I just got in my lee 50 bmg loading press. I do not have a reloading bench i use one of the little stands midway usa sells and it worked good for everything so far. Today I mounted the lee press to a board and cut it to fit on the midway stand and use 2 C clamps to hold it in place. I am using imperial sizing wax.  the first shell I tried to re-size didn't work so well. couldnt get it to size the case or even go in far enough to deprime the case. I even used all my weight and still couldnt get it to work the stand was moving around to boot.  I then got the idea to back the sizing die out about 5 turns so I am just sizing the neck, then turn the die in another 1/4 turn and size a little more and repeated it until the case is fully sized  i am raising and lowering the ram a lot but it is not a lot of stress on the stand and is working great so far.

Ok,
First you are doing a lot of extra work with the die, screwing it out and then back in. Many of us experience the same problem. But what we do is lower the handle as far as we dare before it wont back back out. Then we raise the handle and then lower again, this time going a bit further. Repeat till you can bottom out on the die.

Just an fyi for a bench for your press. I just put together this bench, http://www.samsclub.com/sams/seville-classics-lighted-hardwood-top-workbench/prod3460484.ip?navAction= , for my 1st reloading press. Its a stout sucker for 200 bucks. Got some weight, thick table top, but not overly big. Didn't feel like building one. Again, not to hijack your thread, just an fyi. Am going to learn 308 before I jump into 50 BMG. Buying those rounds is getting to expensive.

You need to do this^^^^^^^

I have broken a bunch of the lee handles where they meet the press and even one handle. Make sure it is lubed inside the neck real good and on the outside too. For me I actually get a running start by jamming it up into the die then putting my weight into it, slowly backing out and putting my weight into it until it gets all the way to the bottom. Even then I had to redo a bunch because they sprung back too far and would not chamber in my rifle. I now get the ram to touch the die then give it three little bounces then rotate the shell a 1/3 turn twice and give a couple bounces.

This is of course for brass fired from a M2. My brass that I bought that was not fired from an M2 I just put in the die and size normally. After doing around a thousand pieces of once fired M2 brass I now only buy the non M2 fired brass.

I had tried to size little bit at a time but didn't work for me. Backing the die out let's me use the full leverage from the press for the little increments at a time.
I am using a midway USA portable reloading stand. It only has a 9x9 top on it so I don't have the option of using my weight to man handle the shell into the die.

As stated above, get something better to size off of. Build a bigger table, build something for the reciveer hitch on your truck/car, something. The 50 case isnt gonna be easy, so your gonna have to get tough with it.

You might purchase a 1/2 inch plumbing round brush cut the handle off so you can remove the sealing compound inside the neck of a lot of once fired 50 BMG cases.  Even with imperial lube it can make it harder.  Just chuck it in a drill and stick it in the neck.  Plus like the above folks said..  You need a heavy table my table top is 2 3/4 inch pieces of plywood glued and screwed together.  If you cast bullets the bottom of the bench is a good place to store your lead ingots to give it stability.  Good luck.

If you are using once-fired brass that wasn't fired in your rifle, the only sane way to go is with a hydraulic press.
That's what we do at our shop.
CLEAN your brass, first! By whatever method.
We use Dillon case lube, as it seems to reduce sticking in the die.

In an effort to increase that stage of production, we tried hand sizing/de-capping. We broke the lower casting on both a Hornady, and an RCBS presses.

If your brass was fired in your rifle, CM4D has (or, at least, used to have) a neck sizing die, and inserts. Yes, you have to knock out the spent primer as a separate step, but it's still a LOT less force required to just size the neck, vs. the whole of the body.

50bmgsupply.com sells prepped brass, that has been sized, swaged, and trimmed, for $1.10/ea, lot of 100.
Prepped and primed, is $1.40/ea, lot of 100. That's a good deal, as, to buy the brass, and primers, you'll be paying 1.50 each. (I'll be pointing this out to management, on Monday)

Now, the above is just my opinion, and the way we do things here, at our shop. YYMV.

Safe shooting, Y'all!

I just did 1000 cases last winter with the same setup.

Took me maybe 12-15 hours total, not counting the run time of the tumbler.

1.  Clean the crap out of the cases.
        I soaked in lemi-brite, overnight then  tumbled, then used a pipe cleaner to clean the inside neck
        I then turned them in my cordless drill using a jig I made that fit in the case neck, I used a scotch brite on the outsides,  then I tumbled them again before I started sizing.

2. Use a good lube, I used the Lee lube that came with the setup.  It worked fine.  

3.  don't try to do the entire case the first time, as stated above, use several shorter strokes to get the entire way down the case.

4.  I actually ran mine through a second time just for good measures.

Steve, back up a little and remember, the op has a LEE press and the CM4D has a different thread size and threads per inch than the LEE Press. This die will not fit.

Hey Ricky,

Welcome to the world of 50 BMG reloading.  It's a whole new animal, and you'll have to learn some new stuff about reloading.

I have a Lee classic with the 50 dies, and it's a FANTASTIC platform to reload on a budget.  Hell, I think it works as good as my buddy's RCBS setup.  You have to make some concessions loading 50, though.  First off, do everything you can to make it an easier job for yourself and the press.  

I deprime the cases with an 1/8" punch and a jig I made out of a 2x4.  Then, wet tumble the cases for at least 3 hours.
Next lube the cases by hand with sizing was, taking care not to lube above the shoulder.  Take a q-tip, and lube the inside of the neck, however.  
Get a 50 bmg case gauge, especially if you're resizing mil brass.  It'll take most of the guess work out, and you'll have piece of mind.  
Remount the press on a sturdier table (bite the bullet, it'll make your job easier).
Screw the die down until it barely touches the shell holder, then adjust accordingly.

I went through the same trials and tribulations that you did, and couldn't get it right until I did ALL the steps above.

for M2 fired brass, take the decapping rod out of the die for the first pass.  Makes it a lot easier to size the case body down.
Once it's sized down, then put the decapper back in and proceed normally.  I use Hornady Unique lanolin lubricant.  Inexpensive and works great...

I found that if I lube the cases (with Hornady One Shot) and let them dry then apply a second coat the cases size fairly easily.  I have the Hornady LNL and their dies.  I do deprime and clean the inside of the necks before lubeing and then sizing.  For depriming I put the shell holder on top (like for priming) and hit the punch I made with a ball pein hammer, the primers fall into a can that I placed the the opening in the frame.