Planning first build. I see buffer tube assemblies from as low as $20 all the way up to $50.

 

I understand this lower part is not as sexy as discussing barrels or triggers but what's the difference between a 20 and 50 assembly? My build is not going to be high tech, mostly plinking. My only real requirement is I want carbine tube because I will most likely get the new Magpul fixed stock.

 

I get it, like how much could I possibly save? 25 bucks? It's more really about learning the differences in stuff. So someone please give me some recommendations.

 

(also I might just post in WTB section if someone has the parts laying around they want to get rid of.)

Price is related to tube construction (6061 aluminum vs 7075 aluminum)  and commercial or milspec,  and whether material is stamped or machined.

 

http://dsgarms.com/ProductInfo/MAG480-BLK.aspx

http://dsgarms.com/ProductInfo/DSGMILSPECKIT.aspx

 

Buffer tube is made from 6061 and milspec, 7075 is tops however.  Plan on 7075 if you intend to participate in carbine training or other intense use of the firearm.

First of all make sure you get a true mil spec receiver extension. Receiver extensions built to Commercial spec dimensions are significantly weaker. Here is a quick diagram that explains it better and faster than I can:

 

Also, the $50 kit is likely not only mil spec dimensions but also mil spec materials and construction process as well, making it even stronger. The receiver extension is an important part of the system. If that part fails you have nothing but a weird looking club...

 

My advice, buy a true mil spec kit from a reputable company like BCM, Ranier Arms, etc...

I do either brownells or vltor depending on what the person wants to spend. Both are 7075. The vltor I get has number on the top to show where your stock is positioned. Useless in nj but in other places is helpful. I honestly use the brownells ones more because when matched up to a magpul fixed carbine stock it's totally covered anyway.

First of all make sure you get a true mil spec receiver extension. Receiver extensions built to Commercial spec dimensions are significantly weaker. Here is a quick diagram that explains it better and faster than I can:

 

Also, the $50 kit is likely not only mil spec dimensions but also mil spec materials and construction process as well, making it even stronger. The receiver extension is an important part of the system. If that part fails you have nothing but a weird looking club...

 

My advice, buy a true mil spec kit from a reputable company like BCM, Ranier Arms, etc...

What the image is describing is cut threads vs. rolled threads.  Rolled threads are stronger as they interface more completely with the receiver extension.  Cut threads are cheaper to produce.  The very best receiver extension that I have come across is the Lewis Machine Tool six position extension.  Forged 7075, rolled threads, dry film lube, nearly hydraulic feeling with the SOPMOD stock.  They also cost nearly $50 each for just the extension but you're paying for quality.  UTG surprised me with their US made extension.  While it's extruded 6061 with a T6 temper, they rolled the threads.  I've mortared a couple test builds before committing to carrying them and they held up.  

Just wanted to add my experience to what High Exposure and T Bill already covered.  I purchased an M&P15 last year and upgraded the stock with the Ace Skelton and put the old tube away. Last month I started assembling my new lower and wanted to re-purpose the old buffer tube which is commercial.The commercial tube didn’t cover the buffer stop correctly in my new Spikes lower, see image below. If I went one more turn it was too much. In the image you see it just barely hit the the edge of the buffer stop. To resolve the issue on the Spikes lower I moved the Ace stock to the new build and returned the old commercial tube (with newly purchased Magpul fixed stock) to the M&P15. The exchange provided the proper coverage for the buffer stop on the new lower however the M&P15 was OK with both....

 

If you buy a Magpul Fixed stock make sure you are getting the Milspec assuming that you will be buying the milspec tube. The commercial tube OD is 1.17” and the Mil Spec tube OD is 1.15” so you want to verify you are getting the correct Magpul Fixed stock to go with your tube. I ordered my stock Amazon and they don’t always specify Commercial or MilSpec so you want to watch for that... BTW I really like the Magpul fixed stock, its very well made.

 

I have put a small bevel in the tube edge at the right place that allows that one extra turn you  desire for the buffer retainer.  Works OK, in fact the contour matches the profile of the retainer and seats better on the tube. 

I buy the BCM kits.

I buy the BCM kits.

Same here