. .. .are there reasons this would not be a good idea ? I'm thinking mostly of machine gun once fired but wondering if it would be helpful in " squaring up " the bases of roughly used brass . I'm a beginner ..please advise . And thank You .

Never heard of such a thing. I do military brass the same as regular brass, just pay more attention to primer pocket reaming.

I see no benefit double stroking. Use consistent adequate lube on every case. I do recommend Hornady's Headspace Gauge set for adjusting your sizing die so you can measure how much you push shoulders back. Also I highly recommend closely checking every military once fired case for signs of stretching to prevent the possibility of case separations. I typically find 5-7 and maybe more cases in a batch of 1000 that shows bad stretch marks inside the case. Different brand cases and military cases sometimes requires the sizing die be screwed in or out to get the same shoulder set back due to expansion differences and brass hardness differences between brands. Sizing will not square up bent rims from violent ejectors. If I see a slightly bent rim I use a smooth file to clean up the case head or scrap a badly damaged rim.

Turn your die down even more. If that doesn't work have (or do it yourself) the shell holder's top surface ground or sanded .005" shorter.

Measure the total height with your callipers. Using emery cloth taped to a smooth surface (mirror, glass etc.) use a figure eight or circular motion remove .004" or more from the too of the shell holder.

This will allow you to acheive more sizing in one stroke. Make sure you adjust your die based on headstamps after modification.

There are some great answers and things that should be done already posted.... but I guess I'm not understanding your question?

I think he means sizing the case and then let the ram down and turning the case a bit and run it up into the resizer again AMIRIGHT?

Anyway I do it sometimes if I feel a hard to size case, I turn the case a little and run it into the die again, just so I feel better about the case being fit to chamber

Sometimes out of a batch esp. military cases there will be some tough ones that are blown out a good bit. IDK if it helps, but the second time up it feels normal going into the die.

Yes . ..dennyd  understands my question . In my limited experience I have run into several cases that just felt like it would benefit the case to spin it 90 degrees or so and run it into the die a second time . All of these answers are helpful . ., thank you , and please continue w/ any
further advice .

If you're resizing cases fired in a machine gun some people recommend running the brass through a .30-06 resizing die (lubed brass, not dry) and then running it through a standard .308 resizing die (I'd lube the brass again.

Two steps, but they say the .30-06 die takes it down to size part of the way and then the .308 resizing die takes the brass down the rest of the way.  Makes for less stress/strength required on the really tough brass.

Get a Dillon case gauge. If it fits that you’re good to go.

I agree with dryflash3 that the Hornady gauge set is great for setting the shoulders, however, especially for those starting out, a full case gauge has some advantages.  Those being that, 1) one doesn't need a caliper to use it, 2) the entire case is gauged so one can see if the base is being resized completely, 3) the neck datum is set between the proper high-low lengths so it will chamber correctly and, 4) the neck length is set between the min-max.

Once one gets the basics down and wants to go more precision like ensuring that the shoulder is bumped back .003", then the Hornady gauge set is a smart addition assuming that the reloader has a proper caliper to measure with it.

I’d recommend annealing the brass before sizing.

Machine gun chambers vary.

This.

Exactly this.

Pay attention to the highlighted portion. I'll usually setup dies to way oversize the shoulders of mixed once fired military brass because of this.

If it's all the same headstamp, you should be g2g, but I'd still size an additional .002 if it were me.

Edit: Dryflash is correct about the Hornady gauge; it's all you'll ever need.

I've run into the same problem with MG fired 50BMG brass.

The commercial cases size easily but the LC .mil brass is oversize and requires a lot of force to size it. I run them in about 3-4 times BUT without pulling the expander ball out of the neck, sizing a little bit at a time instead of trying to size them the whole length all at once. Puts less stress on the press and keeps me from getting a stuck case.

Once it's been fully sized (shoulder bumped) I turn them 90* or more, still without pulling the ball through, run it all the way in again, like you are saying. I don't know if it really does anything but it doesn't hurt and gives me peace of mind that they are fully sized.

