Like what was said, if you could find a quality AR for regular price (under or around MSRP) then get that.
I would suggest a S&W M&P15. Walmart does not carry these though (I don't think). The Smith is a "budget" rifle with many of the features of more expensive rifles. It is only lacking a chrome lined barrel and a M-16 bolt carrier.
Do your research on ARs before you purchase one, I did not but I lucked out (mostly). They are not all the same. DPMS, in my opinion is not a good rifle. Also, there is articles out about how Walmart products are subgrade to fit their lowered prices. I do not know if this carries over to the firearms, but it might.
Another option would be to buy a lower. This is the "firearm" so other parts should be easily purchased later on.
Since Walmart wouldn't stand behind firearms if they thought they couldn't make (enough) money off of it, I would also suggest you doing business with a LGS. I know this is not always possible, Walmarts are in almost every town and usually better priced.
If you are going for a shotgun I would suggest a Rem 870 (my preference) or a Moss 500 or 590. These have proven to be tough shotguns. There is also many aftermarket parts to customize the shotgun to your desired setup. If I am not mistaken, some 500s come with two different barrels, a "home defense" short barrel and a longer "hunting" barrel.
