Does yours flex? How do you know? How did you check?
spinoff from this thread
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Does yours flex? How do you know? How did you check?
spinoff from this thread
Rob, I can't really do much to back up my statements but here is my opinion (based on some mechanical design education/background and my own experience):
What you're seeing is more the barrel flexing than the handguard flexing. The barrel "tube" is relatively small in diameter and as a result can be flexed surprisingly easily. The handguard tube, while made of aluminum and pretty thin, is relatively large in diameter so it's actually pretty rigid. In order to 'flex' noticeably, it has to deform some (meaning the tube diameter gets squished in one direction), and I don't think you're applying enough force to really see that happen.
I don't know if you saw my comment in the other thread, but my concern is the mounting method of the TRX and the interface between the handguard and receiver. I actually did the same thing on one of mine that you did in the video. What happened for me was that the handguard would move on the barrel nut and not return to original position. I could measure the change in the relationship between the end of the handguard and the barrel. I don't like this one bit, especially if you're running a low profile gas block and rail mounted sight.
FWIW, doing the same thing you did in the video on an upper with, for example, a Larue handguard will yield similar results regarding apparent flex - if the barrel is similar in profile. The difference for me is that I can do that on an upper with a handguard using a different mounting system (such as the Larue) and there's no issue with repeatability due to the handguard moving on the barrel nut. No, this 'test' isn't something the gun would normally experience, but neither is a hard drop onto the handguard. And if I can fairly easily make the handguard tube shift permanently with my fingers, then I'd expect a hard drop to have the potential to do the same.
Last edited by krichbaum; 02-07-11 at 06:57. Reason: Clarification
Funny, it was not that long ago I was laughed at when I said I easily "flexed" weapons while slung in (two point) and torquing down for a long shot. You could actually see the flex as I got "set"... obviously changing point of impact. I believe at the time the topic was pencil barrels vs. my "heavy" little SBR running a 11.5" Hbar. Good job with the vid.
Last edited by Zeus; 02-07-11 at 06:50.
Remember, the Founding Fathers reason for the 2nd Amendment... to combat tyranny. Sadly, when people discuss the 2nd Amendment, the opponents erroneously think it was meant to stand alone... it HAS a preamble:
"...But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security..."
— John Hancock
I only posted the video to spur discussion. I don't want to make this only about this one example. It's about worst-case scenario with a 13.0 tube and a lightweight barrel.
So in terms of barrel flex vs. rail flex, how do we measure or determine rail flex? and does it actually matter if the point, in fact, of the free float rail is so that bumps, bangs, sling tension, etc. do not translate to or effect the barrel?
Sorry...I can delete the above post if you want.
I don't think what you're seeing in your video matters as long as everything returns to it's original relative position when no force is applied. Unless you have one really thick barrel, it's not going to take much force to flex it. The strength of a tube to resist bending goes up greatly as the outside diameter is increased - much more quickly than the diameter is increased.
Measuring the rail flex in a quantifiable way is going to be very hard for the avg person to do. I don't think any of the widely used handguards have enough flex in the tube itself that it's a problem.
No, don't delete the post, I just want to encourage others to add to your post and see the thread grow to talk about the larger issue.
I'm asking these questions more as a way to spur discussion than I am because I'm personally looking for answers to my own problems, that's all. It's an offshoot of another discussion. I probably should have skipped the video to avoid confusion.
On that note...
Looking at the larger issue I tend to agree with you but would like to hear from some others. IMHO if the barrel returns to the same place every time what you are seeing is barrel flex, which the tube system is designed to isolate. If the barrel moves within the tube and doesn't return to the same place what you're seeing is weakness in the tube mount and that *could* be a problem. Which then leads to the question of exactly what the repercussions of this problem might be.
This very topic is what makes me question Troy's new 15" TRX Extreme rail. I've got their 11" version already, and will be doing a 14.5" PA build, using their 13" rail, over the next week or so. While shopping for the 13" rail, I ran across the new 15" rail on thier site.
Now, I have absolutely zero complaints about the 11" version, and I'll hold my judgement for the 13". However, at what point does the length of the free-float tube become too much for the barrel nut? I know that LWRC ran into the same problem with their new REPR style forend.
Their solution was to do away with the barrel nut altogether. The base of their new rail threads directly onto upper receiver, cutting out the weak point on the longer rail lengths. I have yet to actually place my hands on one of these rails, but this is the gist of what I've gathered from their forums. I'm sure that there's actually more to it, in terms of reinforcement. SMGLee has posted some pics from the shot show, and you can see the example of this rail on their SPR rifle.
Last edited by PatrolRifleGroup; 02-07-11 at 07:26. Reason: Grammar
This is why i LIKE the Samson design as it uses the stock barrel nut which we know works fairly well as a method to secure the barrel to the upper receiver.
The barrel itself is chucked up and secured into the upper receiver via chamber extension and barrel nut. From there on out, provided you are using a free float rail, the entirety of the barrel is hanging in space.
Unless your receiver is out of spec or something is installed improperly no manner of movement via the rail itself should impact your barrel UNLESS It is causing the barrel nut itself to move.
The units that use aftermarket barrel nuts MAY allow undue flex due to improperly securing the barrel in the upper receiver.
That is pure speculation as it is not proven but the theory is sound.
Rob, i think you're over thinking this one as i dont see you experiencing any "problems" with your current setup. As the video displays, yes, the barrel is indeed moving but it is returning to its point of origin.
Go get a piece of steel, rebar, whatever. Ram it into the ground somehow and pull on the top of it. By virtue of its length you're GOING to be able to get it to bend slightly.
I think the impact of flex is far less important than shift. If the barrel and forend are both correctly secured at the rear and any flex encountered doesn't create a permanent shift, the impacts are negligible. We all know that barrels flex and it's easily seen in hi-speed video footage of the firing cycle. It's the main reason we studiously avoid resting the barrel itself on anything when firing. This is also the reason that hi-power rifle shooters equip their rifles with free float tubes. On a military KD qual course, so long as sling pressure is consistent and dope is properly applied, even rifles without free float tubes can deliver accurate hits at range.
Forends like the TRX Extreme are probably going to flex more than a heavy rail forend. There's just not as much material there to resist flex. The real question is whether the attachment point is solid and dependable. If it is, then the rail will return to it's original position as the pressure is relieved. I doubt that many TRX Extreme users are going to use traditional sling aided shooting positions for long range shooting using rail mounted BUIS. So long as they're not applying excess force to the forend of the rifle, zero should be unaffected by the minimal amount of flex encountered.
If it's not secure and reliable, then you may have issues with zero on rail mounted BUIS. The only way to really know would be to chuck up a rifle in a vice and apply gradual force to the forend, while frequently checking barrel clearance measurements until failure was detected.
I can see flex between the 11" TRX Extreme and barrel on my Noveske Rogue Hunter. Whether it's the barrel or forend, or both that flex isn't really relvant. It doesn't seem to affect accuracy of the rifle so I don't worry about it. If I discover a POI change then it will be an issue, but for now it seems to be doing it's job. If it was a part from a less reputable manufacturer, I might be a little more concerned.
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