The standard 1/4 x 28 nipple for T/C uses a #11 cap,,that's why your having trouble with the #10.
Don't know why the hammer was bent,,lot's of guy's think they want a scope mounted on the thing and bend the hammer to make it fit,,and 3/4 of those guy's make a mess of the job
.
The slow twist barrels are marked with a stamp on the barrel <Roundball Twist>,,some of the very old t/c's hawkins had a slow twist.
Here's a quote I found here about twist rate measurement;
"" Screw a cleaning jag tightly on a cleaning rod and just start it down the barrel with a cleaning patch on the end of it.You will need the barrel pretty well locked in a stationary position for this. Place a vertical mark towards the top of the
cleaning rod, and another mark at the muzzle.
Start pushing the rod down the barrel, allowing the rod, jag, andcleaning patch to follow the rifling. If the vertical mark rotates a full 360 degrees before the jag bottoms out, place another mark on the rod at themuzzle. Pull the rod out of the barrel and measure the distance between both the marks you made at the muzzle. If it's 20", guess ya know what
ya got.
On the other hand, if the jag bottoms out long before the vertical markmakes a 360 degree rotation, start over. This time, place your second mark at the muzzle when the vertical mark rotates only 180 degrees, andpull out. If the distance between the marks is now 14", you have an acceptable 1:28 twist. ""
Now of course this is for a faster twist, most barrels for 1-48 or slower aren't long enough to get a full revolution. So with carefull marks and measurement you can expect to see a 1/4 revolution of the mark and get either 12" or 15" travel