Dissipator conversion questions

[CamoDeafie]

Let's say I want rifle length sight radius on my old M4 profile carbine gas barrel; should the 2nd FSB be a clamp-on/set screw type, or pinned old school way? Torn between those two options for doing a Bushmaster styled "A3" flat top dissipator on my SARCO barrel...
I figure I can take the original FSB off, then Dremel or hacksaw it down to a super low profile setup, then reattach the gas block after taking end cap off; that would give me an extra A2 front sight post assembly to install on the 2nd FSB tower mounted at rifle length; but it's the mounting I'm trying to figure out. Set screws, cross-bolt (anyone have old Armalite ban era clamp on front sight towers?) Or buy virgin FSB and send to someone to pin?


Bushmaster original Dissipator setup, looks like it used a M4 barrel and pinned FSB


The 16" carbine length bottom, the one I want to turn into a dissipator.


Additional photos of the 16" M4 upper with detachable carry handle

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[monkeybutler]

I'd send the barrel to be reprofiled to .625 where the forward FSB would be, and have that size mounted there. I think John Thomas doe that kind of thing. I know he mounts FSBs, pretty sure he will reprofile it too.
https://retroarmsworks.com/

 

[CamoDeafie]

I'd send the barrel to be reprofiled to .625 where the forward FSB would be, and have that size mounted there. I think John Thomas doe that kind of thing. I know he mounts FSBs, pretty sure he will reprofile it too.
https://retroarmsworks.com/

Hm. @Velzey did the pinning on my 12" 6.5 Grendel; but the Bushmaster pic does look like they used a M4 profile barrel and a .750 diameter F marked FSB.

 

[monkeybutler]

Yeah it does look like that. I wonder of there is any impact to the .040 variance. Probably would if it were over a gas port. Probably not just pinned as a sight.

 

[CamoDeafie]

Yeah it does look like that. I wonder of there is any impact to the .040 variance. Probably would if it were over a gas port. Probably not just pinned as a sight.

FWIW, the upper that I'm using with the M4 barrel is a flat top with RRA Dominator sight/Eotech mount so even with a F-marked front sight, the 0.040 height shouldn't be affected that much with the rear sights. I could still get a 0.750" diameter "Non F-marked" virgin front sight from either JSE Surplus or Retro Rifles... and get it pinned.

 

[monkeybutler]

I think we are talking about different things. I meant the diff between the barrel outer diameter and the the FSB bore. .710 I think and .750. I don't think there has ever been an issue of f marked or not with the rifle FSB placement, just with the shorter carbine position. And even then it is often not a real problem.

 

[CamoDeafie]

I think we are talking about different things. I meant the diff between the barrel outer diameter and the the FSB bore. .710 I think and .750. I don't think there has ever been an issue of f marked or not with the rifle FSB placement, just with the shorter carbine position. And even then it is often not a real problem.

I'll need to measure mine then. If it's actually straight .750 diameter between the grenade launcher ring and muzzle...

 

[monkeybutler]

Oh now I get it. I did not realize the M4 had the step back up to .750 by the muzzle. Suprises me I didn't. The times I have considered making a dissy it has been with middy barrels with the way simpler contours.

 

[CamoDeafie]

I just remembered. I may still have a cast set screw .750 diameter FSB in one of my storage totes either here or in storage unit with my brother's business inventory; it was from when I was planning the 6.5 Grendel pistol build... which got a virgin FSB from Retro Rifles that Velzey installed for me. Can't remember exactly what I did with the set screw one.

 

[CamoDeafie]

Oh now I get it. I did not realize the M4 had the step back up to .750 by the muzzle. Suprises me I didn't. The times I have considered making a dissy it has been with middy barrels with the way simpler contours.

From the muzzle threaded part to a step up, about 5/8" long and 0.71875" diameter and then .750" diameter straight for 3.125" length to grenade launcher step down ring, then back up to .750" diameter to the front sight rear ring. Grenade ring diameter is 0.625"

 

[CamoDeafie]

So. I ordered two FSBs. One, a non-F marked virgin FSB that would be drilled/pinned, and the other, a clamp-on FSB with set screws from Dez Tactical. The clamp on will do OK as a temporary setup, or until I decide to use the virgin FSB on a different barrel &receiver for the A2 lower build, depending on how things goes. Now I just need to source a set or 2 of A2 rifle handguards. I also ordered a YHM Clamp On Gas Block for the dissipator build, less messy that way.. actually, this would enable me to keep my original FSB and pins to return the Dissipator to M4 Carbine setup if I just use Dez set screw FSB..

Edit. Confirmed, Guncrafters in Salem doesn't offer AR15 front sight pinning services. Kinda figured going off their site and what they usually have in shop (mostly wood and blued steel vintage stuff)