Question about multiple uppers for the same lower

[Doc Safari]

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]

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]

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?"

[tehpwnag3]

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]

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]

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.

[markm]

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]

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]

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/BCMG...-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.

[GH41]

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/BCMG...-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.