Rifle upper on carbine lenght lower?

[1986s4]

I've looked around for information and read the Brownells catalogue but I am still unsure of this question. Maybe it isn't done very much at all? I have a LMT Defender lower with a standard Colt carbine [16"] upper, it works great. But I would like to add a rifle length upper to the LMT lower. Is this done and what if any modifications/changes to the buffer spring and buffer do I need to do to make this combination work? Or will it?

I am rather new to AR 15 modifications.

Thanks,
1986s4

[Diamondback]

Isn't one of the common field mods for soldiers needing ultra-short LOP (stubdudes like me, or super-thick body armor) slapping an M4 buffer/telestock assembly onto the M16?

[RobertTheTexan]

I've looked around for information and read the Brownells catalogue but I am still unsure of this question. Maybe it isn't done very much at all? I have a LMT Defender lower with a standard Colt carbine [16"] upper, it works great. But I would like to add a rifle length upper to the LMT lower. Is this done and what if any modifications/changes to the buffer spring and buffer do I need to do to make this combination work? Or will it?

I am rather new to AR 15 modifications.

Thanks,
1986s4

By rifle length upper, do you mean a longer barrel? If so, you can add whatever upper you want as long as it’s a mil spec AR-15 upper receiver. What length barrel? I would also recommend changing out the buffer system on the lower to an A5 system (research Vltor A5 buffer systems) that way you can keep your adjustable stock. Also you could go with a rifle length buffer system and use a fixed stock like an M16A1/A2 or a Magpul PRS or any other AR-15 fixed stock. However if you want to keep it light and more maneuverable, I would go the A5 route. There are a few ways to get there, so if your interested in knowing more let me know.
ETA: I should reiterate my A5 buffer system recommendation is just that a recommendation, not mandatory.


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[ChoppinFatTony]

I've put together some in country builds at the Company level that took standard A4 uppers on M4/M4A1 lowers and had no issues in reliability and function with either M855 or Mk262.

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[ColtSeavers]

Recoil system is completely independant of gas system length.

[1986s4]

I've put together some in country builds at the Company level that took standard A4 uppers on M4/M4A1 lowers and had no issues in reliability and function with either M855 or Mk262.

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OK, that's interesting to me. Am I correct in assuming lower gas pressures with a rifle [20"] length barrel and gas system? If so then lower pressures don't have a adverse effect on function?

[MistWolf]

A rifle upper will work on a carbine lower. The rifle gas port is located where gas pressure is lower, but has a larger gas port to make up for it. The rifle buffer is heavier than an H or H2 buffer, so you may want to play with buffer weights. I didn't have to with mine. The rifle upper is very forgiving.

[1986s4]

OK, thanks all!
I might get that rifle length upper.
Thinking Colt, FN or BCM.

[GH41]

OK, thanks all!
I might get that rifle length upper.
Thinking Colt, FN or BCM.

You know that an extra upper laying around at some point will require a lower for it?? Why not just buy a fixed stock 20" rifle now. I would at least do the math.

[cd228]

I've put together some in country builds at the Company level that took standard A4 uppers on M4/M4A1 lowers and had no issues in reliability and function with either M855 or Mk262.

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Hmm, I remember your in theatre M4A1 conversion, I don't remember seeing your homemade M16A5s? Where you using the USGI H buffers?