Interchangeable Upper Receiver Reliability/Headspace Questions

[Gutshot John]

I tried searching this and while I got some information nothing seemed to directly address the question so my search-fu is lacking.

Given the modularity of the AR I'm curious as to who uses interchangeable uppers say one dedicated to CQB and the other to DMR.

What are the technical implications for headspacing and the selection of a recoil buffer?

I would imagine it's preferable to have two separate BCGs each properly headspaced to the dedicated upper.

Are there reliability issues using one lower for both an SBR with carbine system and another with a longer gas system and heavier barrel of an SPR? or is it no big deal?

What are the implications if either upper is used with a suppressor?

Thanks in advance.

[87GN]

Are you referring to items like the MGW QCB?

Or standard receivers?

Headspace relates to how the barrel extension is attached, and the bolt that interfaces with said extension - in a standard receiver, this is set and doesn't have anything to do with swapping the upper assembly as a whole.

Recoil buffers should be selected based on caliber, barrel length and gas system, ammunition to be fired, etc.

The military has encountered problems with 20" rifle length 5.56 uppers on lowers with collapsible stocks. These problems normally manifest themselves on full auto.

But if you're referring to the MGW QCB, most of what I've said is irrelevant to you.

[M4Fundi]

Gutshot check out my poo-storm on this thread I think it will answer your question;)

https://www.m4carbine.net/showthread.php?t=39902

[Gutshot John]

Are you referring to items like the MGW QCB?

Or standard receivers?

Standard receivers.


Headspace relates to how the barrel extension is attached, and the bolt that interfaces with said extension - in a standard receiver, this is set and doesn't have anything to do with swapping the upper assembly as a whole.

It was more about the BCG, I presume that two different uppers would require two different BCGs to insure a proper headspace between the two uppers. The concern was more whether one had to change the buffer as well.


Recoil buffers should be selected based on caliber, barrel length and gas system, ammunition to be fired, etc.

The military has encountered problems with 20" rifle length 5.56 uppers on lowers with collapsible stocks. These problems normally manifest themselves on full auto.

But if you're referring to the MGW QCB, most of what I've said is irrelevant to you.

I'm pondering an 18" barrel for the SPR setup but I wasn't sure whether a buffer that functioned with that setup would be adequate for an 11.5" if one went the CQB route for the other upper.

[87GN]

Standard receivers.



It was more about the BCG, I presume that two different uppers would require two different BCGs to insure a proper headspace between the two uppers. The concern was more whether one had to change the buffer as well.



I'm pondering an 18" barrel for the SPR setup but I wasn't sure whether a buffer that functioned with that setup would be adequate for an 11.5" if one went the CQB route for the other upper.

The short answer is that a barrel should be allowed to wear in with one particular bolt. This would be a very very good idea if high round counts are encountered with one upper but not the other. You could have two bolts and one carrier, if you want, but it wouldn't be as fast to swap between the two.

Buffers - well, I've used my ST-T2 buffer successfully with 10.5", 14.5", 16" and 18" uppers of the carbine and midlength varieties. However, there might be better choices for 16" or 18" midlengths.

[Blankwaffe]

As said it would be advisable to have a bolt or a complete BCG for each dedicated upper.For a cost of $70.00-$150.00 that is a minimal amount in the scheme of things to have in order.
As far as info on using a single collapsible stock carbine lower for multiple uppers of differing barrel lengths and gas systems,do a search for "C7A2".There have been several of the "Subject Matter Experts" and "Industry Professionals" comment on this type of configuration.
I personally prefer the collapsible stock carbine lowers for basically all my uppers as its more functional for my needs as in offering flexibility.I run 16" to 20" barreled uppers,with the varying gas systems of each different assembly without issue.

[Gutshot John]

As said it would be advisable to have a bolt or a complete BCG for each dedicated upper.For a cost of $70.00-$150.00 that is a minimal amount in the scheme of things to have in order.
As far as info on using a single collapsible stock carbine lower for multiple uppers of differing barrel lengths and gas systems,do a search for "C7A2".There have been several of the "Subject Matter Experts" and "Industry Professionals" comment on this type of configuration.
I personally prefer the collapsible stock carbine lowers for basically all my uppers as its more functional for my needs as in offering flexibility.I run 16" to 20" barreled uppers,with the varying gas systems of each different assembly without issue.

Awesome thanks I knew I just needed better search terms.