50BMG bullet seating problems

[co17]

I was loading my first 50BMG rounds today and got a little worried about the force required to seat the bullets.
I have a Hornady 50BMG kit and I have FL sized the IMI brass which I have trimmed and chamfered.
The bullets are 800gr. drive band solids from Lutz Möller.
.50 " BMG
I loaded 20 rounds and there was much difference between the amount of force required to seat them.
Some of the rounds took so much force that the press was flexing so the C.O.L varied .02".
I measured all the bullets. They were perfect.
What am I doing wrong?

 

[lazylabs]

What does the I.D. of the brass measure before you start the seating? It may be that you are getting too much spring back after the brass passes over the expander ball. I would measure the expander ball and then the I.D. of the brass before seating.

 

[co17]

The I.D. of the brass measures .5047". This is done with a digital caliper but they are always a bit off when measuring internally.
The actual I.D. is probably more like .5079" but I will check with a micrometer when I get home.
What should it be and what is safe?
I'm not concerned about group size right now but I AM concerned about blowing myself up.
I have a great respect for +250gr. of powder