I new to building ar-15s.  I recall the first time I considered building roughly around 12 years ago and the prices were ridiculous.  Now the price is right and I'm looking forward to my first build but I have many questions.

1.  upper reciever:  Is there a difference between the a3 and a4 recievers?  what features should I look for in an upper reciever (not complete upper with barrel, just the complete upper reciever)?  what are m4 ramps? and are there different recievers for carbines, mid-size and full-size?

2  lower reciever:  I understand the pre-ban/post-ban info for the ar-15.  are there any features that I should look for or avoid in a stripped lower reciever other than forged?

3.  barrel:  I think I've gotten that down with a future purchase of a bushmaster barrel at 16".  Is the chamber part of the rear portion of the barrel?  I take it I should go with a .556 chamber in order to use military as well as civilian rounds.

right now I'm in the parts selection phase of my build.  I want to build a tactical carbine-sized ar-15.  is there anything special I should be aware of in comparison to the full-sized ar-15?  Thanks for your patience and future info!

1.  A3 and A4 are both flattop, just different names by different companies.

2.  Nope.

3.  Yes, you are correct on all accounts.

Clarifying #2, Look for forged, avoid cast.

thanks guys, I didn't want to waste anyone's time.  but what are m4 ramps?  

Here is more detail than you ever wanted to know:

[url]http://www.roderuscustom.tzo.com/cgi-bin/ib3/ikonboard.cgi?s=3f188d3f0518ffff;act=SF;f=11;st=0[/url]

thank you!  it looks like I've got some reading ahead of me.  thanks you guys for the helpful info, I'll keep ya'll updated on my build progress.

Your serial munber is the difference in a pre/post lower.  http://www.mp5.net/info/preban.htm#Colt

Try this link and it will tell you about your serial number and if it is in fact a pre or post ban.  Hopefully in September this will all be a moot point!

Good luck with your project.

DG

I hoping it will be too.  it would be nice to have an ideal carbine configuration without paying twice the amount for one.  what are pre-ban lowers running these days?

Re things not needed on an AR-15:

I am pretty sure that a forward assist, while fine in a battle rifle, is pretty much useless on the range and probably not worth the effort in a defensive carbine.  

IN FACT...

Anyone who wants to chime in with the makers of receivers without forward assists, the information would be welcomed.

The AR has a potential design flaw that allows the weapon to fire out of battery.

Sometimes when a fully loaded mag is inserted and the boltcatch is released the momentum of the bolt is not enough to strip the round, chamber it and rotate the bolt.

When in doubt - the Forward assist is a ratcheting finger used to force the boltcarrier forward. This was also requested / used if a cartridge was slightly deformed and would not smoothly chamber on its own.

Another use was to be able to load the weapon in a stealthy manner by controlling the movement of the bolt carrier with the charging handle then using the Forward assist to complete the movement of the Bolt carrier to rotate the bolt.

Air Force doctrine at one point said that if the round would not chamber under the natural action of charging the weapon that round should be discarded by cycling the action again. (not forcing it)

so it sounds like I do want a forward assist.

I disagree.  If the bolt does not rotate (cam closed), the firing pin will not protude from tha bolt face.  Don't believe it ?  Take your carrier/bolt assembly out and try it.

How can it fire out-of-battery if the firing pin can not reach past the bolt face ?

My quess is that most out-of-battery instances and slam-fires are from high primers.  Or maybe KaBooms from improper powder charges which is blamed on slam-firing.

The decision to have a forward assist if all yours, none of my ARs have them, that just my choice, actually the only AR's I own are Sp-1 or A1 configuration, again personal choice. Forward assists are useless to me, some people say its good others don;t care, do some research and find out the benefits/shortfalls of each.

Forged good, cast bad

a 5.56mm chamber will allow you to shoot both 5.56mm and .223. A .223 chmaber will also allow you to shoot a 5.56mm, but I wouldn't trust it too much. Too many variables when you get into a non-standard setup.

Tropic

what are the downfalls of a forward assist?  

None really.

The way I look at it is the original Stoner design didn't have one, so why is one needed now? so you can jam a dirty case into an already dirty chamber? The best way to go forward if you have any reistance is to pull the charging handle back and cycle the rounds.

Somewhere I read that the idea of the forward assist came from all the higher-ups that felt they needed to be a way to "assist" the bolt forward to chamber a mangled round, just as they did when they were privates in WWII slapping the op-rod on their Garands after every clip change.

Tropic

got it.  thank you for the info.  do you have any suggestions on tools for assembly?  I know it's been done to death but what tools am I absolutely going to need?  

If you are ever in a position to "press check" your AR15, the forward assist will come in handy.  Aside from that, I have never had to use it.

So the simple answer is to buy a receiver that has it a forward assist and IGNORE IT.If you need it some day it's there.

The AR I built for prairie dogging does not have a forward assist. Sometimes I wish I had one but really by the time I need it it's a good indicator that it's time to stop shooting and clean a little.

I think we strayed off the original question a bit.Our friend needed advise on building an AR....
A3 receiver has a flat top for scope mounting, and you can put a carry handle on it as well.
A2 receiver has a carry handle and you can put a sight on top of the carry handle.
I have two A2's with sights on top, one red dot, one Small scope.
A4 is the same as A3.
The receivers are the same for all size barrels.
The lower receiver, get forged not cast.
Go for 5.56mm not .223.Then you can shoot both types of ammo.5.56mm is loaded hotter than .223.
The chamber is the rear part of the barrel that the bullet sits in when it is fired.
There is no differance in a carbine or full lenght build exept for the lenght of the barrel.
If you buy Bushmaster look at some of there tools.You can call them and they will help just like they helped me when i built my first gun.
Hope this helps.
The forward assist...its there and part of the design of the gun, and if you use clean ammo and clean your gun after shooting you should never need it.