View Full Version : Question about multiple uppers for the same lower
Doc Safari
07-11-17, 11:33
I'm thinking of having one rifle with multiple complete uppers so I can try out different optics, iron sights, barrel profiles, etc., because it's less expensive than having a bunch of complete rifles.
Due to the differences in gas port sizes between manufacturers, I think I will stick to all BCM uppers for my BCM lower to try to avoid having to swap out buffers too.
Are there any technical issues I should anticipate?
Should I expect all uppers with the same rough specs as the original to be 100% reliable on the shared lower?
militarymoron
07-11-17, 11:47
I have more uppers than lowers, and typically use the same couple of lowers no matter which upper I'm using (I lean towards my fully ambi lowers). I have had no issues swapping uppers and lowers, with most of my lowers having the Vltor A5 tube/buffer kit on them.
Doc Safari
07-11-17, 13:06
Thanks for the reply especially since you have a lot of experience with this.
I'll tell you the source of my concern.
Back in the ban days I bought two identical Colt ban style carbines on the same day. At that time I did care about consmetics. One lower had a couple of blems (typcial Colt), and the other upper had a couple of blems.
I thought I'd be clever and put the nicest two halves together and let the upper and lower with blems be my "shooter".
I immediately started having reliability problems and switched back. The reliability issues went away.
My dad looked at me and said, "Not as interchangeable as you thought, huh?"
I often swap combinations of uppers and lowers, and it's generally not a problem. These are carbine, A5, and rifle-configured lowers with all sorts of upper configs. The only issue I've had is a 24" barreled rifle-length-gas upper on a carbine-length RE lower. Some loads are really finicky through that setup.
militarymoron
07-11-17, 13:47
I thought I'd be clever and put the nicest two halves together and let the upper and lower with blems be my "shooter".
I immediately started having reliability problems and switched back. The reliability issues went away.
LOL, funny how that happens. Did you swap any other components other than the uppers? If I had wanted to persist, I'd have swapped bolt carrier groups, buffer springs and buffers to see if that would work. I think I recall that my enhanced LMT bolt carrier would work well in some rifles, then not as well in others. The bottom of the carrier would drag on some magazine feed lips. Turns out that the bottom of that particular carrier was ever so slightly wider than the others, and certain upper/lower/mag combinations would change the height of the carrier to the top of the magazine feed lips ever so slightly. Even a couple of thousandths of interference can cause some drag. So, sometimes it's just tolerance stackup and the the less common chance that they all stack up a certain way.
Doc Safari
07-11-17, 13:49
LOL, funny how that happens. Did you swap any other components other than the uppers? If I had wanted to persist, I'd have swapped bolt carrier groups, buffer springs and buffers to see if that would work.
Back then I didn't know diddly squat about tolerance stacking or swapping buffers or anything. I thought there was probably just some quirk or that Colt wasn't the all-knowing source of quality I thought it was, and blah blah.
I'm guessing what happened to me is relatively rare, if I'm not mistaken.
I tune all my uppers to run standard buffer systems... with the exception of BCM 14.5 mid length. Those run h buffers best.
Doc Safari
07-11-17, 14:39
I tune all my uppers to run standard buffer systems... with the exception of BCM 14.5 mid length. Those run h buffers best.
My 14.5 midlength worked best with a carbine buffer. I even emailed BCM and that's what they told me to use. Of course I shoot mostly .223 and not full horse 5.56 so I suspect that's the difference between my experience and yours.
Doc Safari
07-11-17, 14:45
To further muddy the waters, I'm buying a NIB complete BCM upper from a friend with one of these installed:
https://www.bravocompanyusa.com/BCMGUNFIGHTER-Compensator-MOD-0-5-56-p/bcm-gfc-mod-0-556.htm
This Compensator was not designed as a gamers comp. It was designed for tactical applications to reduce muzzle rise, flash signature, noise, and lateral pressure.
The BCMGUNFIGHTER Compensator Mod 0-5.56
Exterior dimensions consistent with Mil-specs for mounting flash hider mounted suppressors.
Tuned slots and interior cone offer maximum in recoil mitigations, compensation of muzzle-rise, and flash reduction.
Designed to have much less side blast and noise associated with typical compensators. Makes it perfect for working in teams for CQB. It is a compensator that is built for the 21st Century Gunfighter!
