Looking to do a build with a CavArms lower and a Polymer Upper.  The plan is to also use a light weight barrel with a Magpul foreend.  

Any comments and suggestions on using this or one of the Bushmaster carbon uppers?

Dont know about a polymer upper, seems it would take more force than the lower and as I recall the carbon thing didn't pan out for bushmaster. Of course the big bang happens in the chamber so who knows. I would worry about heat and wear though.

As for the lower, might look into Plumcrazy(i know....). I've got one of their lowers on a delton LW upper and I've had people tell me it handles better than any of their AR's. Holding up great so far.

Don't bother with the polymer uppers, there's not nearly as much material in the upper as in the lower, so the weight savings is negligable.



Also, that's where your optics attach to your barrel.  It will not be as accurate and consistent as aluminum. Period.



And there's a good bit of heat coming off that barrel extension if you do a lot of firing...

The one you linked to is from the much-bitched about blackthorne.

I have no experience with that upper ... but I would suggest you stay away from that company ... they are blackthorne.

See this link

Here's my view on the Polymer stuff... When Glock first came out, everybody said "I'll never own a 'plastic' gun". Maybe a couple of those people still own nothing but steel handguns, out of spite; but I'll wager that most have a 'plastic' gun of some sort now. Either Glock, XD, or any of the very very many polymer-framed guns that are out there.

My take on it is, what do you really have to lose with the poly upper? If it breaks, replace it! It's a $75 part. Just like people replace broken bolts (hell, a lot of people carry a SPARE bolt with them wherever they go).

Pmags are plastic, and they're the most durable magazines I've ever seen. They're literally the Only magazine I will own. And yes, I HAVE driven my Tahoe over more than one of them, just because...

I'd say try the poly upper, and see how it works. If all else fails, buy a forged stripped upper for another $65-$70, and transfer the parts to that.

I'd imagine a poly upper would shoot a lot more rounds in its lifetime, than most people who build these things will shoot in theirs.

Who knows though. I don't have any particular experience with a poly upper, so I could be way off. It might be a really crappy polymer that they used to build these things, that doesn't even compare to the Pmag polymer...

But again, if it wears out/breaks. Replace it. Unless you're an 'operator', and this some sort of a combat weapon, who cares if it breaks? If you're anything like me (And 95% of shooters out there; mall ninjas included), the real-world story of if it wears out/breaks, the worst thing that will happen is that you cut a range day a little short, and replace it with another $65-$70 part.

Or spend the extra $10-15 and not worry about it...ever...

stay away. you can get an Aero Precision upper from Surplusammo.com for $59.99 right now
AP upper

I'm pretty sure it wasn't a money issue. It was a weight issue.

RIF.

And i'm pretty sure i explained how there's basically zero to be gained weight-wise by going polymer with an upper as compared to a lower...



RIF, back at ya...





 

Correct me if I'm wrong, but the rails on most polymer guns are metal.

If there were, say, a steel lining for the barrel to be seated in and the bcg to run in, then I'd go for it. Assuming it was done right

Then it would weigh MORE than a forged upper



 

It says its half the wight, a little 20 gauge steel won't add that much

God no, please.



Metal receivers have a tendency to split and shatter if the gun kabooms...use a polymer receiver, and you'll end up with an extractor in your eye...

"Basically Zero" is actually probably 3oz, or so... Is that huge weight savings? No, not in and of itself. However, if you combine it with a polymer lower, it adds up pretty quick. A lightweight barrel will help weight reduction the most, but there is certaily some weight to be saved in the upper and lower. A combination of shaving a few ounces here and there can add up quick, to some substantial weight savings.

I've handled the Carbon-15, and between those things, the rifle is VERY light, compared to its aluminum sisters.

You can blab and say "well, it's not very much, so it doesn't count", but weight is weight. Taking 5 oz off a lower is good, but taking 8 oz. between an upper and a lower is even better.

Again, mall ninjas such as yourself will almost certainly require "Mil-spec", if not titanium or tungsten lower/upper combos; but to pretend that there's no point, because it's only a few ounces, is a little disingenuous.   The question is about weight savings. Not whether or not you think cutting weight is important.

But I reckon your opinion on everything is so important, that you simply must make sure it's heard; relevant to the question, or not.

It does say half the weight (which I doubt) but it doesn't say there is metal reinforcements (which there aren't).  That would be a rather more complex injection molding process that blackthorne wouldn't be capable of pulling off.   If Magpul or one of the other polymer wizard companies made one, I would buy it.  But not blackthorne.

Does anyone make a polymer forward assist or a plug for the hole?  That would save you a few ounces

Yes, there are plugs out there. I have a buddy who bought a .22 upper from AKpartskits.com, which came with that upper, and they had plugged the hole with a polymer plug, that appears like it might be a sling mount (never looked too closely at it).

Goes bang every time, and makes a decent group for an off-the-shelf .22lr... I haven't seen it used for a 5.56.

I don't know how much a forward assist actually weighs, but as I said before, weight is weight. If it's 5 oz. or 1/2 oz., it all adds up in the end.

There is something to be gained: weight reduction... which may simply be a higher priority than durability, for some shooters.

