hello im new to the forum and i also just recently bought a smith and wesson m&p15 sport its a pretty basic AR as you all know (no forward assist or dust cover) i am a left handed shooter fireing a right handed gun and i was thinking about upgrading the upper receiver, is it possible to replace my right handed upper with a left handed upper? and also my AR has a fixed stock if i were to replace it i would need a new buffer tube correct?

I think you will need to replace the upper and the bolt carrier assembly.  Your entire stock assembly will have to be swapped out to make it adjustable.  You will need a receiver extension, a lock plate, a castle nut, a carbine recoil spring, and a carbine buffer.  Make sure you get a receiver extension that fits the stock you are putting on.  You are talking about $350-600 depending on what you want.

As stated below, make sure you don't break any laws.

Might depend on what kind of fixed stock you have.  Pics could help.  There are some compliant M&Ps with pinned collapsible stocks, which would make it a carbine buffer extension.  If its an A2 style stock, it's rifle length and you'd need the stuff mentioned above to change it to collapsible.

*Edit* Just noticed you may be in MA.  If you're still only allowed to have a fixed stock, your options may be different.

Bigger question is - WHY?  I've been a shooter (police, SWAT, competition...) for over 30 years - left handed - and never ever noticed my rifles were right-handed.  I install ambi-safeties on every AR I purchase and that has been the only thing I've ever needed to change.  If you want a left-handed rifle, buy a left-handed rifle instead of doubling up on the money you already have in your right-handed rifle.

If you want a collapsible stock in MA it HAS to be pinned. I took all my collapsible stocks off before I moved here and put the MOE rifle stocks on them. I actually like these better then a collapsible stock, I'm a tall guy, and I have long arms. Your buffer will work with your a2 stock, IF you get a collapsible stock, you will need a carbine buffer and a carbine spring.


As for the new upper. I would see if you could just buy a stag left handed upper.

Also is your MB pinned?

OP, you should be just fine keeping your right hand setup (though the nuns may slap you with a ruler for using your left hand).  Your case deflector will do it's job if the rifle's pitching brass too far back.  The only things that you'd probably find to be minor annoyances are the safety (which I just go from safe-semi-burst by curling my left knuckle and scraping the lever into the correct position, or using my thumb), and the bolt release (bring right hand to mag well, use index/middle fingers to release bolt carrier group).  Been shooting the black rifle left handed since 1999, and actually qualifying on the M16 family prior to deployments since 2008, never had any problems, hell, the catm instructors were dumbfounded that a lefty could even shoot.  

after looking into it further it would be really expensive ive putt about 200 rounds through my M&P so far and havent had any issues with casings hitting me in the face. but i definitely need to invest in an ambi saftey. thanks for the insight

how do you add pics to this forum

Use Photobucket or similar.
Upload to Photobucket, then copy and paste the IMG code here in your thread.

As a lefty myself the things I've done that helped a lot were:

1. Ambi safety as others have mentioned
2. Magpul B.A.D lever... allows you to release the bolt with your right thumb as you insert a magazine (~$28)
3. Ambi charging handle.  There are less expensive ones but look at the reviews and there are none better than the Ranier Raptor.  I LOVE mine.

Just food for thought.  

Being lefty can get expensive, but my kids and gf are all right handed so I didn't want it to be a strictly lefty gun.  This way anyone can use and enjoy it, and I kept the standard upper orientation as the modifications above "fixed" all the issues leftys can have.  Only limitation you can't fix on a standard upper is a bullpup conversion as empties will hit you in the ear, but otherwise you just get to see the empties flung out in front of you.

Enjoy your gun.