16" middy barrels with .75" at muzzle for FSB - dissy

[rob_s]

chart

Brownells also sells, or sold, .625" FSBs. I bought one from them and chopped it down to ultra-low profile for a project.

The BCM Dissipator prototypes, at least the one I have, are govt. profile barrels.

[DHart]

chart

Brownells also sells, or sold, .625" FSBs. I bought one from them and chopped it down to ultra-low profile for a project.

The BCM Dissipator prototypes, at least the one I have, are govt. profile barrels.

Rob... your charts are wonderful resources!

On the BCM 14.5 barrel shown in your chart, where it says "Beyond" that doesn't mean close to the muzzle, does it? Because on my BCM 14.5" middy barrel, the diameter close to the muzzle where one might place a FSB measures .75", then for the last like 3/4" before the muzzle it steps down slightly to like .71". But a .75" base FSB could definitely be mounted at the end on the .75" section, though the 14.5" barrel isn't long enough to take a rifle hand guard. <EDIT - I just realized that your chart is for LIGHTWEIGHT barrels and my BCM 14.5 is not a LW, but a "regular" BCM barrel, which they don't actually identify by a specific profile type. It seems BCM barrels are either labeled as "LW" or not-identified as to profile.>

The BCM Dissy prototype you have... you say is a govt profile. Does that mean the barrel diameter where the FSB sits is .75" or .715? Is there a way you can measure the barrel diameter where the FSB is sitting?

I've asked BCM twice in e-mails this week if any of their 16" barrels could mount a FSB (whether .75" or .625") close to the muzzle (to allow use of a rifle length hand guard) and they told me the barrel diameter on all of their barrels is too small to host a FSB.

[rob_s]

You are correct, these are all lightweight barrels. I posted it because the discussion turned that way for a bit and I wanted to show people that none of the lightweight barrels outside of the Colt have a .625" diameter forward of the FSB.

BCM barrels that are not specified at "lightweight" are either M4 profile or GI/A2/government profile (people use the terms interchangeably sometimes, I prefer the term "A2" but have probably used all three). The M4 profile are obvious due to their gas tube length and the 203 notch.

If you are serious about this project, what I would suggest is using a lightweight barrel and a .625" FSB and having a local shop turn a spacer (or two, one for each pin location) to make up the difference. Having over 700 rounds on the A2 profile, mid-length, BCM Dissipator I can tell you that it is muzzle heavy IMHO over time. If I were buying one I'd want the thinnest barrel profile I could find for weight savings. The only concern I'd have at that point would be whether or not there is enough meat in a .57x" barrel to drill a cross-pin through without hitting the bore.

.572-.223=.349/2=.175" wall thickness of barrel. That ain't a lot of meat to be drilling across. 1/8" is .125" so .175" is 11/64" which is awefully thin.

Alternately, if you could find someone adept enough with a lathe, would be to have the A2 mid-length barrel turned down to an honest .625" from shoulder to muzzle. Or smaller once past the gas block and with two raised areas for the FSB. That might be the best bet, given a talented enough lathe operator who knows ARs.

Hope this is making sense. I just really believe that the major downside to this configuration is muzzle weight and the way it affects balance. Sure, one can make it up with a heavier stock, but that seems counter-intuitive to me.

[Biggy]

Thinking about getting a 16" barrel with a profile that ends up with .75" at the muzzle end so that a FSB can be positioned there (at rifle length) and pinned.

Would this barrel profile be considered a "standard"? Any suggestions on top picks or sources for such a barrel? (The barrel would be used to make a mid-length gas dissy upper).

Anyone know of any FSBs made to fit smaller diameter than .75"? Something that could mount at the end of a smaller diameter barrel?

You may want to check with www.Adcofirearms.com . They make a 16"mid-gas dissy barrel assembly. Barrel specs are, CM/V 1:7 MP tested, pinned gas block and pinned F height FSB. I imagine they could reprofile this barrel anyway you like, or make you up a custom one using a .625"pinned FSB.

[DHart]

Rob... Thanks for the detailed reply. I think your second suggestion makes the most sense if I were to move on a custom build. And I agree that turning down the barrel wherever possible to reduce weight is very wise.