Nitride treated for maximum corrosion resistance
Includes a Crush Washer
Should be installed by a qualified armorer.
Threaded 1/2 x 28 for 5.56mm AR15 platforms.
Please note:
The combustion gasses exiting the muzzle of a rifle are extremely hot, traveling at extreme velocity, and contains not only burning gasses but solid particles of unburnt powder as well as metals from the projectile. Due to the inherent nature of steel, it is nearly impossible to survive unaffected in this environment when directly exposed. The high temperatures begin to make the steel more malleable, which allows solid particles and pressure waves to deform and erode surfaces upon exit.
Due to the new and unique design of the BCM compensators, the user may notice small pits on the interior surfaces of the compensator. This is normal. Most designs do not impede the exit flow very much, while the BCM approach is exactly opposite. These surfaces are specifically designed to operate within the environment found at the muzzle, and are intended to allow significant amounts of deterioration before any loss of performance is seen/felt. These compensators have been tested through many thousands of rounds, and while the wear surfaces may give pause to some users, trust when we say that it's normal.
Made by Bravo Company MFG, Inc. (BCM®)
I'm wondering if that will have any effect on the performance since their write up talks about the difference in the way it controls flash and muzzle movement compared to the normal A2 flash hider.
Bottom line: change any one little thing and you have to test it to make sure. I may need to drag out a different weight buffer.
To further muddy the waters, I'm buying a NIB complete BCM upper from a friend with one of these installed:
https://www.bravocompanyusa.com/BCMGUNFIGHTER-Compensator-MOD-0-5-56-p/bcm-gfc-mod-0-556.htm
I'm wondering if that will have any effect on the performance since their write up talks about the difference in the way it controls flash and muzzle movement compared to the normal A2 flash hider.
Bottom line: change any one little thing and you have to test it to make sure. I may need to drag out a different weight buffer.
Everything a MD does it does after the projectile leaves the barrel.
Doc Safari
07-11-17, 15:55
Everything a MD does it does after the projectile leaves the barrel.
True that, but from their write-up I was scratching my head if the expelled gases are being held in the FH and acting upon the recoil impulse.
Maybe it's my reading comprehension. I looked at it as having maybe some characteristics of, for example, the can-shaped "booster" FH on a Krinkov or something.
I may have been reading it wrong. They may not be implying anything of the kind.
Build a lower with a Vltor A5H2 set up, and you can pretty much run the field. At least in my limited AR experience.
Build a lower with a Vltor A5H2 set up, and you can pretty much run the field. At least in my limited AR experience.
Word up.
I tried the one lower and multiple uppers thing years ago. All my uppers grew lowers and then produced more stripped lowers which then matured into rifles.
**** lead farming, I'm a gun farmer.
I tried the one lower and multiple uppers thing years ago. All my uppers grew lowers and then produced more stripped lowers which then matured into rifles.
**** lead farming, I'm a gun farmer.
Exactly. That crap never works for long.
Build a lower with a Vltor A5H2 set up, and you can pretty much run the field.
My 20" rifle has an A5H2 R.E./buffer and a G2S trigger. I have really been tempted to build a light weight 16" middy upper so I can switch configurations and still enjoy the nice lower.
Andy
I had problems with VLTOR MUR's not taking non-USGI magazines (which still required gorilla strength to seat) on a closed bolt when mated to any of my Mega lowers. No such problem existed using that upper on a shitty R-Guns lower, or my buddy's BCM or DD lower. The problem resolved when I switched over to standard Mega forged uppers.
All of my lowers are built with A5 systems and usually run A5H3's, but I shoot my short uppers almost exclusively.
Not really sure what the upper would have to do with it, unless it was grossly out of spec. I have several MUR uppers and have built dozens of uppers with MUR's with no issues at all. My oldest MUR is probably 7 years old.
I had problems with VLTOR MUR's not taking non-USGI magazines (which still required gorilla strength to seat) on a closed bolt when mated to any of my Mega lowers. No such problem existed using that upper on a shitty R-Guns lower, or my buddy's BCM or DD lower. The problem resolved when I switched over to standard Mega forged uppers.
All of my lowers are built with A5 systems and usually run A5H3's, but I shoot my short uppers almost exclusively.
Doc Safari
07-12-17, 09:00
My issue is I just flat out can't decide on ONE optic or sight system. I like using irons, Aimpoints, ACOG's, and I've even considered getting a Meprolight just because.
