Rigid Rails and Deflection - Practical?

[Ironman8]

Something I've been thinking about lately is rigid free float rail systems for the purpose of mitigating deflection to prevent POA/POI shifts with lasers and irons.

I understand the concept behind it, but when, in actual/practical use, would you be able to flex a rail and cause POI/POA shift?

The only real scenario I can imagine is loading a bipod or bracing a forward grip against and over the top of a barrier, which would cause a downward flex and (I think) a high POI... but only if one were using lasers or irons in this case. If you are using a receiver mounted optic instead, then I don't think rail flex comes into play.

However, in a run and gun scenario where you're bracing AROUND cover, AND using secondary rail mounted sighting systems, I don't see how (or why) torque would be applied enough to induce rail flex. When I post around cover, my support hand is bracing/grabbing cover and the rail simultaneously for support...I would have to really torque the grip of the rifle with my firing hand to induce rail flex in relation to the barrel.

So again, I'm failing to see how this happens in real world use. Can anyone "prove me wrong" here?

[opngrnd]

Haven't done much to test free float change of impact, but the Marine Corps shooting team did a post on deflection using the M16 and M4. It wasn't an insignificant amount of change. I've experienced changes of 10" at 200 yards from the kneeling position with M16A4s when alternating between gripping the front of the receiver and the handguard.

[Ironman8]

Haven't done much to test free float change of impact, but the Marine Corps shooting team did a post on deflection using the M16 and M4. It wasn't an insignificant amount of change. I've experienced changes of 10" at 200 yards from the kneeling position with M16A4s when alternating between gripping the front of the receiver and the handguard.

Right, I've heard of that study, which is why I specified FF rails. I can definitely see how a non-FF rail would have much more effect on the barrel POA/POI...for both rail and receiver mounted optics.

The whole rail flex issue seems to have become a fairly hot topic over the last year or so for guys running lasers, possibly starting with the Primary & Secondary crowd...and possibly because of a comment by one of the P&S gunfighter dudes like Chuck or Blowers. Not sure exactly where it originated, but seems to be taken at face value with no thought toward the practicality of it.

[opngrnd]

Right, I've heard of that study, which is why I specified FF rails. I can definitely see how a non-FF rail would have much more effect on the barrel POA/POI...for both rail and receiver mounted optics.

The whole rail flex issue seems to have become a fairly hot topic over the last year or so for guys running lasers, possibly starting with the Primary & Secondary crowd...and possibly because of a comment by one of the P&S gunfighter dudes like Chuck or Blowers. Not sure exactly where it originated, but seems to be taken at face value with no thought toward the practicality of it.

I scratch my head a little as well. There are a number of respected trainers running BCM handguards, which seem to be more susceptible to flex than competing designs. But you don't seem to hear much complaining.

[Ironman8]

Anybody else want to play?

[Defaultmp3]

I scratch my head a little as well. There are a number of respected trainers running BCM handguards, which seem to be more susceptible to flex than competing designs. But you don't seem to hear much complaining.To be fair, the deflection issue isn't a big deal until you start using MFALs, so for most uses, the BCMs are perfectly fine.

I distinctly remember that Pressburg would demo deflection when bracing against barricades during his NVG courses. Don't remember if I read it in an AAR for Fight Club or if it was after he established Presscheck Consulting.

[Randall]

Like you said, the only time I can see it really being an issue is when you load a bipod.

This was quoted from the “FSB in 2020” thread, and he’s obviously talking about rail mounted sights, but I’d think a laser would be in the same realm.


MSG Jared Van Aalst (KIA, RIP) was one of my NCOs, formerly of the 3rd Ranger Battalion, the USAMU, the infantry sniper school, and Delta. He stated there was definitely a shift with the SPR's rail-mounted front sight if you leaned into the bipod for distance shots.

[Ironman8]

To be fair, the deflection issue isn't a big deal until you start using MFALs, so for most uses, the BCMs are perfectly fine.

I distinctly remember that Pressburg would demo deflection when bracing against barricades during his NVG courses. Don't remember if I read it in an AAR for Fight Club or if it was after he established Presscheck Consulting.

So back to my OP, was he demoing normal bracing of the gun on the barricade? Or was he intentionally inducing torque from the back of the gun while bracing the front to show that it’s possible? If it’s the second one, then I’m not sure why that became a “thing” since it’s not normal technique for barricade shooting (at least not how I’ve ever done it or been taught).

