I have notices people discussing different lowers, A1 ar15 and xm15

exactly what are the differences between these and the positive / negative to having one over the other

There is no difference, really.  Lowers are lowers.  The differences you are talking about are jsut the names that different manufacturers have for the same product.

There are SOME differences, in terms of manufacturing technique.  Occasionally a few companies in the past would make cast lowers, which are considered inferior.  Pretty much everyone these days make forged lowers - to the same specs out of the same materials.  (Except CavArms, who make a lower out of a lightweight composite kind of material, and possibly bushmaster's carbon line.)   Every once in a while, some manufacturer might make a certain hole (for a pin) a different size or something, so you have to buy their spare part, instead of the standard one, or something like that - but that's pretty rare. Some companies try to differentiate themselves by the finishing (how pretty is the finish, how smooth at the edges, what custom finish has been added, etc.)

But ultimately, lowers are lowers - and all work the same.



ETA:  I just realized you were probably talking about something compeltely different that what I answered above - so ignore what I just said, since I was talking about MODERN commercial lowers.

There have been some differences that have changed over the years from the original design to the current lower.  So an early A1 lower has some slight but subtle differences from a modern A2 lower, and some variations also differeed - like the lack of a mag relase fence and the pin holes and stuff like that.  If you search through threads where people talk about building "retro" AR-15s, you can often find discussions of this - and also check the information section of the site, where I believe there is a guide to different variations of AR-15's.  There's probably also a thread here somewhere that shows pictures of different historical variations over the years.

Sorry - misunderstood the question

AR15, XM15, etc... are just model numbers of various manufacturer's.  Various Colt copyright's kept others from using the "AR-15" designation, so they came up with their own.  This also helps with various AWB states because they may specifically ban an AR-15, but the Bushmaster model is the XM-15, so there is a bit of a loophole.  Follow me so far?

As far as the A1, A2, etc., these are types of uppers...

A1 -- earlier style upper, preferred by many, full carry handle, rear sight adjustable for windage only
A2 -- full carry handle, rear sights adjustable for windage and elevation
A3 -- removable carry handle/flat top model

Does this help any?

both rep;ly do help, I had noticed occasional reference and just put it to inexperiance with this firearm. What really got me thinking was this thread http://www.ar15.com/forums/topic.html?b=3&f=118&t=247446

this posting

If they would just make us some XM16 and A1 lowers.

This past week i have been tossing around the possibility of building my own so ive been trying to read all i can. Obviously the place to start is the lower so when this came across i had to ask so I can make the best choice when it comes time for me to order

You brought up something that I had completely forgotten about...

The early models (A1 and, before that, the SP1) had different lowers than the A2 style.  About the biggest differences would be that the mag release button did not have the protective "fence" around it.

Another difference on the A1 upper is the Forward Assist...some models had no FA, others may have had a teardrop style or even the round style...alot depends on the year, etc.

These days, lowers can be made to look like the orginal A1 style, but personally, unless you are looking for a specific "retro" look, I wouldn't bother.  I'm all for function over fashion, though.  If I like it and can use it, I will.

Don't be afraid to admit a lack of experience with the AR.  Last year at this time, I barely knew anything about them and I owned one...this site has been a huge help to me.

Differences in lowers:

XM16E1 - no mag fence, old buffer extension
A1 - has mag fence, old buffer extension
A2 - mag fence, reinforced buffer extension