I decided after a fashion that instead of putting wear and tear on the rails and optic mounts by constantly mounting and dismounting things that I would keep each upper dedicated to its own sight system and simply swap out uppers depending on which one I wanted to take to the range.
I can certainly appreciate that uppers can grow into complete rifles, but when you intend to spend a good chunk of money on optics you get to where you want to save funds somewhere.
@Iraqgunz I have no earthly clue why it was that way, but it was a problem that only existed between my MUR's and three Mega Gator forged lowers. The uppers all had Noveske barrels and LMT Enhanced Carriers with Noveske bolts. Lowers were built with BAD-ASS, BAD-EPS, Geissele triggers, White Oak bolt catch and mag release from ADCO. I swapped the BCG's out for standard LMT and BCM, tried QMS and regular mil-spec triggers, mil-spec safety, and mil-spec takedown pins. Nothing helped. As far as I could tell, the tolerance stacking was just tight enough between upper and lower to cause that specific problem.
Like I said, they all work perfectly with standard Mega Forged Uppers now. It still bugs me, and I'd really like to know if you have some insight. I still have the two MUR's in my parts box if there are any measurements that might shed some light.
Doc, just continue swapping optics and sights around until you shoot them enough to figure out what configurations you like. That's what I'm currently doing with Aimpoint Micros, an NF 2-10x, Leupold 10x and Primary Arms 1-6 along with swapping around muzzle devices and a suppressor. The uppers I'm playing are 10.5", 11.5", 14.5"' and 20" Hbar profile. I had a 16" upper but it was traded away.
I'll share some observations, if I may. First, I prefer not having an FSB when using an optic. It's not so bad when using an Aimpoint because I can take advantage of the Aimpoint's unlimited eyebox to look over the top of the front sight, whether I'm using 1/3 or absolute co-witness. But the smaller, limited eyebox of the magnified optic means the front sight always blocks the bottom half of the reticle rendering the hold over marks useless. Therefore, any upper that gets dedicated to magnified optic use will not have a fixed front sight. Even when using an Aimpoint, my preference is to not have a fixed front sight.
Second, heavy optics will make themselves felt. Changing from a 4 ounce Aimpoint Micro to a 26 oz (with mount) LPV will turn a fast handling carbine into a slug. There's no denying that an LPV is more versatile than a non magnified Micro, but do yourself a favor and buy the lightest scope and mount you can afford.
Third, when using a variety of uppers on a single lower, or when swapping uppers between different lowers, it's important to have everything sorted out. Use a conservative buffer weight (a buffer that falls between an H and rifle buffer in weight) and a quality recoil spring. Either use barrels with the correct port diameters or use an adjustable gas block. Don't use generic extractor springs. I've installed Colt extractor springs in ALL my uppers and have seen a marked improvement in reliability.
Fourth, I find there is a difference in how a 14.5" upper and a 16" upper handles and shoots. It's subtle and I can't quite put my finger on it, but it's there. Enough that I much prefer the 14.5" upper.
The only QD mount I'm using is the Scalar Works mount on a T1. I don't trust the durability of the flip lever QD mounts. For the other mounts, I keep a box end, Torx bits and Allen keys in the range bag
SomeOtherGuy
07-12-17, 13:27
Due to the differences in gas port sizes between manufacturers, I think I will stick to all BCM uppers for my BCM lower to try to avoid having to swap out buffers too.
Are there any technical issues I should anticipate?
Should I expect all uppers with the same rough specs as the original to be 100% reliable on the shared lower?
1) All BCM is a good bet, not so much because of gas port sizing but because you are more likely to find all the upper receivers fit the lower. I have had some surprising incompatibilities between top brands, in particular Daniel Defense uppers vs. a Noveske lower. Each of these fit nearly all my other counterparts without issue, so I don't think either one was far out of spec, just at opposite ends of the spec. Even with quality brands nothing is guaranteed, you must test.
2) Other than the chance of physical incompatibility, no.
3) No guarantees. However, using an A5 recoil setup with the A5H2 buffer may minimize the chance of issues.
Doc Safari
07-12-17, 13:52
Here was my checklist for attempting to maximize upper to lower compatibility. Keep in mind all my bullet points are true for both the upper that came with the rifle and the new upper I purchased from my friend (I helped that friend order the upper in the first place so I know exactly what the specs were). The lower came as a complete BCM rifle with all BCM parts.
