View Full Version : Is AR-15 reliability more dependent on the upper or the lower?

C Philip
February 9, 2011, 12:53 PM
I am thinking about building my own AR-15 lower half, then buying a complete upper to put on it. It will be my first time putting one together, and I've read that a lot of problems with reliability come from home built AR's. If I'm only building the lower though, and using high quality parts, how much risk is there of me ending up with an unreliable rifle?

Bartholomew Roberts
February 9, 2011, 12:58 PM
In the lower, assuming quality parts, the things I would look at would be:

1. Make sure the lower receiver extension (aka buffer tube) is threaded far enough in to hold the buffer retaining pin in.

2. Make sure the hammer spring is correctly installed to prevent the pins from walking.

3. Make sure the mag catch is properly tensioned and installed.

I'm pretty non-mechanically inclined; but even I can build an AR. The key is to take it out and shoot it before you actually start relying on it - and that is true whether it is a home build or a factory build.

February 9, 2011, 01:01 PM
Most of the accuracy and reliability come from the upper, particularly the barrel and bolt carrier group (magazines play a big role in reliability, but since quality GI style and Pmags are very reasonably priced, there's no reason to have crap mags).

So long as your lower passes function checks (the safety works, the disconnector works, the trigger resets, etc.), the end result of accuracy and reliability should be unaffected. I wouldn't use the cheapest lower parts kit I could find (personally, I recommend RRA or Daniel Defense kits; not much more expensive than the real lowball stuff, but they go in easier and they work), but buying a cheap bolt carrier group is more likely to leave you with a nonfunctional rifle than going DPMS instead of Colt on the lower.

February 9, 2011, 04:43 PM
I've done three lowers now and all function reliably. Just follow the suggestions above and you'll do fine if you are handy with common tools (which is all you need for building a lower receiver). Check the fire control group for proper function and the buffer/spring assembly. Make sure to catch the hammer with your thumb when you test the trigger action. Don't let it slam against the bolt hold open catch.

Go slow, look at the online guides and videos and you'll be fine. Don't mix up the small springs (BHO and disconnector springs look similar).

As far as reliability, I put quality factory built uppers with good BCGs on my lowers and no functioning problems at all.

February 10, 2011, 03:03 AM
It would seem that rifle reliability will be more dependent on your ability to build a decent lower.

February 10, 2011, 03:07 AM
Go to you tube and search building ar-15 uppers and lowers. You can watch it done and explained by the builders. This is 100% better than printed instructions:)

February 10, 2011, 09:09 AM
Brownell's has a excellent set of AR assembly vid's, they walk you through all steps.

Buy a quality LPK. G&R makes one of the best I have seen, a few more bucks but well worth it.


The most common problem I see with home built lowers is incorrect installation of the FCG springs.

February 10, 2011, 12:44 PM
Is AR-15 reliability more dependent on the upper or the lower

Yes. Is your life more dependent on your heart beating or you breathing? You have to have both. FWIW mags generally have the most influence.

February 10, 2011, 02:09 PM
The upper is the more vital part(s), but problems can occur on either end...

February 11, 2011, 12:18 PM
The biggest issue with assembling an AR isn't so much getting the parts together right, It's getting the right parts together.

There's way too much marketing and hype that distorts what improvement you really get. Want more accuracy, will a $250 free float deliver, or maybe that money could go to a 1MOA barrel and some practice? The internet is full of photos of closet queens dressed out in all the latest Brand parts, none of which are required or even necessary on a two way range. Worse yet, how many tag themselves with the line, "haven't taken it to the range yet."

Then again, people build rat rods and leave them in the garage, they might get dirty.

February 11, 2011, 01:18 PM
The upper is where most of the magic happens. From what I've seen, it's also the place where the home-builder has a lot more opportunities to make a mess of things. As for the OP, putting a good upper on a home built lower is a pretty good strategy. People have already posted some potential problem areas on the lower, all of which are pretty easy to catch if you're doing it wrong.