Hi Adrian, so you have an AR pistol, not a rifle. You cannot legally add a stock without registration and a $200 tax stamp, because that would make it a short barrelled rifle. I think you mean a pistol brace, which is ok to add.
If you want to add an adjustable brace, you would need to replace the buffer tube with a slotted one. budsgunshop.com/product_info.php/products_id/146750/tacfire+mil-spec+buffer+tube+black+hardcoat+anodized+aluminum+for+ar-15 ,
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Then you could add this brace budsgunshop.com/product_info.php/products_id/143778/sb+tactical+sba3x-01-sb+sba3+brace+synthetic+black+5-position+adjustable+for+ar-platform+tube+not+included ,
.
Stay safe & have a great day!

" the one that came with AR pistol is all smooth, ( i was gifted a stock) " please get advice from someone local IE FFL as you can not legally put a stock on a AR pistol without a lot of paperwork and a NFA tax stamp. you can install a pistol Brace

I would stay basic for now with a Magpul Pistol Grip and stock: budsgunshop.com/product_info.php/products_id/10300/magpul+mag415-blk+moe+pistol+grip+aggressive+textured+polymer+black ----------------------------------- Buffer tube needed: budsgunshop.com/product_info.php/products_id/146750/tacfire+mil-spec+buffer+tube+black+hardcoat+anodized+aluminum+for+ar-15 ------------------- Brace needed: budsgunshop.com/product_info.php/products_id/143778/sb+tactical+sba3x-01-sb+sba3+brace+synthetic+black+5-position+adjustable+for+ar-platform+tube+not+included -- and if you have an MLOK Handguard, MLOK Rail slots to add accessories: budsgunshop.com/search.php/q/mlok+rail+section ------------------------- Light for handguard: budsgunshop.com/product_info.php/products_id/722056817/xtreme+lumens+tactical+weapon-mounted+light+w+remote+pressure+switch+-+lon --------------------------------------------- Red Dot Sight: budsgunshop.com/product_info.php/products_id/131518/crimson+trace+0102030+cts-25+compact+4+moa+illuminated+red+dot+black+anodized -------- Cheers, stay safe and enjoy the AR-15 Pistol Bug!