Criteria apply to both uppers:
1. BCM barrel (to try to get the same gas port size since BCM probably buys their barrels in bulk)
2. BCM upper receiver (also an item BCM probably buys in bulk, and I know Paul's quality control is excellent so it's probably GTG)
3. Carbine length
4. Bravo Company BCG
5. Government profile barrel (although that probably hasn't got anything to do with reliability other than it might affect how the gun flexes during fire).
The only major difference between the upper that came with the rifle and the aftermarket upper is the compensator. IIRC Bravo Company did not offer an upper with the standard A2 compensator at the time this was purchased.
This weekend I plan to test the upper with five or six magazines that have been thoroughly vetted with the original upper. I also plan to use ammo that has been 100% reliable with the rifle as originally purchased.
If I experience any stoppages, then I will experiment with buffers, springs, and maybe BCG's.
I honestly expect everything to work right out of the gate since I did my homework and tried to make everything as compatible as possible at the time of purchase.
christopher.dow
07-12-17, 17:53
I tried the one lower and multiple uppers thing years ago. All my uppers grew lowers and then produced more stripped lowers which then matured into rifles.
**** lead farming, I'm a gun farmer.
The same thing happened to me!
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I tried the one lower and multiple uppers thing years ago. All my uppers grew lowers and then produced more stripped lowers which then matured into rifles.
**** lead farming, I'm a gun farmer.
Same here. Its a cool idea though.
Not to hijack, but I'm working on a similar project just more limited in scope. Looking to build a lower that will run a criterion 18" barreled SPR upper with magnified optics, occasionally suppressed along with a Sionics 11.5 RGP barrel with either an MRO or USO 1-4 with suppressor 50-75%. Lower is currently running an A5H2 on my recce but I'm looking at the new UBR to help balance the weight of the longer barrel on the 18 and suppressor on the 11.5. Currently set-up with SSA-E, but I might swap out for a Tricon - I love the trigger bow! Runs like a standard SSA but I can reduce the 'felt' tigger pull by moving to the flat section for more precision oriented shooting if I want.
Apologies for the long post, but I've been collecting parts and am almost ready, just looking to see if this is feasible or if I should keep them as separate builds all together as the differences in the function of the rifle and RGP systems wouldn't play that well together on a single lower. Thanks
Lower will be register as SBR and I have one lower already SBR'd with the A5H2 for function testing.
I never really tried the multiple uppers, but I've switched and swopped out lowers and uppers 25 times with no issues. I've got a bunch of MUR uppers and some of their monolithic stuff, no issues. Stick with the usual suspects Colt BCM LMT etc and you will be fine. Start deviating from the first tier stuff and issues MAY ensue.
Very strange you got burnt on two Colts, one must have been BLEM PLUS P
PB
Doc Safari
07-13-17, 09:18
Very strange you got burnt on two Colts, one must have been BLEM PLUS P
PB
My experience with Colt in the 1990's soured me on them for years. It was a running joke that I couldn't buy a Colt that didn't have a rusted spring on it somewhere. The final straw was a ban era full-length rifle that had an obviously deeply-pitted handguard spring that had been re-blued. I decided Colt was using the civilian market to clean the closets of parts that wouldn't pass milspec. When I returned to AR's a few years ago I still gravitated toward BCM ahead of Colt because of Colt's QC issues. I think Colt is much better now. I have a Colt that shoots just fine and doesn't have any suspect parts that I can tell.
I tune my uppers by using a gas port insert or enlarging the gas port so they all use the same buffer. This allows me to be able to swap lowers and uppers but it would allow you to use one lower for multiple uppers.
I tune all my uppers to run standard buffer systems... with the exception of BCM 14.5 mid length. Those run h buffers best.
Same for me. I've tried every weight too. Currently running White spring and H buffer. Smoothest naturally aspirated AR I have LOL.
Build a lower with a Vltor A5H2 set up, and you can pretty much run the field. At least in my limited AR experience.
That is some great advice.
Here was my checklist for attempting to maximize upper to lower compatibility. Keep in mind all my bullet points are true for both the upper that came with the rifle and the new upper I purchased from my friend (I helped that friend order the upper in the first place so I know exactly what the specs were). The lower came as a complete BCM rifle with all BCM parts.
