20" upper on a carbine lower

[pistolman1974]

I'm thinking of buying a BCM 20" upper to use on a carbine configured lower. The buffer is a standard carbine buffer. Do i need to switch to a different buffer and spring to have the gun run reliably?

[Frens]

I have a carbine and a rifle...once I tried the 20'' upper on the carbine lower and worked 100%

[tiger seven]

I'm thinking of buying a BCM 20" upper to use on a carbine configured lower. The buffer is a standard carbine buffer. Do i need to switch to a different buffer and spring to have the gun run reliably?

Not in my experience. I've seen several 20" rifle uppers run just fine on lowers using standard CAR buffers.

Derek

[beavo451]

I'm thinking of buying a BCM 20" upper to use on a carbine configured lower. The buffer is a standard carbine buffer. Do i need to switch to a different buffer and spring to have the gun run reliably?

Put it together and shoot it first.

[greene]

You'll be good.

[ForTehNguyen]

i dont see why it wouldnt work. The gun is going to feel front heavy however if its an M4 style stock.

[NickB]

The ideal carbine buffer when using a 20" barrel and rifle-length gas system is an H2, specifically one using tungsten carbide weights rather than pure tungsten.

[Pilgrim]

I have Bushy V-Match 20" upper in pre-ban configuration, that has been used on 3 different 'carbine' lowers, both commercial and mil-spec, never had a problem.

[Failure2Stop]

In my experience it works, but feels really weird.
The only adjustable stock that feels acceptable to me with a fence-post upper is the UBR, and even that still feels unbalanced once anything else is attached to the gun forward of the receiver.

[blade_68]

I use a old non forward assist upper on m-4 lower no problem. but as stated feels odd.

[Detmongo]

i ran a 20" gun with a calaps. stocked lower after 9/11 to shorten the gun up a bit. i ran fine, but had to switch to a company m5p about six months later.

[sundance435]

I currently have a BCM 20" upper on a Colt 6920 lower with an H buffer. The 20" upper on an M4 lower is not a new or novel idea. The Canadian military issues it as the C7. Maybe I'm the exception, but I don't think it balances very awkwardly. It is a little front heavier, but I like that about it.

http://www.coltcanada.com/Images/c7-annot.gif

[NickB]

I currently have a BCM 20" upper on a Colt 6920 lower with an H buffer. The 20" upper on an M4 lower is not a new or novel idea. The Canadian military issues it as the C7. Maybe I'm the exception, but I don't think it balances very awkwardly. It is a little front heavier, but I like that about it.

http://www.coltcanada.com/Images/c7-annot.gif

FWIW, the Colt Canada C7A2 uses the H2 buffer I mentioned.

[sundance435]

FWIW, the Colt Canada C7A2 uses the H2 buffer I mentioned.

Correct. I should have mentioned that. From what I've gathered the Canadians tested different buffer weights and found that the heavier the buffer, the better this combination performed.

That being said, I believe an H buffer would still work just fine.

[ksa464]

A simular thread was just posted a few weeks ago. I will post again on this topic.

As a couple have said already said, the C7 (which this essentially is) uses the H2. I have an "M4" LMT Defender lower on a Colt 20" rifle barrel and BCM upper and auto BCG. It runs great and is "correct". Also understand that the rifle buffer (M16) is heavier than an H3. Take a look at the sticky that maybe here that shows the various buffer weights.

[sundowner41]

I have a 18" varmint upper receiver and use it on a Bushmaster lower with an H buffer. No problems whatsoever.

[pistolman1974]

Today i received my 20" BCM upper bolt and charge handle and installed it. The rifle is a little barrel heavy. This upper will eventualy be in stalled on a spikes tactcal lower with an acs stock or a solid stock, not sure yet. I'm looking forward to sighting it in..