Buffer weights for Huldra and BCM?

[UnaStamus]

BCM has since changed their recommendation of the 14.5" middy to standard carbine. They experienced some cycling issues with the EAG rifles due to people running H buffers with .223 ammo. You can run quality 5.56 in the 14.5" middy and it will run on an H, but .223 wasn't 100% reliable.

[Hmac]

BCM has since changed their recommendation of the 14.5" middy to standard carbine. They experienced some cycling issues with the EAG rifles due to people running H buffers with .223 ammo. You can run quality 5.56 in the 14.5" middy and it will run on an H, but .223 wasn't 100% reliable.

What size gas port are they using for that middy barrel? My 11.5 does fine with an H2 buffer running PMC Bronze despite the significantly shorter dwell time. That gas port is .073 inch.

I wonder what the dwell time difference between that 14.5 inch mid-length and my 11.5 inch carbine gas system. I suppose I could do the math, but my brain hurts right now.

[Tronster]

Sounds like the easy solution (at least for 14.5" and 16") is to run an H1 buffer with full power 5.56, and swap in a carbine buffer to run .223 plinking ammo. Takes 10 seconds to swap and doesn't take up any room in a range bag.

[UnaStamus]

What size gas port are they using for that middy barrel? My 11.5 does fine with an H2 buffer running PMC Bronze despite the significantly shorter dwell time. That gas port is .073 inch.

I wonder what the dwell time difference between that 14.5 inch mid-length and my 11.5 inch carbine gas system. I suppose I could do the math, but my brain hurts right now.

No clue.

[Tronster]

Some searching for actual BCM middy gas port measurements shows a .076" gas port.
I don't know the dwell TIME due to the various muzzle velocities of different barrel lengths, but we can easily find barrel dwell LENGTH.

Rilfe length gas system 12" from chamber: port pressure about 19,000 psi
20" barrel has 8" dwell length

Mid length gas system 9" from chamber: port pressure about 27,000 psi
16" barrel has 7" dwell length
14.5" barrel has 5.5" dwell length

Carbine length gas system 7" from chamber: port pressure about 34,000 psi
16" barrel has 9" dwell length
14.5" barrel has 7.5" dwell length
11.5 barrel has 4.5" dwell length

So the dwell length of the 14.5" middy is 5.5" vs the 11.5 carbine is 4.5". Slightly longer dwell with lower port pressure in the 14.5 middy.
We can see that a 16" carbine length gas system is severly overgassed due to long dwell length and high gas port pressures.

As the gas length gets shorter it needs less dwell length to cycle the action due to higher port pressures if the port sizes are all the same.
If the port size gets smaller as the gas system gets shorter, then the dwell length can be closer to the ideal rifle length gas system.

[Uffdaphil]

Here is a good explanation of dwell TIME and why more is good from BCM. I am curious how much dwell time is added using the Claymore closed tube muzzle device on my 10.5" . By the following definition it looks to be about the same as an 11.5" with birdcage.

"A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.

Hope this info helps,
Paul "

[Hmac]

Here is a good explanation of dwell TIME and why more is good from BCM. I am curious how much dwell time is added using the Claymore closed tube muzzle device on my 10.5" . By the following definition it looks to be about the same as an 11.5" with birdcage.

"A: Dwell time.
Dwell time is the time that your gas operated weapon maintains pressure to continue the cycling of the weapon. It primarily exsists from the time the bullet passes the gas port in the barrel to the time the bullet exits the muzzle. When you pull the trigger and fire the weapon the movement of the bolt carrier group unlocks the bolt, extracts, and ejects the spent casing. Then it cocks the weapon, feeds, chambers the next round, and then locks again. One of the thing that can make SBRs finicky is the dwell time (or lack of).

The 11.5" carbine is approximately 4% longer weapon than its' 10.5" counterpart, but this extra inch gives the barrel a 40% increase in length for dwell time. IMHO, this is an excellent trade off. This additional dwell time (all other things being equal) will allow the carbine to be more forgiving to different ammo types, extremes in air temperature, weak or worn extractor springs, worn extractors, buffer weights, etc.

Last Statement: For those folks who have a 10.5" that works very well, I would reply; me too. (see first statement)

If I had to "go to war" with a AR15 Carbine, I would grab the BCM 11.5".
The BCM 11.5" Runs Like a Dream.

Hope this info helps,
Paul "

That was an old post from Paul Buffoni on Arfcom and represent the reason why many SBR builders choose 11.5 over shorter barrels. Certainly, if I were buying a 10.5 or shorter from a quality mfgr's like Noveske or Colt, I'd have no worries that they were selling me a rifle that was well-tested and would run well. If I'm building my own 10.5, now I have to worry about gas port size because that 40% decrease in dwell time eliminates a lot of the margin for error in gas pressures that makes an 11.5 more forgiving of ammo choice .

[Holland&Holland]

So is there any negative to running a standard carbine buffer in your SBR if it functions fine? I realize perceived recoil will be more.

[Hmac]

So is there any negative to running a standard carbine buffer in your SBR if it functions fine? I realize perceived recoil will be more.

Theoretically more wear stress on the bolt, BCG, receiver, and buffer. In addition,cycling too violently could induce enough bolt-bounce as to impair reliable cycling.