Anything I need to look for in a BCG for a 20 inch barrel as opposed to a 16 inch barrel? 

No.  Upper receivers and BCGs are the same as far as 5.56/.223 barrels go.  No experience with pistol caliber setups. 

1 hour ago, Regular Guy said:

No.  Upper receivers and BCGs are the same as far as 5.56/.223 barrels go.  No experience with pistol caliber setups. 

This is for a 5.56 build with a 20 inch barrel. I purchased an upper without a BCG or CH so was just looking for a little advice. I'm going to use it on my S&W M&P 15 lower or an Anderson that I built a few years ago. Eventually I would like to build a better lower with a higher end drop in trigger group.

BCG is like the heart of AR  try not to buy the cheapest one out there.  jmho

I've put together a couple ARs.  I have shot a lot of them in the military and as an LEO, but I am not an armorer and not an expert gunsmith so my build knowledge is more limited.  Upper receivers and all associated internal components for AR-15, M4, M16 platform weapons should be the same dimensions so that the upper can fit any lower receiver of the proper spec and function properly with the fire control group (aka trigger, hammer, etc).

Don't get bent out of shape on "mil-spec" parts.  A mil-spec part will meet the required dimensions AND be made of the specified material identified in the original military contract.  This includes material alloy composition, any heat treating processes, hardening, etc. There are aftermarket parts like the bolt carrier, that will meet the required dimension, but could be made of a different material like Nickel-Boron.  This is considered an improvement over the mil-spec component.  Do your research on the quality and type of material the parts are made of and decide what you want to use.  There are also aftermarket parts that meet mil-spec dimensions, but the quality of material could be substandard compared to mil-spec.

 

Also, I do know there are four different lengths of gas tubes. 

Full or Rifle length - for barrels 20+"

Mid length - for barrels 14" to 20"

Carbine length - barrels 10" to 18" 

Pistol length - barrels less than 10"

The length of the gas system should correspond to the weight of buffer and spring you use.

 

There is a difference in the receiver extension in which the buffer and spring are install, but this is on the lower receiver.  There is a mil-spec diameter extension which has an outside diameter of 1.146" and a flat back.  Both Mil-spec and commerical stocks will fit on this tube but the commercial stock will be loose.

The commercial extension measures 1.170" outside diameter and sometimes has a slanted back.  Only commerical stocks are to be installed on this tube.  Don't force a mil-spec stock on it.

 

Get a BCG that was designed for the original 20" bbl 5.56 rifle.  What you want is the most common BCG, an M16 BCG.  Don't overthink it, go with standard equipment.  An M16 BCG works in almost every standard 5.56/.223 build type with barrels from 14.5" to 24".

 

It's a 20" barrel with a rifle length gas tube. You can probably run everything from a full weight m-16 style carrier to a lightened carrier and have it run. It's just a question of how violently it will cycle. 

Right now it is looking like you can't be too picky about what. right now joe bobs says they have spikes BCGs in stock for $140, whihc are good. I'd snag that. Every place else I looked was out of stock of anything but overpriced stuff or esoteric lightened/adjustable stuff that is ! $240+

 

 

 

 

Psa had them the other day

25 minutes ago, Golf battery said:

Psa had them the other day

I treat psa in stock stuff as 4-6 weeks out and out of stock stuff as never existing. Also, kind of like optics planet despite ordering from them, I never recommend them to people who expect a store to behave like a store. 

I have heard that alot about psa.  But oddly enough.  I have never had a delay or cancellation from them.  

I have never had an issue with PSA.

6 hours ago, Golf battery said:

I have heard that alot about psa.  But oddly enough.  I have never had a delay or cancellation from them.  

I've never had a cancelation, but a lot of  10+ day waits for something to ship. In fact the only things I have ever gotten from them in a timely manner were rcbs items back when they would have sick discounts on their stuff. 

If you are having trouble getting a complete BCG, I have an extra, new PSA premium BGC laying around. 
 

If you need it to get your rifle running, I’ll  let it go for what I paid for it.