Question about multiple uppers for the same lower

[ScottsBad]

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.

[ScottsBad]

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]

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.

[ScottsBad]

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]

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.

[ScottsBad]

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]

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]

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.

[JusticeM4]

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.