I put together a R700 last year and I'm getting into precision shooting.  I saw how things were starting to dry up as far as reloading so I picked up the last box of primers at a local store (Federal LRM) and ordered dies that were in stock (Lee Ultimate Set) I've reloaded pistol in the past, but this is my first time working on rifles.  The first time around I just used the Lee Collet die to neck size but I'm reading more about FL sizing and shoulder bumping. I don't want to spend the money on a new bushing die/ mandrel setup but I was wondering how it would work if I took the decapping pin out of my FL die, resized to get a shoulder bump, then ran it through the collet neck sizer?

In for Info.

Using the very same die set, also for a R700.
I've managed to get 3 reloadings with neck sizing only, now my brass became due to annealing.
After that I'm looking to resize the way you described it.

Although it is lowest possible budget, I can't say anything bad about these Lee dies.

Neck sizing works until it doesn't. As already mentioned, you can get away with it for two or three firings, then full length sizing will have to be performed.

I want clearance when I close the bolt on any rifle. I consider .0015" an absolute minimum clearance, but I can't hold that tolerance over a large batch reloading. I set my dies up to bump the shoulder .002" and the end result is I get -.001" to -.003" over a long run of identical headstamp brass.

Virtually all long range competitors are full length resizing every time.

I get that, that's part of why I'm asking the question.  It also appears that most LR precision guys use a FL bushing die and expander mandrel. My question is, would I get a similar effect or at least an advantage by using the FL sizer without the expander followed by the collet neck sizer.

That's exactly my idea. From my understanding, I see no reason why this shoudn't work out.
Unfortunately, right now my life is being coc by the chiflu-despotism so much, it'll take me until spring to report about my results.
Maybe I can do a small batch as a test run.
But even getting to the range is a pita right now.
Anyway, this isn't GD. So maybe someone else with more experience can confirm this plan.

I'll try some after Christmas and let you know what happens.  My range is about 30 yds from my front door.

Here is another approach you can consider.

A set of Redding Competiton Shell Holders and a Redding Body Die.
https://www.redding-reloading.com/online-catalog/35-competition-shellholder-sets

You use the body die and shell holder to control your body and shoulder bump, and the Lee Collet Die to set the neck tension.

This has been a very popular combination for decades.

With the Lee Collet Die, you skip body sizing for a few cycles until you perceive some tightness on the brass, then carefully begin to body size with only enough bump to make feeding easy.

Nothing wrong with the comments above either.

If you stay with this hobby long enough, you owe it to yourself to try a few methods to learn which one works for you.

That means having the other tools in your repertoire for a while even of you don't end up keeping them for the long term.