Criteria apply to both uppers:
1. BCM barrel (to try to get the same gas port size since BCM probably buys their barrels in bulk)
2. BCM upper receiver (also an item BCM probably buys in bulk, and I know Paul's quality control is excellent so it's probably GTG)
3. Carbine length
4. Bravo Company BCG
5. Government profile barrel (although that probably hasn't got anything to do with reliability other than it might affect how the gun flexes during fire).
The only major difference between the upper that came with the rifle and the aftermarket upper is the compensator. IIRC Bravo Company did not offer an upper with the standard A2 compensator at the time this was purchased.
This weekend I plan to test the upper with five or six magazines that have been thoroughly vetted with the original upper. I also plan to use ammo that has been 100% reliable with the rifle as originally purchased.
If I experience any stoppages, then I will experiment with buffers, springs, and maybe BCG's.
I honestly expect everything to work right out of the gate since I did my homework and tried to make everything as compatible as possible at the time of purchase.
Here are my comments about your criteria.
Criteria apply to both uppers:
1. BCM barrel (to try to get the same gas port size since BCM probably buys their barrels in bulk)
There are differences between a 16" and 14.5" BCM. I run mid-length everything, but the 14.5" BCM BFH LW is gassed differently than a 16" BCM Stainless. The length of the barrel also matters. However, both BCM barrels are gassed less than my other barrels. I prefer the way BCM does it.
2. BCM upper receiver (also an item BCM probably buys in bulk, and I know Paul's quality control is excellent so it's probably GTG)
The BCM uppers are very good with really tight tolerances, especially around the barrel extension. But I have also had very good luck with Vltor, Mega, and Noveske. Not happy with Aero.
3. Carbine length
If you are talking carbine length gas systems, I would urge you to rethink that. I personally think that mid-length and longer gas systems tend to be more forgiving.
4. Bravo Company BCG
You cannot go wrong with anything BCM, in my humble experience.
5. Government profile barrel (although that probably hasn't got anything to do with reliability other than it might affect how the gun flexes during fire).
Disagree. Government profile barrels are only adequate, they do the job. But there are many better profiles to choose from these days. I only own one Gov profile barrel (out of 10) and it has a mid-length gas system. The Gov. profile puts the skinny part of the barrel in the rear and the thicker part in the front. Not good for the balance of the rifle. If you like BCM look at their Standard ELW or BFH ELW.
The opinions expressed are my humble but honest opinions based on my limited experience.
Your best bet, as Iraqguns says, is to build all your lowers with the A5 buffer systems running A5H2 buffers. BUT, its expensive to go that way and I've had very good results tuning with carbine buffer tubes (receiver extensions).
I personally have had very good luck with interchangeability if my lowers use carbine buffer tubes running Spinco Blue springs and H2 buffers. The BCM uppers seem to like an H buffer best but will run full power ammo with an H2. The BCMs are a little iffy with this setup (H2) if you are using weak ammo like PMC Bronze or Steel Case Wolf. If you decide to put weak ammo in your mix then I suggest you go carbine buffer tube, H buffer and Sprinco Blue. This works for mid-length gas 14.5" and 16" barrels of various types/brands YMMV.
I dare say that almost any good mid-length 14.5-16" barrel quality upper will run with a carbine buffer tube, H buffer, Sprinco Blue (or fresh standard carbine spring), with full power 5.56 ammo (say M193, M855). And it might run a bit smoother if you change out the H for an H2 buffer.
Doc Safari
07-13-17, 17:16
Your best bet, as Iraqguns says, is to build all your lowers with the A5 buffer systems running A5H2 buffers.
I personally have had very good luck with interchangeability if my lowers use carbine buffer tubes running Spinco Blue springs and H2 buffers. The BCM uppers seem to like an H buffer best but will run full power ammo with an H2. The BCMs are a little iffy with this setup (H2) if you are using weak ammo like PMC Bronze or Steel Case Wolf. If you decide to put weak ammo in your mix then I suggest you go carbine buffer tube, H buffer and Sprinco Blue. This works for mid-length gas 14.5" and 16" barrels of various types/brands YMMV.
I dare say that almost any good mid-length 14.5-16" barrel quality upper will run with a carbine buffer tube, H buffer, Sprinco Blue (or fresh standard carbine spring), with full power 5.56 ammo (say M193, M855). And it might run a bit smoother if you change out the H for an H2 buffer.
