How good is a polymer lower compared to an aluminum? How reliable are they? Any problems or issues with them? Found a good deal on one and wasn't sure how good they are.

Unless it's a CavArms, not interested.  

They are way better than forged aluminum.

Make sure you have a sharp file and set of drill bits if you expect to actually assemble it.

Just say no, for the love of all thats Holy...I beg you.

If you want a lightweight (or at least lighter-weight) receiver, check out Mag Tactical.  They put out a receiver made up of some sort of alloy that is lighter than aluminum, but not as fragile (and shitty) like polymer lowers, just be ready to spend $170-200 for it pre-panic.

It's magnesium. Does that sound like a good idea to anybody?

Actually it's made of magnesium alloy, so yes.  It does.

Learned the hard way .......... stay the fuck away from polymer ......... at least the PLUM CRAZY P.O.S.

Rear take down pin fell out during live fire .......... didn't notice till the FTE, then saw the front pin was jacked and broke from carrying the entire recoil load.  Ended badly with a cracked mag well.

what a piece of shit ........... worst money spent ever.

By the BY ................. can this thing be repaired or is it bin bound?

Stay away from all poly. Doesn't matter who makes it. Unless you want to be worried about shooting it, limited to upper choices and gas systems, then stay away. Prices for lowers are back down to $119 for a PSA lower, so price isn't an issue. Just say no to poly..

yep.

I still find all this information rather amusing.  I have the NFA lower and rather like it.

regarding the fitting: it did take some light filing on the rear takedown pin hole, and a little on the mag catch slot.  in total it took about 7 min with a fine small round file.  inconvenient maybe, but everything now fits perfect with zero slop anywhere.  from my personal experience with, I'll take it as a win.

for the durability issue: I still do not understand all the hate.  it holds my mega arms upper and setup in 300BLK with no issues and no signs of wear after about 800 rounds now.  I can not speak to the plum crazy or the cavarms but my NFA is rather nice and would personally not hesitate to own another.

as always YMMV, so make your own decision.  I bought it on a whim just to test one out, and i don't forsee replacing it anytime soon unless it does end up developing an issue..  if it ever does.

I don't own one, but as a neutral observer, NFA offers a lifetime warranty.  I would check on your other lower choices and see if they do the same.  I can't imagine any manufacturer offering a lifetime warranty that thought the product would be coming back broken.

Just some thoughts.

While I don't personally own a poly lower (at least not yet).  I have been reading quite a bit about them in various forums and I will say this:  If you are going to get one, get an NFA.  NFA seems to have the least amount of problems with theirs, and they do back them up with a good warranty.  Plum Crazy, ATI, others don't seem to be very durable in general.  Bushmaster's Carbon-15 is interesting, but not really the same thing as it's some sort of Carbon Fiber material, not the same as the Polymers others have used.  That being said, this market is driving innovation and you will start seeing more low cost options made from various materials.

My last three builds have been with NFA.  1000 rounds through the first with zero issues.  No fitting needed, although others have stated some may be required.
Lifetime warranty.

Paired with these uppers:

.300 Blackout
.223 20" target
.300 Blackout fluted barrel

Great fit and finish.  I will buy more.

I can understand some of the skeptics that have seen lots of gimmicks over the years. Some just don't see a reason to stray from convention. But there will always be new materials and it just takes a long time to establish a solid reputation. I am sure some people still don't like magpul pmags.

I think about cracked BCGs and how people react not by saying steel is bad but how to improve the process. My NFA is working well so far, I would get one again but not while I'm in NYS.

I purchased an NFA complete lower before the panic. Upon receiving it the gun would short stroke and burst fire, so I sent it back for RMA. Since that seemed to be an issue with the LPK instead of the lower, I still didnt really regret the purchase. While waiting for RMA the panic started and my only panic buy was another NFA complete lower for $150 ($50 more than before). One of the first times I tried to swap an upper on the 2nd lower the pivot pin broke its holding detent and came all the way out of the gun, just from pushing on it. So I decided to get rid of them ASAP and actually made a few bucks. FWIW new frontier was awesome with the CS in both experiences, they shipped parts to me quickly and the RMA was pretty quick and hassle free. I plan to buy a G-15 lower from them once they again become available

Of course it's an alloy, almost all industrial metals are.

Well you made it sound like it would burst into flames or something...

It's beefed up in the critical areas, since it's weaker than 6061/7075, but still lighter overall.

Are you sure it won't?
I'd like to know the specific alloy.

ETA: It's almost certainly not an alloy that will burn. But I'd still like to know.

Magnesium Aluminum alloy. Source: I have one and that's what a card that came with it said.

The same questions about poly lowers comes up each week by someone else.  I wonder if we can get a tacked poly lower thread?

The main concerns I see about poly lowers is durability of the poly material and the design of the lower itself.  Polymers are durable, however they are not 1 for 1 replacements for metals.  If you look at the lower designs that are exact copies of alloy lowers, a common failure is the collar that the buffer tube threads into breaks off by the rear takedown pin.  If a lower was die cast, it might suffer the same failure, which is why we only see hammer forged and billet lowers.

The Cav15 lowers don't suffer from that problem, due to how the buffer tube, stock, and grip being integrated into the lower.  The web between the pistol grip and stock is thicker, and the integratred buffer tube/ stock doesn't place the same stress on the back of the lower as a threaded in buffer tube would.  I wouldn't be afraid to use the mortar technique to clear a jam with a Cav15 lower.  I wouldn't try it with a PCF lower.