Mid-length upper on a rifle lower?

[sonny22]

I was thinking about putting a BCM 14.5 mid-lenght on a colt MT6700 lower, just wanted to know if I needed to
put a H or H2 buffer and a carbine buffer spring in it and would they work in a receiver extension for a rifle.

[Toyoland66]

No

The buffer (type, not weight) and spring are dependent on the receiver extension, not the upper.

[sonny22]

ok, thanks for the info. I learned something today.

[markm]

Definitely not. Some tards on other forums have run this, but it'll beat the crap out of your weapon... and the back of the gas key could slam into the RE and damage it.

The 14.5 middy has not worked so hot for me with a rifle buffer system. Short strokes pretty bad.

[sonny22]

ok, I appreciate the advice.

I was thinking about just changing the receiver extension on my rifle with a carbine receiver extension,
is there any reason this wouldn't work?

[Cowtown556]

I cut a 2 5/8" plug to make the rifle tube the same lenght as the carbine tube and run the rifle stock with a carbine buffer and spring.

[Failure2Stop]

The 14.5 middy has not worked so hot for me with a rifle buffer system. Short strokes pretty bad.

Interesting.
Care to go into detail, or is that about it?

[markm]

Yeah... it's the T&E upper from Bravo. It's just weird. When we first started running it, there were no problems with the rifle buffer system.

We even ran it full auto as seen on the video on Bravo's home page. But after a thousand rounds, the gun just doesn't cycle the rifle buffer system very well at all. It'll run... but it often won't lock back, and you get that slow bolt group cha-chunk from the cycling of the action really slow.

Runs great on an H buffer carbine though. I posted that video of the A5 stock here in full auto. The A5 effectively being a rifle buffer system... the cyclic rate was crazy slow.

[DC3]

Im not a big fan of the 9 inch gas tube on a 14.5

[SA80Dan]

I was thinking about putting a BCM 14.5 mid-lenght on a colt MT6700 lower, just wanted to know if I needed to
put a H or H2 buffer and a carbine buffer spring in it and would they work in a receiver extension for a rifle.

As the others have explained you can't put a carbine size short buffer in a rifle length stock tube, otherwise you will rapidly find yourself beating hell out of the bolt carrier key and the end of the tube.

You could however just give the upper a try on that lower as-is and see how it goes. If you found it short stroking (and its my guess you might, especially if using anything less than hotter ammo types) you could try to reduce the buffer weight by taking apart the rifle buffer and replacing some of the steel weights with either aluminum rod or delrin rod pieces of the same diameter. If you don't like the sound of the DIY bit, you could try one of these instead (although they are quite expensive and basically consist of what I just described):

JP LMOS rifle buffer http://www.jprifles.com/buy.php?item=JPLMB

ETA: I run a fixed rifle stock on a 16" midlength. With decent ammo it runs well with the standard rifle buffer. If I use weaker cheap steel cased ammo it will short stroke on occasion with the standard buffer, but a quick switch out to my JP low mass rifle buffer and that then runs perfectly (although as stated, knowing what I know now if I was to do it again I'd buy another spare rifle buffer for $12 and get some aluminum rod from Lowes as described and make my own and save some $$).

[markm]

ETA: I run a fixed rifle stock on a 16" midlength. With decent ammo it runs well with the standard rifle buffer.

That extra 1.5" is all the difference in the world.

[SA80Dan]

That extra 1.5" is all the difference in the world.

Roger that Mark, I know it is....hence my "likely to short stroke comments'. Similar principle though...if he felt like experimenting he could give it a try. If mine will run with shitty ammo with the lightened buffer, his may at least run with good ammo with the lightened buffer.

[DC3]

Here is a good link..

http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system