I’m asking to be educated here. Maybe I’m bracing wrong :confused:

ETA: I’ll have to disagree slightly with what you said though, it’s not just MFALs. If you deflect enough to affect an MFAL, it will be the same for a rail mounted front sight.

[Ironman8]

Like you said, the only time I can see it really being an issue is when you load a bipod.

This was quoted from the “FSB in 2020” thread, and he’s obviously talking about rail mounted sights, but I’d think a laser would be in the same realm.

Agreed...although regarding the quote by Sinister, I’m not sure why a rail mounted front sight would be used for distance shots with a (probably) 10x scoped SPR...unless it was just for training and something that was noted.

[Failure2Stop]

I can get real weird and into the weeds on this, but I'll try to keep it simple and short.

For most practical purposes, unless an aiming device (including a front sight) is attached to the handguard, you won't see a difference regardless of how rigid the tube of the handguard is.

IF, however, you are able to generate a few hundred pounds of force at the junction of the barrel and upper receiver, you will be able to mechanically bend that junction, causing POI to change. This amount of force can be reached with a sturdy bipod with the feet constrained, attached at the furthest end of a long handguard (leverage), with a heavy forward force (loading the bipod). Changing these variables can significantly change the effect on POI. This phenomenon DOES NOT effect systems that do not allow the user to input force to the barrel junction of the receiver (such as most modern bolt gun chassis).

Further, if you have aiming devices attached to the handguard, the mechanism/methodology of attaching that handguard to the upper receiver will directly effect the amount of shift incurred by a sharp blow to the handguard, such as that encountered when the gun is struck by or is intercepted by something during rapid movement, transport bumps and dumps, and drops. Handguard tube rigidity itself has little to do with this. As little as 10 ft/lbs of impact force will make a lot handguards shift a significant amount.

All of these things can be easily verified by the individual user without firing a shot. Just put a red dot sight on the end of the handguard 12:00 rail and rezero the dot so that when you look through your primary sighting device it is in a hard reference location to the reticle. A decent mid-range magnification optic that allows you to match parallax between the two and has some kind of measurement reference in the reticle is really good for this application, though most LPVOs and red-dots will have a 200 yard fixed parallax. Just apply different pressure to the handguard, and see where the dot goes, and by how much. You can also smack the handguard with a dead-blow hammer and see what happens. If you want to get really weird, rest just the barrel on a table and look at how much shift happens naturally in the gun when it's in your hands that you never thought to think about.

Enjoy.

[docsherm]

Anybody else want to play?

It is a real thing. Barricades, bipods, or simply adrenaline can cause a rail mounted front sight or laser to be off with a less ridged rail. I have seen this at night with lasers and poor quality rails.

Not asked but also with a rifle on a bipod with a lightweight rail and a PVS22 it will obscure the view like you do not have the proper eye relief on a scope. It will black out a portion when looking through it.

If you don't use any of those things then it really does not matter. People that simply have a optic and maybe a light on their rifle can use any rail they want. If you are adding something to the rail that needs to be sighted in then it really does matter.

This is why when people post a picture of there Reflex sight on the rail, and not the Receiver, the entire intern melts down and people freak out and start to YELL TYPE.

[Defaultmp3]

So back to my OP, was he demoing normal bracing of the gun on the barricade? Or was he intentionally inducing torque from the back of the gun while bracing the front to show that it’s possible? If it’s the second one, then I’m not sure why that became a “thing” since it’s not normal technique for barricade shooting (at least not how I’ve ever done it or been taught).

I’m asking to be educated here. Maybe I’m bracing wrong :confused:

ETA: I’ll have to disagree slightly with what you said though, it’s not just MFALs. If you deflect enough to affect an MFAL, it will be the same for a rail mounted front sight.It was the former, although he states that he jams his gun in pretty hard into barricades to help brace it, often against a WML, sling, etc.

I agree with the irons issue, but since it's so rare for people to use them these days on a free float tube that I don't see it as a big deal. Although now that I think about it, there are probably more of those out there than lasers being used, so... yeah.

Interestingly, my friend has noticeable POI shift when loading the bipod on his LPR, so the URX4 may have minimal deflection of the handguard itself, but seems like it could still cause issues?