Fine, you do your thing, and i'll do mine.  And i'll still be doing it when you've replaced/repaired the plasticy bits.





Sure polymer has it's place, like with the Cav lowers, which are designed AS POLYMER, not taking an aluminum part design and making it from plastic.





I shoot my guns, all of them, probably more than most.  I'm not going to buy anything that I won't use and when i use it, i don't want any doubts as to it's reliabilty...  You think that's mall ninja-ish? Well whatever...


Stop namecalling and whining and maybe more people will take your opinions seriously.





 

I've said it more than once (back to that RIF thing), I haven't used a polymer upper. Weight savings is not a huge deal to me. But my personal preferences are irrelevant to the guy's question.

I own very many different ARs, intended for very different purposes. Making one as light as possible hasn't been super high on the list yet, but it's on the list somewhere.

I'm VERY tempted to buy a polymer upper, just to run it through its paces. I've been quite clear that I haven't personally tried it, and it seems that you haven't either. So your saying that it will require replacement is pure speculation on your part. You've never tried it, you don't know. But it also seems that you don't know that you don't know.

The mall ninja thing was meant as a joke. But reading it again, it seems just rude, rather than a joke. Not the way I meant it. It never works out the same way written down, as it does in my head.

My point is, there are people who want to WAY overbuild these things. And just because something wasn't possible with polymers 63 years ago, when these things were designed, doesn't mean it's not possible now, in the year 2010. If it was Magpul putting these things out, people would be falling all over themselves to try it. And any naysayers would be poo-pooed by saying "Hey, it's a $70 part. If it doesn't do what I expect it to, I can always just replace it". And a whole lot of people would jump on the bandwagon.

If somebody has actually, first-hand, seen one of these things fail, bring it to the table, and lets see it! I want to know about it, honestly.

But just speculating that it's junk, or that "it's going to explode", because it's polymer, is silly; and frankly, a little bit ignorant.

Sorry, it's not the kool-ade opinion that will get mad props, but whatever. I'm not interested so much in everybody liking my answers, as I am in actually answering the questions posed. Cheers.

Point taken, mostly.





I don't consider AR's to be overbuilt as designed.  In fact the receivers have been modified over the years for increased strength (ribs and such).


There have been cases of case ruptures in carbon15's, can't seem to find the pics at the moment, but the front half of the upper basically turned to splinters.  A similar failure in a quality aluminum receiver will probably grenade the magazine and may bulge the upper and/or lower a bit, but it should mostly contain the chaos.





And I'm just not a fan of plastic wear surfaces...





I'm not opposed to "plastic rifles" if they are designed as such, but when someone takes a design, and substitutes another much different material without a significant re-design, that's a bit off-putting.



(Cav-Arms lower, ok.  PlumCrazy lower?...not so much...)





I own an RFB, it's got a whole lotta plastic in it, but the bolt carrier rides on steel rails and there's two layers of 16ga steel between the .308 in the chamber and the shooter's face...





 

I owned a bushie carbon 15. It was really light but had some issues. The barrel nut is not a standard thread ( compared to other AR's). The polymer got a bit chewed up from splitting the upper from the lower during cleaning ( I was running a collapsable stock). The brass deflector fell off and was only held by a set screw that stripped the upper reciever. The muzzle brake was a quick detach that would readily during rapid fire.
 Overall it was not horrible, however I would not own another.

A KB is a KB, and it's not caused by upper receiver EVER. Period. If a weapon KBs, its because either the round was over-charged with powder, the barrel was somehow blocked, or the headspacing was WAAAY off, or some other problem with the bolt or barrel.

In any case, you can pretty much count on the upper (and lower) being trashed. How pretty it is afterward is irrelevant to the discussion, because ruined is ruined. Period. When I build a rifle, or reload rounds, or fire a rifle, I do what it takes to ensure that my rifle does not become a bomb.

If you're more worried about your rifle blowing up in your face, or how well your upper is going to contain the explosion when it blows up in your face, than how much it weighs, or how much it cost, or really much of anything else for that matter; my advice would be two fold:

1:) DON'T FIRE THE WEAPON, UNTIL YOU CAN TRUST THAT IT WON"T EXPLODE IN YOUR FACE.
2:) Perhaps consider finding a different hobby, besides rifle building.

I know you're a competent builder, as I followed your build of the pistol shown in your profile pic, and I respect your level of competence in that area; so I don't think you really have any need for concerns of your weapon exploding in your face. So I'm left to wonder what it is you're getting at.

When I ask for examples of failures, I'm talking about failures of the upper, caused by the upper. Not caused by a KB, or being run over by a tank, or dropped out of a jet, or placed in a furnace... Normal failures, caused by normal use (Even heavy or "extreme use", is fine by me). I really would be interested to see what level of use it takes to make these things fail, be it a lot, or a little.

I do agree in regard to wear surfaces. I'd like a metal wear surface too, based on what I'm traditionally used to. But unless we really establish how many rounds it takes to wear one out, we're really just speculating on what we think MIGHT happen. Will aluminum outlast today's polymners? Almost certainly. Will the rifle realistically ever be able to fire enough rounds to actually wear it out? Well, we don't really know that, and I would like to know. My guess is that a large majority of shooters will not fire enough rounds to wear it out, but again, that's speculation...

