below... thinking wow this is awesome.
Then shoot at 200 yds, not EVEN on the target. About a foot to the right?? and about a foot high (according to BDC reticle...I know BDC not accurate)
Confused, back down powder to 85 grains. 100 yd shot still on the money, hitting exact same place where as the 100 grain shot....
Shoot to 200 yds with 85 grains, now about 4-5 inches to the right? Correct elevation for BDC reticle. About a 3 inch, 3 shot group.
Shoot 300 gong which is 24x24.. aim at the far left corner, shoot...GONG, three shots GONG..hard to tell where I hit on the gong (range does not allow paper targets at 300 yds)?
So my question is, why such a tight group at 100 yds with 100 grains, then a foot right at 200 yds? Did back down powder really affect the bullet flight that much?
Not canting the gun, scope is level?
I've had to return to your 1st post and read it over a few times, trying to get a grasp. From that post, it appears you indicated that the target that you attached, was your 3 shot group at 100yds. If this is correct (?), then you are shooting left of center on the target, which is ok, you're trying for groups.
However, all things being equal and shooting at 200yds, with that 100yd group, the bullets shouldn't have went a foot right and a foot high because of the bullets or load. Didn't happen because of the bullet or load. Bullets just don' travel like your target indicates at 100yds, then make a sudden turn to the right and upwards a foot each way at 200yds, unless you're hitting something, shooting through a tornado, shooter error or, something amis with the rifle/scope. Its clear in your photo of that target, that the bullets are punching a good hole, so they're flying straight, even though left of center a bit.
Something else is happening...... I'd forget about the BDC and use the crosshairs, get it off the bi-pod and on a ROCK SOLID rest, supporting the front and rear of the rifle and shoot that again. If you're heeling and anticipating recoil with the heavier load or, thumbing (squeezing with the thumb) or possibly putting to much finger on the trigger, it will also cause right and high shots.
Sorry, but you have me thinking and reading this over a few times.......