Gas block minimum clearance question

[MoxyDave]

Hey all, I'm trying to use a Wilson Combat adjustable gas block inside an Aero Precision COP upper receiver.

The Wilson block is taller and slightly wider than a standard low-profile block because of the adjustment screw.

The fit is super tight. Even after chamfering the (oddly) square corners on the top of the Wilson gas block, there is maybe 1/16" clearance from the top and sides of the rail interior.

I am using a very high quality 416 stainless 16" barrel with a medium contour. The COP rail is extremely stout.

My concern is that the gas block will hit the inside of the rail when a shot is fired.

How much does a barrel move inside the handguard when a shot is fired? Anyone have a link to some slow motion footage of an AR being fired in controlled conditions (i.e. mounted firmly to a stationary object)?

http://shopwilsoncombat.com/images/750-004.jpg

[RiverRat]

Try this.

ar-15-platform-barrel-whip-discussion (http://vuurwapenblog.com/2011/05/22/ar-15-platform-barrel-whip-discussion/)

The barrel whip he shows is perceived at the muzzle and it looks pretty noticeable at that position. His overall conclusion (that the muzzle device is applying the force that deforms the barrel) seems reasonable and is supported by his testing with other devices.

So if you're going to run a comp, you're more likely to see up-down deflection than if you run a symmetrically ported flash hider. But either way, the barrel vibrates some and probably moves in all directions after the initial push.

Extrapolating to how much movement you might see at the gas block, there are too many variables even beyond the muzzle device design to even consider making guesses. Flex in the receiver itself, looseness in the fit between the receiver and barrel extension, contact design of the barrel nut and physical flexibility of the barrel itself all probably contribute to the amount of movement you might see. Case in point, I can create that 1/16" deflection (at the gas block) of my Adams Ultralight middy-barreled build using downward pressure on the flash hider. All of that flex appears to occur in the barrel only, but I knew it could be whippy when I chose it. A 16" Rainer medium profile barrel shifts far less under light pressure but does move. Who knows how much it really moves under fire?

IMO, given your situation (where the upper/rail has value far beyond simple replacement cost ), I would be looking at other gas blocks with more than 1/16" clearance.

D.

[FortTom]

RiverRat,
I'll be damned. I would have never guessed that you'd find a picture of that. In fact amazed. And it looks like my "theory" is completely wrong. But you brought up all sorts of things I hadn't considered, such as flex in the receiver, the barrel nut, etc. and I knew it was an apples to oranges comparison given the differences in designs between the firearms.

Glad you caught that before anyone read that, and thought it to make sense, or be valid..so I deleted the post...

Thanks,
FT

[MoxyDave]

Thank you both for your input. Fascinating video! I knew (assumed) a barrel would flex with some sort of sinusoidal motion but I did not consider the effect being primarily based on the muzzle device. I didn't think it would be as dramatic either. It makes sense since it is at the end of a lever arm. My Engineering basics are coming back to me ... [:)]

Still, with the gas block at the carbine-length position, the motion will be reduced. And it seems you can limit this behavior with your choice of muzzle device.

The engraved COP upper is nice and I'm grateful to have such a unique item for sure, but it's not like someone put in 1000 hours of custom labor. I'm not too worried about wear and tear so I might just assemble it, shoot for a while and take it down later for examination.

Does anyone else have any thoughts or experience to share?

[GOST]

I don't have much experience with adjustable gas blocks but the the Seekins may be shorter since the adjustment isn't directly underneath the tube.

[MoxyDave]

I received an email from Dan Lilja (owner of Lilja Barrels) this morning, he says 0.060" (roughly 1/16") should be fine.

[FortTom]

I received an email from Dan Lilja (owner of Lilja Barrels) this morning, he says 0.060" (roughly 1/4") should be fine.
Damn, that's the figure I put, but just as a WAG, based on experinces free floating barrels on hunting rifles. My guess was .030 per side, in a SIN wave oscillation if you sketched it out. (I knew that part was correct).

But all the other factors I didn't weigh in, well I just deleted the post, not wanting to look like a complete fool. 0.0625 would be 1/16 , not 1/4", or roughly .030 per side. I was dumfounded though when I saw that a muzzle break could cause that much flex. It seems like that would kill a barrel in short order. I'm still confused there.

Live and learn...
FT:o (embarrassed) and 4 years of engineering school, shot to hell.:P

[MoxyDave]

Whoops, yeah 1/16" not 1/4" ... my mistake.

FT, you're fine. Don't worry about what other people think.

[FortTom]

Ah, it's all good. If I can't laugh at myself, who am I going to laugh at?...

FT.[:)]

[UWone77]

Dave,

I've never seen a gas block as tall as that Wilson. I would just shoot the heck out of it, and if something happens it happens. All the more reason to tell us what went right/wrong. Then again, I don't mind screwing up a piece of gear from time to time. I guess I got used to it when I first started building up a couple of AR's and put idiot marks on them. After that, it didn't phase me to beat up my gear.

[RiverRat]

I've never seen a gas block as tall as that Wilson.

I had been thinking the same thing, even to the point of wondering if a non standard gas tube were supplied with the block :)