I'm going to start reloading for my S&W M&P AR-15. Do I need a small base sizing die set? Thank You

I never needed an SB die.

I have them, but rarely use them. Usually a solution in search of a problem. If you don't need one, they just overwork perfectly good brass which shortens useful life.

If your brass source is the gun for which you're loading you'll never need a SB die.

Sometimes individual pieces acquired via bulk brass purchases or range scrounging are found pretty stretched. Easiest way to identify them is to use something like a Wilson gauge. These pieces can be made to chamber using a SB die. Lacking a SB die some choose to run the pieces through a std sizer a couple times. The much better approach is to recognize these pieces were badly overstretched and simply cull them. When you load questionable source/stretched brass you can get a partial or complete separation.

I've never had to use one.

What is important is that the shoulder is set back .002" to .003" from the dimension it took when fired in the rifle.

Crush fit and a floating firing pin isn't the best combo.

Thank you gentlemen. All my brass is coming from Winchester White Box that has been shot through my rifle.

I use a SB die set with absolutely no issues whatsoever. Most of my chambers are CLE though.. If you ever get a barrel with a tight chamber, you may wish you got the SB die set instead..

I've loaded thousands of rounds of range salvaged brass fired in rifles other than my own and not once needed a small base die.

Count me in the "Old Fart Syndrome" faction! LOL!

When I started reloading back in the early 70's, I was faced with a gent who had a Rem M742 .30-06. If it loaded a round, it wouldn't extract ....or ....it wouldn't chamber the next round.
He was using RCBS FL dies. Set him up with a set of SB dies. Problem solved!
I know lot'sa folks who reload for semi autos using FL dies and never have a problem. I've had a few "chokes" with FL resized brass. All my semi autos have SB die sets.
Are they necessary?
Uhhmmm.....I dunno? ....but my "Old Fart Syndrome" sez reloading for a semi auto requires a SB sizing die! 🙂

I've loaded thousands of rounds for various AR15 calibers. Never had to use SB dies

Standard dies work fine for me, even sizing range pick up AR brass for my rather tight chambered 527.

Several years ago I sized a bunch of 5.56 brass with a new standard die set to touch the shell holder. “Assumed” all was good without checking. But half wouldn’t chamber in either a Mini 14, S&W, or Rock River. Gives you a really stupid feeling So I got to do it again as I got around to using them. Second go-round was with a SB die, just in case. Also did around 2k different once fired with the SB. Some must have been fired in a 7.62 chamber while some were easy.

If I’m not dead by the time I cycle thru this brass I’ll try the standard die again, now that I know what it has been fired in. But basic once fired and new to me will go thru the small base sizer. Just in case.

I come from the same background, thus my question. LOL!!!

Set the die up right and you don't need a sb die. Run the case up into the die, now look under the die between it and the shell holder. See that gap? That is the flex in the die and press. Screw the die in a little more at a time till the brass chambers in your gun.