I recommend getting both the Hornady headspace gage AND a case gage. The Hornady will let you get a precise measurement for setting the die and for quick spot checks. After a batch is run use a case gage to check each one to ensure you don't have any defective rounds going into your finished ammo. Lever and bolt guns allow you to feel if a round isn't quite right. Once you insert the mag in a semi-auto and let the bolt go you lose that luxury. The recoil spring has enough momentum to slam a faulty round into the chamber and lock the gun up tight. Do you want to be ''that guy'' that has to mortar his rifle at the range trying to get a defective round out ? Once or several times ? I don't. I run every round for a semi through a case gage to catch those defective rounds that we all make from time to time. They get pulled down later but they don't get anywhere near my good ammo.

I always do this with rifle rounds.
Some cases will drag harder against the neck expander and that can pull some headspace back out of the case.  
The second sizing, the expander always feels much smoother coming out of the case.

This, makes a difference. And some of then fat cases will start to expand as soon as you pull em out the sizer, put em back up and they stay a little better.

The neck on the out stroke is always something you can feel, if it fights you pulling the case back out, always run it up again or you might have 0 headspace.

And I always lube inside the case neck with a 22 caliber bore cleaning brass with my favorite lube, chapstick. Don't believe sprays get in good enough for me.

I just resized around 800 Lake City 5.56 brass.  They would almost fit in my Wilson gauge but not in my Sheridan.  After sizing, 100% of the cases needed trim.

That is all I do, I don't know what you mean by the additional work you describe.

I would use the Dillon or Wilson gauges for length checking only.  For go/no go chamber test, Sheridan is better in my experience.

Thanks Fellas . , that's a lot of great advice .  I am familiar w/ separating commercial from military brass , along w/ different brands ; lots; and years etc . . And checking for stretch marks and the difference in volume between commercial and mil. brass . Haven't stuck a case yet but I know if I forget to adequately lube (inner neck etc. ) I will join the club . Thank U for the reminder . Probably can't be said enough . Anything more on the sizing / loading of Mil. .223 /.308 would be welcome here .

I suppose while i'm asking how do you guys feel about the need for using small base dies for batches that may be shared by two or more different action typed rifles ( as in bolt vs. semi ) or several different rifles ? I know if you are shooting through one particular bolt rifle  you should be able to get away w/ just neck sizing ( aside from length trimming ) as the shells  "chamber form " ( ? ) to THAT rifle  . Yes ?

Initial sizing of Mil Surp Once Fired brass would just benefit from pass through a small base sizer die, rather than anything else.
Most Mil Surp is fired through a sloppy chambered MG.  So, the small base works best for that application.

The only time rotating a case may help is seating a bullet.  Start it into the neck, then drop the ram and rotate the case 180 degrees and finish seating the bullet.

If you think about the sizing of brass in a die, there is no difference between the initial sizing and a re-sizing after rotation.  After all, the die is cylindrical and uniform in dimensions.

Any irregularities in the sized brass are likely doe to non-uniform wall thickness of the brass.  Even then, rotating the brass is not going to change anything because the non-uniformity is in the brass, not the die.  As you spin the brass, the non-uniformity also spins.

If there is some significant misalignment between the ram and the die, this will impress itself every time you run a piece of brass into the die.  If you see this, realign your die with the ram.

I have purchased some large quantities (by my standards) of military brass.  I have never once needed a small base die.

I have a SB die and most of the LC mg fired stuff makes two trips through it. Lube each time also. If i don’t relube them it takes 3-4 strokes to get them bumped enough. Typically those cases grow around .020” in length during sizing. This is specially 308 LC mil brass. Never had any trouble or extra steps with 223.

It depends, if they come out from the expander ball really tight like I said, you will pull your case shoulder up. I turn em and that makes my greasy fingers rub over the case, maybe even over the case mouth if I'm in a good mood, LOL

And run em up again. I use a SB die, always have always will in semi-autos. These are my habits wrong or right, Amiright? I doubt it.LOL

I do exactly as OP describes.
I will turn the case 90 or so and run it back in.
I do this without pulling it over the ex ball though.
It helps to knock down any bumps on the case head caused the the ejector or any swipe.
Remember, there is a portion of the case head not in the shell holder.