AR-15 cross-brand incompatibility? Will BM and RRA upper/lower match?

[MatthewVanitas]

Greetings,
After much, much agonizing, I'm just about to order a Bushmaster Dissipator upper, M4-profile barrel on A1 receiver. BM will put the pieces together, drop in the boltparts, warranty it as BM, and sell it to me for just over $500.

I'm thinking I'll slap some A1 furniture on it, and plunk it onto a Stag Arms lower receiver. A compleat Stag Arms lower, sans buttstock, is $145. The same deal from BM is $259. So I thought I'd save myself $114 of ammo-money buy buying generic, especially as I hear that Stag is the company that makes receivers for RRA and others.

But the infamous "guy I was talking to at the range" warned that such a BM upper / Stag lower AR-15 might not work at all, or might actually be unsafe. Something about incompatible tolerances, "not MILSPEC", etc.

He also argued: "there's no point to buying a generic lower; you'll probably keep the same lower for decades, so buy a good one."

So, is he right or no? Will putting a stock-BM upper on a stock-Stag lower turn an AR into a hand-grenade or a wallhanger? Is it worth the extra $114 to have a snake on the side?

Are there any infamous "don't combine brands X and Y" out there that I should be aware of?

Thanks for any info. I was feeling pretty good about the whole "modularity" concept, but then along comes someone to fill my mind with doubts. Thanks for your time, -MV

 

[CleverNickname]

FWIW, I have two Bushmaster lowers with RRA uppers on them, and they have yet to blow up. In fact, all my ARs are combinations of multiple brands of parts. Get the Stag and save your money.

 

[dmftoy1]

He's full of crap.

I've mounted an RRA upper on my BM lower, and my BM upper on my Stag lowers. They all work fine. (never a single malf of any kind) I think the only thing you have to worry about is if you use one of the colt's that use a wierd pin size . . even then I think there's a work-around.

Just my .02

Have a good one,
Dave

 

[browningguy]

Should be no problem at all, the only issue you may see is a little looseness between the upper and lower but that has nothing to do with safety or accuracy. There might be a little difference in trigger quality, but to be honest you will want to do a trigger job on most any lower that has the standard trigger in it.

 

[RavenVT100]

Whoever it was that told you that is feeding you a load.

While it may pay to get a quality lower, the entire process of firing the gun aside from the point where the striker hits the FP occurs in the upper receiver. Anything that can go "boom" is going to go boom independently of the lower. The parts that need to fit together "just right" are the chamber and the bolt; that's why you check headspace when you switch either of these parts out. Lowers and uppers require a far more granular tolerance.

 

[g56]

He didn't know what he is talking about! It shouldn't be a problem, sometimes they are tight, but most of the time no fitting is required. As far as quality, a Stag Arms lower receiver is as good a receiver as you can get, Stag is CMT's house brand, and CMT does the machining on all of Rock River Arms lower receivers, it is identical in quality to a Rock River Arms lower receiver, the only difference is that RRA gets their lower receivers un anodized from CMT, and CMT does their own anodizing, the quality of the finish is the same.

I have both and the only difference is the markings.

The only real compatibility problems are with Colt parts, some Colt receivers use a different size pin than every other brand.

 

[MatthewVanitas]

Groovy. I assumed it was fear-mongering, but I just wanted to make sure. He was a very pleasant fellow, and let me try out his hi-speed RRA AR, which he was about to add $500 of custom parts and $400 of optics onto.

I'm very lo-speed for a guy in his twenties, so it's A1 upper and 1927-model 1911s for this boy.

If I do have any looseness between the upper/lower, do I just buy one of those little rubber wedge thinggummies?

Thanks for all the reassurance. If my AR-15 does become a handgrenade, I'll be sure to post pics / coroner's report / etc. -MV

 

[Gary G23]

I have owned Armalite, Bushmaster, Colt, Eagle Arms, Rock River Arms, and Stag. I plan on using Stag lowers for ALL my future rifles.

 

[DMK]

But the infamous "guy I was talking to at the range" warned

One of the reasons I leave my earplugs in even when nobody's shooting.

 

[dch1978]

M4 profile

just a question.

Isn't the only thing that an M4 profile adds is the slot for the grenade launcher? Over any other threaded 16 incher, with a comp and bayo lug?

So with the Dissipator wouldn't you in effect cover that cut-out?

As far as fitting the two differing brands there is no problem saw 3 RRA lowers with Bushy uppers built not to long ago.

Isn't there a difference in pin size with some colt models? Or has that changed?

DCH

 

[Bartholomew Roberts]

I am aware of two instances of different uppers and lowers not matching. I wasn't present for either one and only read Internet reports about them and both happened around three years ago, so not a common occurence; but possible I suppose.

From my own person experience, I've never had trouble mixing and matching lowers in 15 or so years of owning and building ARs.

He also argued: "there's no point to buying a generic lower; you'll probably keep the same lower for decades, so buy a good one."

Stag Arms is certainly a good lower and there is not $114 worth of difference between it and a Bushmaster (speaking as an owner of several Bushmasters).

 

[Thin Black Line]

You get what you pay for. The B and Colt receivers are better, but you
can make a good AR-15 with the RRA lower. I've used mix n match
parts often. The B upper will fit fine on a RRA lower (in fact I just
finished putting a very used B upper on a RRA lower yesterday and the
tightness of fit was still excellent).

