I'm loading 110gr by vol of FFG H777 with a 240gr XTP Thompson sabot and a CCI plain #11. 24" barreled Thompson .50.

When I adjust the MV input for JBM Ballistics to fit the observed trajectory, I get a MV of about 1275 fps. The 777 jug says I should be getting about 1800fps.

Any idea about the reason for the discrepancy?

How old is the 777?

It's old, but this has been a problem from the very beginning.

Best practice is to throw out last years powder and buy a new jug. Both 777 and Pyrodex degrades and that is a typical symptom. With BH209 going for $50-$60 for 8 ounces, I have gone back to regular black powder. 777 and Pyrodex do not offer any thing all that extra.

I would at least try a different lot of powder. How about your Chrony. Do you trust it? Might check it with a 2lr maybe.

BH209 cannot be used in Sidelocks and certain older in lines like TC Black Diamonds

Because of #11s with the TC BDs?

I believe it is because the blow back you could get up thru the nipple(not sure about that) . On the Black Diamond, it did not have a solid member in back of the breech plug breech plug like inlines today have. There is bolt that is held in place by a screw in plug in back of it

I wouldn't trust the accuracy of a velocity calculated from the short-range trajectory of a pistol bullet. Do you have access to a chronograph?

I believe it's because it doesn't reliably ignite with anything less than a 209 primer.

Yes, I clocked the 300gr XTP one time with the same charge at 1,700fps. The trajectory indicated the velocity was only a bit more than 1,200fps. The 777 was fresh then.

My old TC Black Diamond used 209's but instructions that came with it said do not use BH209. You are probably right about the #11 cap not igniting the BH209,but I don't think I will try it.

Sell that ML and buy either a break-action or fall-action ML with 209 primers.

I don't think that the bc of an XTP is worth much. Essentially, you just shoot it and see where it hits. Mine is a sidelock, 29" barrel, 100gr loose 777, and a 240gr XTP. On at 100, 7" low at 150, 24" low at 200. Once they slow down, the tables aren't accurate, in my experience.

Your results are very similar to mine. Have you chronographed your load?

I have not. Don't have a chronograph. Sure wish I did. Anyway, I don't shoot a muzzleloader that much anymore. Nothing works as well as actually shooting, as far as I can tell!