I think my issue lies with my short throat.
No, I don´t think so.
A short throat increases the starting pressure and will increase the velocity a bit.
You might try a powder which is a bit faster than the one you are using now.
If the powder is to slow for your barrel length, it might be, that not all powder is burned when the bullet leaves the barrel.
From that moment, the bullet is no longer accelerated, the gas streams elswhere.
That being said, with a powder which is not already burned when the bullet leaves the barrel, you are losing velocity.
The optimum would be, when the powder is just all burned directly before the bullet leaves the barrel.
We call it the "burn-end" over here.
Quickload shows that on the charge graphics, as a rating.
Another thing to take care of is the bullet / barrel diameter.
If the bullet is to small, or the barrel to wide, the bullet can´t caulk firmly and some pressure will get lost.
To work this out, you can slug your barrel with a lead bullet which is somewhat bigger in diameter than your caliber.
You must add oil to the slug and the barrel.
Starting slowly from the barrel mouth with a wooden or a rubber mallet.
Then use some wooden, brazen or bronze stick to, one shorter at about 5" and then a longer one, so that you can push your slug through the whole barrel.
There you have a footprint of your rifling, measure it.
Your bullets should be as wide ore even up to 1/1000 of an inch wider than the widest measured diameter on your slug.
But first you should try the powder thingy.
