I originally posted this over in sights and optics, but I think this may be a better place for it.

I just got a new upper from Bushmaster, and had them install their BMAS 3-rail gas block on it. The gas block appears to be secured with three set screws on the bottom, and isn't drilled for taper pins. They included a regular FSB with the barrel which looks like it's been installed before, so I assume that they removed the FSB from a barrel which was drilled for taper pins and clamped on the three-rail FSB, as opposed to drilling and pinning the FSB onto an un-drilled barrel. Up until recently that distinction would have escaped me, because I was not aware that the FSB and barrel are drilled for the taper pins as a unit.

I was conversing by email with one of our board's esteemed vendors, and he recommended against hanging a weapon light off the side of my gas block as I had intended since it's just secured in place with set screws. That kinda makes sense to me... if I'm gonna hang a pound or so of light off the gas block, then I figure it ought to be pinned in place so it doesn't shift under recoil.

Since I have the FSB that was apparently mounted on the barrel originally, in theory I ought to be able to measure the pin locations and drill my three-rail gas block so I can pin it onto my barrel. I have a CNC mill so I should be equipped to be able to do this, but I could definitely use some advice on how to go about doing the job right. Maybe a better option would be to see if I can order an un-drilled barrel from Bushmaster, drill and pin my 3-rail gas block onto it, then reinstall the plain FSB on my first barrel and set it aside for use with that MGI QCB upper that I'd like to get before long.

Should I have reasonably expected Bushmaster to have drilled and pinned this gas block onto an undrilled barrel since I ordered them as a factory-installed pair? If so, maybe I'll call them up and see if I can exchange my upper. If not, then I guess I'm on my own trying to get my 3-rail gas block pinned in place.

I haven't contacted Bushmaster about this yet. I'm also still waiting for the stock which was supposed to be in the box with this upper...

OK...where to start.

I recently got bored with the look of my M4gery, which was my first build ever. An Eagle Arms so-called "preban" lower (built in '93) and a Colt M4 upper with Bushie bbl./3-rail gas block. I've got this sick need now for a basic, "no-frills no-rails plain-Jane" look to all my weapons these days, including a renewed fondness for the AK side of my nature which I'm fighting tooth-and-nail daily. I attribute this to a mid-life, (probably going to have my nails done and my hair colored next, then go buy a new Corvette...but I digress) so I had the same barrel basically that you have. Decided to put the original FSB back on the ol' girl.

My 3-rail gas block is exactly as yours, has 3 set screws on the bottom side of the unit. These I later found out were there on my particular unit basically for show, as the f***ing gas block's tolerance-to-barrel was so tight that I have no idea how Bushmaster was able to get the thing on. I swear they must have used a hydraulic press to get the unit onto my barrel in the first place. After using every penetrating oil I had, a blowtorch, and performed a voodoo chant to the gods of gunsmithing, I gave up and put it in the freezer and went to bed. Next day, the sumbitch came off with just a bit more than a "harumfph" from me. Good to go. And, not damaged, so it could be reused.

As yours, Bushmaster shipped the original FSB with my bbl. Putting back the original FSB/gas block was a snap, everything lined up perfectly. It obviously was the one that had been on there prior to the 3-rail assy. being attached by Bushmaster.

My point to this whole treatise was that if your 3-rail was ANYTHING like mine, you could hang a dump truck full of diced gorilla assholes off the thing and not worry about it stressing the 3-rail too much. Believe me, those set screws on my gas block were there for mental masturbation, that sucker was on there but good ! Can't say for certain, but I suspect yours is probably the same.

Good luck.

You may be in for some trouble if you attempt to remove and reinstall the gas block.

The BMAS parts are actually made for Bushmaster by Yankee Hill Machine.

www.yankeehillmachine.com/store.html

Here are the installation instructions from the YHM website on how to install their railed
gas blocks.
yhm.net/store/media/4Rail_LoPro_SingleGB.pdf

Note that the YHM Factory method uses a Thread Locking compound.
Bushmaster may use the same procedure.

I doubt that your new gas block would move or come loose with anything approaching normal
usage.

IMHO , If you want a Light on your rifle , the Handguard is a better place to mount it.

Although anything is possible, I did not see any residual thread locking compound or remnants of anything left behind when I finally pulled it off. On mine, anyways, it appeared to be just a "press fit". YMMV.

IMHO , If you want a Light on your rifle , the Handguard is a better place to mount it.