Biggy... I asked Adco about doing some custom barrel work to make a dissy barrel and he told me to buy his pre made one. When I asked for more detailed specs on his pre made one, he never replied. Something in my gut is telling me not to go this route.

Indications seem to be that BCM is likely to do this, hopefully in the next few months. So after all this exploring and inquiry, Im starting to lean toward waiting to see if Paul comes up with one. I'd like to see it with a carry handle A2 rear sight configuration, lightweight barrel, and MOE rifle length hand guard. I would buy one if Paul offered them.

[9DivDoc]

You are correct, these are all lightweight barrels. I posted it because the discussion turned that way for a bit and I wanted to show people that none of the lightweight barrels outside of the Colt have a .625" diameter forward of the FSB.

BCM barrels that are not specified at "lightweight" are either M4 profile or GI/A2/government profile (people use the terms interchangeably sometimes, I prefer the term "A2" but have probably used all three). The M4 profile are obvious due to their gas tube length and the 203 notch.

If you are serious about this project, what I would suggest is using a lightweight barrel and a .625" FSB and having a local shop turn a spacer (or two, one for each pin location) to make up the difference. Having over 700 rounds on the A2 profile, mid-length, BCM Dissipator I can tell you that it is muzzle heavy IMHO over time. If I were buying one I'd want the thinnest barrel profile I could find for weight savings. The only concern I'd have at that point would be whether or not there is enough meat in a .57x" barrel to drill a cross-pin through without hitting the bore.

.572-.223=.349/2=.175" wall thickness of barrel. That ain't a lot of meat to be drilling across. 1/8" is .125" so .175" is 11/64" which is awefully thin.

Alternately, if you could find someone adept enough with a lathe, would be to have the A2 mid-length barrel turned down to an honest .625" from shoulder to muzzle. Or smaller once past the gas block and with two raised areas for the FSB. That might be the best bet, given a talented enough lathe operator who knows ARs.

Hope this is making sense. I just really believe that the major downside to this configuration is muzzle weight and the way it affects balance. Sure, one can make it up with a heavier stock, but that seems counter-intuitive to me.

Good question to pose to Paul..so I just did

When I hear back...will post...

Is there enough "meat" on that light wt barrel to support cross drilling
for the pins of the front sight base re-located to the rifle length position

I got a cut down .625 fsb to use as a gas block under the hand guards
and would then swap out the fsb to the rifle position if there is enough
meat....

[HeavyDuty]

Good question to pose to Paul..so I just did

When I hear back...will post...

Is there enough "meat" on that light wt barrel to support cross drilling
for the pins of the front sight base re-located to the rifle length position

I got a cut down .625 fsb to use as a gas block under the hand guards
and would then swap out the fsb to the rifle position if there is enough
meat....

I don't see this as being practical.

A LW has a 0.625" journal at the gas port; it's reduced forward of this (one of my LWs here is about 0.590" forward of the FSB and then bells out to about 0.610" at the muzzle).

The only practical way of making a Dissipator-type barrel right now from currently available BCM components is to profile a 0.750" barrel down to create a 0.625" journal at the new FSB position. You'd probably have to stay 0.750" for the low profile gas block - how else are you going to get it over the new front sight journal during assembly?

[DHart]

The only practical way of making a Dissipator-type barrel right now from currently available BCM components is to profile a 0.750" barrel down to create a 0.625" journal at the new FSB position. You'd probably have to stay 0.750" for the low profile gas block - how else are you going to get it over the new front sight journal during assembly?

Yeah, it seems this is perhaps the best approach. But, if you went with .625 at the gas port, then reduced profile down to say .57 to shave some weight, then back up again to .625 at the new FSB position, couldn't you then slide the lo-pro gas block back to position fairly easily? Obviously, there's a bit of re-profiling work to be done.

I wonder if Daniel Defense has a barrel that might be more suitable for adaptation to a light-er weight Dissy barrel.

[rob_s]

In all of my rambling from my previous post, what I was trying to suggest was taking an A2 profile barrel and turning everything forward of the gas port down to .57x with the exception of the location where you want to install the FSB.

[DHart]

In all of my rambling from my previous post, what I was trying to suggest was taking an A2 profile barrel and turning everything forward of the gas port down to .57x with the exception of the location where you want to install the FSB.

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