I'm hoping I won't have to mess with any of this. My goal was "push two pins and change uppers." That's the reason for going all BCM, same barrel length and profile, BCG, and upper receivers.
I don't plan on having a closet full of uppers (famous last words), just one for irons and one for optics (all of which I plan to make quick detachable).
The original carbine was all stock with an H buffer and only had malfunctions out of 6,000 rounds with a bad magazine.
I may post something differently on Monday, but having stuck to the same specs between uppers, I fully expect the gun to run 100% with the second upper.
(I should point out that I mostly shoot .223). I did have issues a few years ago when switching to a pinned-and-welded 14.5" barreled upper on the same lower that we are talking about here. I emailed BCM and they said switch to a standard carbine buffer for .223, which I did and they were right that all reliability problems went away. (IIRC it was a mid-length but I may have forgotten).
Bottom line: if I went mid-length or 14.5" I'd have to do it with all uppers for that lower so I (hopefully) don't have to experiment with buffers and springs.
I'm hoping I won't have to mess with any of this. My goal was "push two pins and change uppers." That's the reason for going all BCM, same barrel length and profile, BCG, and upper receivers.
I don't plan on having a closet full of uppers (famous last words), just one for irons and one for optics (all of which I plan to make quick detachable).
The original carbine was all stock with an H buffer and only had malfunctions out of 6,000 rounds with a bad magazine.
I may post something differently on Monday, but having stuck to the same specs between uppers, I fully expect the gun to run 100% with the second upper.
(I should point out that I mostly shoot .223). I did have issues a few years ago when switching to a pinned-and-welded 14.5" barreled upper on the same lower that we are talking about here. I emailed BCM and they said switch to a standard carbine buffer for .223, which I did and they were right that all reliability problems went away. (IIRC it was a mid-length but I may have forgotten).
Bottom line: if I went mid-length or 14.5" I'd have to do it with all uppers for that lower so I (hopefully) don't have to experiment with buffers and springs.
Maybe I made this too complicated. You can run a 14.5" carbine or mid-length from the the same lower if they are all BCM. IF you have approximately 6000 rounds on your lower, put a new BCM buffer spring in with your existing H buffer and you'll be able to run a 14.5" carbine or mid-length. You'll also be able to run a 16" mid-length and probably a carbine.
The reason I said replace the buffer spring is that the standard ones wear out way before 6000 rounds.
Doc Safari
07-14-17, 08:52
The reason I said replace the buffer spring is that the standard ones wear out way before 6000 rounds.
Mine still runs fine. I keep spare parts on hand. I did replace the cam pin at about 5,000 rounds though "just because" it was looking a little beat.
Mine still runs fine. I keep spare parts on hand. I did replace the cam pin at about 5,000 rounds though "just because" it was looking a little beat.
I don't want to tell you what to do, but regular PM for a gun with 6000 rounds would probably include the buffer spring, extractor spring, cam pin (you already did that), and gas rings. And a good inspection. Cheap insurance. JMHO.
Doc Safari
07-14-17, 13:09
I do look it over regularly. I perform the upside-down-BCG gas ring test nearly every time I take it out. The extractor spring was next on my list to replace, but I made three range trips last week and all worked well so I'll probably let it go until the end of summer.
Doc Safari
07-17-17, 10:04
Just in case anyone's curious, I took the new upper mated to my main shooter's lower out this weekend. It was threatening to rain, so I didn't shoot more than 100 rounds before packing my stuff up. I was careful to lube the BCG of the new upper since it was right out of the box.
Just as expected, it fit perfectly and worked perfectly without a hiccup.
Thanks for the reply especially since you have a lot of experience with this.
I'll tell you the source of my concern.
Back in the ban days I bought two identical Colt ban style carbines on the same day. At that time I did care about consmetics. One lower had a couple of blems (typcial Colt), and the other upper had a couple of blems.
I thought I'd be clever and put the nicest two halves together and let the upper and lower with blems be my "shooter".
I immediately started having reliability problems and switched back. The reliability issues went away.
My dad looked at me and said, "Not as interchangeable as you thought, huh?"
If had similar issues with different manufacturers and receiver configurations. Not malfunctions, but with fitment. Some lowers will not match up with certain uppers.
I assume if you stick with the same brand e.g. BCM for the receivers though you should be ok.
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