Maybe I'll come to the same conclusion after I try hanging a light off my gas block, but for now I'd prefer to keep the nice, simple stock handguards on this particular upper.

if your 3-rail was ANYTHING like mine, you could hang a dump truck full of diced gorilla assholes off the thing and not worry about it stressing the 3-rail too much

   

i had BM take off my FSB and replace it with a 3 rail block when i purchased a patrolman's carbine upper from them a few years back.  for some reason they sent me the barrel w/FSB on it and the 3 rail block separately.  i called them up and they said they screwed up, just send back the upper and they'll do it right.  so a week and half later i got the upper back with the 3 rail block on it, and the original FSB+pins were in the box as well.  

i'm trying to keep said upper as light and as KISS as possible, however...
i too am looking at hanging a light off the 3 rail block, so i guess this is a long-winded "tag" to see how this story plays out.  

ar-jedi

'Splain this to me, folks. What did they intend the three rail gas block to hold if not lights, lasers, etc.? Am I missing something in all this? I realize that some vendors et al may say not to hang a light or other item off the rails on the side of the block, well, why were they put there then? Is this not common sense? Did YHM intend the rails to be used to hang Christmas ornaments? If indeed the manufacturer didn't want you hanging lights (or whatever) off, don't you think there would be a supplement to the instructions or a warning from Bushmaster?

Just curious, I suppose.


I'm waiting for the keyboard commandos to chime in any minute with their grandiose stories of how they were clearing a room in Fallujah when "all of a sudden their gas block fell off 'cause their weapon light caused soooo much stress on the rail, then they were forced to fight hand-to-hand with their custom Benchmade USMC fighting knife with the "blood groove", and killed 10 insurgents before the CIA was able to send in Jack Bauer with his posse and cellphone to kill off the rest..."

To be continued.

Quoted: 'Splain this to me, folks. What did they intend the three rail gas block to hold if not lights, lasers, etc.? Am I missing something in all this? I realize that some vendors et al may say not to hang a light or other item off the rails on the side of the block, well, why were they put there then? Is this not common sense? Did YHM intend the rails to be used to hang Christmas ornaments? If indeed the manufacturer didn't want you hanging lights (or whatever) off, don't you think there would be a supplement to the instructions or a warning from Bushmaster?

i actually never even considered it could be a problem until i read

Quoted: I was conversing by email with one of our board's esteemed vendors, and he recommended against hanging a weapon light off the side of my gas block as I had intended since it's just secured in place with set screws. That kinda makes sense to me... if I'm gonna hang a pound or so of light off the gas block, then I figure it ought to be pinned in place so it doesn't shift under recoil.

383green,
what vendor made this statement and on what basis did they make it?
did they personally have a "Fallujah room clearing" experience go bad a la above?
are you sure they weren't just foaming at the mouth?

thanks,
ar-jedi

ps: Jack Bauer doesn't work for CIA.  he was previously a CTU operator.  keep that straight ace.  

Jedi,

I wasn't serious about the Fallujah part, it's just the sort of thing you see written on these boards from time to time, especially when it comes to a piece of equipment and its purpose.


Got it on JB, I would NEVER disparage the good Jack Bauer, 'cause I don't want to be killed by him somed...

what vendor made this statement and on what basis did they make it?

G&R Tactical. I don't know whether they've ever seen one shift.

The "what are they for if not to hang lights" argument makes sense to me... so does the "two pins are better than three tiny set screws" argument.

[Edited to add:] BTW, since I've mentioned G&R Tactical by name now, I'd like to point out that the items that I ordered from them when this gas block discussion came up arrived promptly and I'm happy with the service I received from them when I was deciding what to order and which company to order it from.

Quoted: what vendor made this statement and on what basis did they make it? G&R Tactical. I don't know whether they've ever seen one shift. The "what are they for if not to hang lights" argument makes sense to me... so does the "two pins are better than three tiny set screws" argument. [Edited to add:] BTW, since I've mentioned G&R Tactical by name now, I'd like to point out that the items that I ordered from them when this gas block discussion came up arrived promptly and I'm happy with the service I received from them when I was deciding what to order and which company to order it from.

well, grant is one of good guys, i'll have to ask him what he meant by that.  ps: +1 on G&R.  

ar-jedi

G&R is good people. They must have a reason for saying this about the 3-rail blocks. Maybe they have some info I'm not aware of.

Whatever it is, I'm sure it'll  be legit.