Here's the big "but" --you will probably still have to trim and file the
various little parts in the kit for the lower receiver no matter what the
brand. I had a much easier time with parts fitting well into the B as
opposed to the RRA receiver. Now was this the kit or the receiver? I
think in the rush to produce for the post-AWB-sunset that we have an
increase in factory production with less attention being paid to the QA
part of it. You're going to see that MORE with a cheap lower kit and
a cheaper receiver. I could tell my new RRA receiver had a couple holes
that were drilled by a well-worn bit and required some extra work that
my B did not.....

 

[chopinbloc]

i have an rra upper on a bushy lower - works great. you should have no problems either.

stag arms is NOT a "generic" part. if anything, they may offer slightly better quality than bushmaster, though the lowers are the same thing.

if you are concerned about accuracy, the first thing is to get a decent trigger. if you want a good trigger on a defensive rifle, get one of the drop in, non adjustable "match" triggers. rra sells a complete lower parts kit, less stock and lower receiver with ergo grip and match trigger for ~$150 iirc. might not be a bad idea to buy the stripped lower from stag and the parts kit from rra but that's just my opinion. that and 99 cents will get you a cup of coffee.

 

[demusn1979]

i have an rra upper on a bushy lower - works great. you should have no problems either.

stag arms is NOT a "generic" part. if anything, they may offer slightly better quality than bushmaster, though the lowers are the same thing.

if you are concerned about accuracy, the first thing is to get a decent trigger. if you want a good trigger on a defensive rifle, get one of the drop in, non adjustable "match" triggers. rra sells a complete lower parts kit, less stock and lower receiver with ergo grip and match trigger for ~$150 iirc. might not be a bad idea to buy the stripped lower from stag and the parts kit from rra but that's just my opinion. that and 99 cents will get you a cup of coffee.

Good advice!

Ive built from three CMT/Stag lowers and i have never used a file once.

 

[Matthew748]

Right now I have a DPMS upper on an Olympic Arms lower. It works just fine.

 

[dmftoy1]

Good advice!

Ive built from three CMT/Stag lowers and i have never used a file once.

Same here. I think the only thing I've ever had to "tweak" was using a drill bit to clean the parkerizing out of the boltstop pin hole on the boltstop itself. (It was too tight on the rollpin)

Maybe I've just been lucky?? (I've only built 3)

Have a good one,
Dave

 

[MatthewVanitas]

if you want a good trigger on a defensive rifle, get one of the drop in, non adjustable "match" triggers. rra sells a complete lower parts kit, less stock and lower receiver with ergo grip and match trigger for ~$150 iirc.

I'm buying a "compleat" lower, since it's practically the same price as stripped+kit, and spares me monkeying around and buying tools. Stag Arms offers an "RRA National Match 2-stage trigger" upgrade for $85 on the built lower.

Is the NM trigger upgrade worth it? My only AR experience is from the issue M16-A2, and I don't recall having any complaints about the trigger. I'm sure the NM is much better, but am I better off being blissfully ignorant as to how inferior the stock AR trigger is?

 

[boing]

...you will probably still have to trim and file the various little parts in the kit for the lower receiver...

Is this the norm? I've only built one AR (Stag lower, RRA parts kit, Colt A1 stock), and all the parts came together perfectly. The lower was made during the post-sunset rush. I would have guessed that any QC issues coming from production crunches would have passed by now, well over a year post-postban.

But, like I said, my sample size is only one(1).

At any rate, Matthew said he was looking at a complete lower, anyway. My Bushy and Colt uppers fit my Stag lower just fine, btw.

 

[boing]

(Matthew posted while I was hunting and pecking)

I have the RRA Match trigger in my Stag lower. It dropped in fine, and it's a very nice two-stage trigger, light and crisp. I've not fired it much, but I've dry fired it plenty, including a lot of taking up the first stage completely without dropping the hammer, then allowing the trigger to reset. I wanted to get used to the first stage take-up, and where the second stage began.

Eventually the trigger started getting "stuck" at the end of the first stage take-up, and wouldn't reset automatically unless I pulled through to drop the hammer. It functioned fine in normal use, but I had to do some polishing to fix the problem.

I'll probably put stock parts in the gun someday.

 

[DMK]

just a question.

Isn't the only thing that an M4 profile adds is the slot for the grenade launcher? Over any other threaded 16 incher, with a comp and bayo lug?

So with the Dissipator wouldn't you in effect cover that cut-out?

The M4 profile has a cutout under the handguard and in front of the sight block. One of the reasons this profile is so popular (aside from those who want a rifle just like GI Joe's), is these barrels are lighter than the heavy barrel (HBAR) most commonly used. Personally, I think the M4 profile is silly and would rather go for Bushmaster's Superlight, but not every AR manufacturer makes that barrel profile and nobody makes it as a Dissapator.

Isn't there a difference in pin size with some colt models? Or has that changed?

True. As far as I know, Colt is the only AR manufacturer who intentionally manufactures their parts to non-standard dimensions. That's OK though. Most fanatic Colt fans wouldn't dilute the purity of their rfles by using non Colt parts with them anyway.

 

[lycanthrope]

I've been mixing amd matching for quite a while and never had to file......

RRA or Stag lowers are my favorites for build because they tend to be tighter with less slop between the upper and lower (I hate rattles). I've used these lowers on high end uppers like my Clark Gator and White Oak Armament.

 

[dch1978]

DMK

Thanks for the point by point reply. That is one of the things that I love about the more senior members here, clear, concise, and to the point.

I try to make good points when I post and do not wish to look like an idiot, even if I may be, but better to hold the tongue and let everyone think it, rather than open my mouth and confirm it.

Have a friend with a true Colt 16 inch, doesn't really say much about it when around the five or six Bushies that were less money and brand new. He paid like $1000, 5 or 6 years ago for a used Colt.

Thanks again,

DCH