if i had an ar-15 pre-ban lower rifle could i permintly attach a 75 round back loading metal mag by welding it to the reciever and put a pistol upper on it with the pistol no stock replacment piece and would it be legal? if not why?

I believe that the reciever must be registered as a pistol.  

what if the reciever came stripped from the factory?

It's still not registered as a pistol, is it?

I could be way off course here, but I think you'll have to get it registered as a pistol first.

how would i register it as one who or where do i contact?

That's something I have no clue about.  Hopefully someone else will chime in here.  Probably in the morning.

btt

I think it has to be built from a stripped lower which has never been part of a rifle. Thats why hesse sells the Ak pistols. Because their recivers are built new and have never been part of a rifle.

www-2.cs.cmu.edu/afs/cs/user/wbardwel/public/nfalist/atf_letter7.txt

keep your preban in tact, they are to valuable to be welding mags in, what you can do is buy a lower registered as a pistol, stripped preban lowers alone will sell for over $600 all day long. I see stripped pistol lowers all the time for $200.00, (check gunbroker.com or auctionarms.com) just put a pistol upper on that, you will have to weld the mag in place, so that the gun only has 2 evil features, but remember it has to weigh less than 50 oz to be legal. so you will want the lightest parts you can find., I hope that helps

also I dont think you can leagally put handguards on it, (all the legal aspects of this makes AR pistols almost impossible to build), anyone know for sure??? arent all of these laws silly?? why cant we just put a pistol upper on and a pistol buffer assembly??? does that make you more likely to commit a crime with the gun? because it is a pitol, or has a collapsable stock, does it make the gun more deadly or dangerous??? (of course im being sarcastic, it drives me crazy!)

Don't bother welding a drum to a preban.

If you want to make a AR pistol nowadays,
Option one, is to buy the Hesse carbon fiber lower and build up a postban pistol, i.e, no handguards and under 50 OZ.
Hesse sells the carbon fiber lowers registered as pistol lowers.
I have not built one myself but have heard that it is possible to keep the weight under 50 oz with a flattop upper and a carbon fiber lower.

Option two is to buy a OA-98 the Oly "swiss cheese gun" direct from the factory.

Option three is to buy the Professional Ordnance AR pistol direct from the factory.

Option four buy a post ban stripped lower and have the yellow form list is as a pistol and manufacture a AR pistol with a mag permanently welded to the lower. Oly had a gun they sold similar to this for a acouple years. I beleive it was the OA-94.  To load you flipped the gun open at the lower and upper receiver and loaded the mag from the top.
With this option you can have all the evil features you want since it no longer has a detachable magazine.

Option five would be to buy a preban AR pistol lower.

Good luck

if i welded in a back opening type 75 or 120 round mag,the kind you wind up after you load it. i would never have to open it up to load it.