Best M4 Lower Receiver

[zollen]

Am I correct that the major function of M4 lower receivers are for regulating the firing mode (i.e. safe, semi, full..etc), nothing more. M4 receivers are the least important part of an AR. M4 uppers (the most important part) regulate the power of shots and the qualities of the shots.

[Hmac]

If you consider the core of the rifle to be lower, upper, and barrel, then there's some truth to that, but I wouldn't take that to imply that one could universally pick up any old receiver. Price variations are often closely related to the degree of quality assurance applied before a rifle mfgr lets it out the door. Some companies are more rigid in the adherence to the specs than others, and as you'd expect the more receivers they throw away because they don't pass muster, the more expensive are the ones they DO sell. An out of spec lower might result in magazines not dropping easily, problems with the bolt catch function, issues with FC pins, clean fit with an upper, etc.

I had a conversation with Greg Sullivan about this this past summer. He had some very interesting experiences to relate regarding his struggles over the years to get consistently good quality receivers in building his rifles - largely related to accuracy of the CNC program files that many of the companies use (he machines his own, now). I came away from that conversation with the belief that all lower receivers are most definitely NOT created equal.

[TehLlama]

They can cause some rather beguiling issues if they're out of spec, but given the large number of quality forged examples - focus on one of those that comes with a quality LPK, and work from there. There are quite a few parts that have to meet a rather exact spec in order for the rifle to work as designed, so it actually has quite a few functions, but that's not to say that paying twice as much for some buys you a whole lot more (*unless you're a lefty looking at an SR15 IWS or something)

[zollen]

I am planning to get the BCM lower receiver. They are pretty good I think.

[Quentin]

BCM is a great choice for a lower. And of course for the upper as well.

To appreciate how important it is for a lower to be "in spec" buy an 80% lower and try to finish that last 20%. Not so easy, and you can see how important it is to buy a good lower. (Yeah, I have one and still haven't finished it. )

[zollen]

BCM is a great choice for a lower. And of course for the upper as well.

To appreciate how important it is for a lower to be "in spec" buy an 80% lower and try to finish that last 20%. Not so easy, and you can see how important it is to buy a good lower. (Yeah, I have one and still haven't finished it. )

Here is my plan: BCM Lower, Noveske QD Plate, Geissele SSA-E trigger.

[TehLlama]

Better plan: BCM Blemished Lower, Noveske QD Plate, and Geissele SSA (or SSA-E). Works great, and for the life of me I can't find the blemish on mine. Matches my SR15 upper's finish quite well.

[rob_s]

I had a conversation with Greg Sullivan about this this past summer. He had some very interesting experiences to relate regarding his struggles over the years to get consistently good quality receivers in building his rifles - largely related to accuracy of the CNC program files that many of the companies use (he machines his own, now). I came away from that conversation with the belief that all lower receivers are most definitely NOT created equal.

Sample of one and all that...

I bought one of those lowers based on what I thought was a very good explanation from Sully. I liked what I saw, and his logic made sense. It sat for a long time in a box while I accumulated the other parts, and once I had them I built up the gun. The problem I have is that it doesn't drop even current-production Pmags free with regularity. I need to play with it some more, but I was very disappointed after all that time to finally get it together and then have the mags stick.

This isn't to say anything negative about Sully, his lowers, or anything else, but is meant simply to point out that there are no guarantees. For anyone reading this that's planning on SBRing a lower I STRONGLY suggest building it up with all of your parts first, sticking a non-NFA upper on it, and getting some testfire with it before you send off the paperwork and engrave it.

[John Hwang]

Out of all the Lowers Rainier Arms sells & builds on (which is a lot), MEGA Arms is the nicest from our experience in terms of fit and finish. That was why Rainier chose MEGA to build our own lowers.

[zollen]

Better plan: BCM Blemished Lower, Noveske QD Plate, and Geissele SSA (or SSA-E). Works great, and for the life of me I can't find the blemish on mine. Matches my SR15 upper's finish quite well.

Blemished lower would mean a used lower. Why would you prefer a used lower?

[Hatter]

Zollen,

Blemished means just that, blemished. Usually during the manufacturing process the metal or finish may obtain a slight cosmetic irregularity. This irregularity makes the part a blemished part and they are typically sold at a reduced price. The blemish is purely cosmetic. If it sacrificed the integrity of the part at all from a mechanical use or safety point the part would be scrapped without question.

It is not a used part at all. Consider it factory seconds.

[Stickman]

Blemished lower would mean a used lower. Why would you prefer a used lower?

No it doesn't.

[freebug]

If you can find one, the Magpul China Doll lower.

I'd pay for one if I could find one. That one is a keeper!

[Stickman]

If you can find one, the Magpul China Doll lower.

I'd pay for one if I could find one. That one is a keeper!

That is not a realistic option for people.

[Army Chief]

OK, I'll bite: what is it about a Magpul China Doll lower that you find so compelling?

Given all of the recent developments we've seen on the market, these old Magpul billet lowers (MPLA, MPLC) have really been outclassed, and short of the fanboy appeal, I can't think of any reason why it would make sense to opt for one of them over what is currently available.

AC