I've got to hurry up and finish paying off my taxes this summer, and then get to work on some business debt, but in the coming months, I fully plan on locating a polymer upper, and doing a little bit of a torture test...

I've wanted to experiment with a few of my own ideas about polymers, with regard to the AR platform for a few years now as well. So you may see something you haven't seen before, in the next year or two. ;)

I think if you are worrying about the KBs then think about this. Polymers are being used to reinforce or build explosive barriers in the middle east and high value targets across the world.

Polymers are more flexible than aluminum and will might absorb more of the explosion before completely loosing it. Aluminum is going to rupture and rupturing too soon is BAD.

Both AL and Polymers are going to rupture and will likely mess up the rifle if this happens. But there is a good chance the plastic will give a little before rupturing allow some energy to be dissipated before it tears apart. The harder the material the less it will give before rupturing. When it ruptures the least amount of energy left to dissipate is better. So if you are designing as  an engineer, where you are designing to kill as few people as possible when it fails(because everything fails) then the polymer MIGHT, MIGHT, be the best bet.

Would I want a plastic AR? No. I don't even like plastic handguards or butt stocks. But weight is an issue. I own a FAL, so my next AR will be light.


EDIT: Editing out some words as I'm not 100% positive of what I speak, this is an opinion.

Are you equating the polymer armor used in military gear/bomb blankets and the plastic used in Blackthorne receivers?


Bomb blankets and such are designed to stop the shrapnel from a blast, much like a kevlar vest.

In the case of a KB, there's not much shrapnel, but there is a lot of high pressure gas that you want to keep away from the shooter.  The current "polymer" uppers just don't have the strength to contain such an event.  A quality aluminum forging will yield significantly before failure, but as you said, they CAN fail as well, but i've only ever seen this happen with a split bolt/bolt carrier.



I personally am not that comfortable with having nothing between my face and the breech of a high-pressure rifle cartridge but an 1/8" of plastic...  How many modern rifles are designed like that?




I'm sure a polymer/composite receiver COULD be built that would be stronger, lighter and more wear resistant than an aluminum one, but it would be prohibitively expensive.  





OP, never buy anything from Blackthorne, you have been warned. (not just their polymer stuff, ANY of their junk)
 

This.  They do not deserve anyones business.

I own a Hesse polymer upper from some years back.  It is light.  AND - it did NOT fit the barrel.  I had to use shim stock to get it to fit.  I'd try to find a Bushmaster if you simply must have a plastic upper.  PLUS - the optice will never be as rigidly mounted as Al upper.

maybe somone with better search skills can find the thread, but there is a thread out there in which the consensus seems to be a factory loaded hot round destroyed the carrier, bulged the upper, & turned a P-Mag into plastic shrapnel. The forged upper held together, lower was ok, & the carrier & mag were obviously toast. Somone commented that they had seen this before with a metal mag, & it just blew the bottom out of the mag without sending shrapnel. I found this post interesting because of my memory of that thread, but I cant find it so help me. wonder how it would have gone with a plastic upper. he was a lefty & didnt get hurt, but you gotta wonder what would have happened had that upper let go.

I have no idea who Blackthorne is, or even what they do so bad. I was saying just because it is plastic, it isn't necessarily bad. But the plastic has to be designed correctly.

Polymers have a place in almost all building/design applications, Especially FRPs(fiber reinforced polymers).

ETA: And I was actually referencing the FRPs being used in retrofitting buildings not personal armor.

Alright, now we're coming together...

Blackthorne/Hesse/Vulcan (all the same company) has been producing horrible crap for the AR/AK crowd for years, under the different names.  The name changes are basically to escape the reputation.



The "has to be designed correctly" part is the hangup.  I think Blackthorne basically took a forged AR upper, created a mold to match, and is now casting them in the same "Polymer" you find in a plastic paintball gun.



Like i said above, it could be done, correctly, with all the advantages you mentioned.  But it's not going to come from Blackthorne and it's NOT going to cost $75...



 

I have a carbon 15 on a Cav arms lower.


I built it simply to have a polymer AR.
A stripped carbon upper is $160 from Bushmaster.
I wouldn't use the Blackthorne product because o the bad reputation.

Unless you want something different i wouldn't bother with the carbon 15.
The weight saving is negligible, it has no forward assist, and no port door.
For a little more money you could get a billet Sundevil slick side upper.

Not that the carbon 15 is a bad product, it's just for the money you can do better.
Mine has run flawlessly for over 1000 rounds.

Cool "proof of concept" bro!  

Amazing amount of plastic in that gun. Seriously, - it is impressive.  Were it my gun, I would also seek out a Colt brand plastic recoil buffer (they are out there) along with a plastic mag catch button, Only Pmags of course, and just for the hell of it, some of that plastic-cased 5.56mm ammo!

Forgot to mention: plastic Magpul backup sights.

And that is what I will do.   Thanks for the Blackthorne heads up.