The first Lee collet die I bought was for my son's Browning A-Bolt in 7-08. I set it up according to Lee's poor destructions, but the bullets could easily be pushed further into the case. The seating force was very minimal. I turned the die further into the threads, lowering the whole die to increase the force on the ram. CABLEWEY went the aluminum top cap!
IIRC the threads were 5/8X 24, but my memory is always suspect. I was taking a machining course at the local tech college, so in doing a personal project, I made a cap out of steel. Threading on a lathe is fun, and not too hard to do. it works quite well.
BUT that is not the problem! No matter how much force is put on the collet, it cannot size the neck enough if the mandrel is too big. Lee makes them too big! They even admit it by offering a smaller mandrel for sale as a solution to not enough bullet/neck tension.
OR you can reduce the diameter of the mandrel as said above. Simply chuck it in a drill motor, use some fine Emory paper, (I used 600 grit), reduce the diameter about .002. Polish with some 1000 grit, try a case to see if it increases the bullet pull enough.
Single shot and bolt guns don't need a lot of neck tension to hold bullets. BUT some slower powders need maximum bullet pull to retard initial movement of the bullet for better powder burning. I would certainly try for max neck tensions on the big belted magnums. That long column of slow burning powder might spit the bullet out of the neck at first pressure build up.