Parallax Tactical FFSSR Rail Brings New Meaning to the Term "Free Float"

AR/M4 AR General Discussion
1

So I thought I found the perfect rail for my current build and more important one that works with my Mega billet side charging upper. Finding a rail (to my liking) without ant-rotation tabs that will work with the billet upper proved to be somewhat of a challenge.
So I ordered the 13” Parallax FFSSR rail and first hand impressions were positive with regard to ergonomics, finish and rail placement.

So I get out the Mega upper, the barrel and slid the barrel nut down to the receiver and hand tighten. At this point I’m just trying to get a feel for how this set-up is going to look. I slide the rail down over the barrel nut and immediately noticed quite a bit of slop between the nut and the ID of the rail. I measured the deflection at the front of the rail and recorded a full 3/16” of play (moving the rail back and forth between the 3 and 9, & 6 and 12 o’clock positions). The amount of deflection was an immediate concern. It just felt way too loose.

Removed rail and nut and get out the mic and caliper. Barrel nut measured 1.3445” and the rail ID measured 1.360 (min) to 1.365 (max) yielding 0.0155” of clearance between the barrel nut and the smallest portion of the rail ID.
The rail uses 6 screws at the 3, 6 and 9 o’clock position to fasten the rail to the barrel nut (2 screws at each clock position). IMO: With this amount of compliance/slop, tightening down the 6 fasteners is just going to stretch the rail, the fasteners, or both. Furthermore, the screw holes in the rail are elongated (front to back) to accommodate installed variation. With sufficient force those elongated screw slots will also allow the rail to “pull”. Now place a bipod on the front end of that rail, 12” away from the attaching point (barrel nut), mechanical advantage/leverage will continue to pull and stretch on those 6 points. These are my thoughts anyway but maybe I’m being too critical.

So I called Parallax and explained my findings and concern and I was sure the barrel nut OD or the rail ID were out of spec and this may have been a simple stack-up issue. The person who answered the phone didn’t have any specs available to him. He said to give him a minute and he’ll go measure some parts. A few minutes go by and he comes back with a barrel nut OD of 1.34”. I stopped him right there, I asked him , “1.34” what? Do you mean 1.340” exactly?” He said yes. He goes on to say the rail ID measures 1.36” and also goes on to say that they sell thousands like this with no issue. So I asked him if he thought 0.020” of an inch of clearance was acceptable to which he answered yes. He said just tighten down the screws, it will be fine.

My instinct and experience told me this was too much clearance. So as a benchmark I pulled out a brand new Wilson TRIM rail and a brand new Diamondhead rail.
Wilson: Barrel nut 1.350”, rail ID 1.354” yields a clearance of 0.004”
Diamondhead: Barrel nut 1.3605”, raid ID 1.365 yielding a clearance of 0.0045” (keeping in mind the Diamondhead rails is slit allowing it to compress around the barrel nut when tightened down)
A far cry from the 0.0155” of free play in the Parallax FFSSR components.

There are other issues as well. The flats across the barrel nut measure 1.232", which is 0.018" short of 1-1/4", yes the wrench was quite sloppy. Oh not to mention the wrench flats on the barrel nut measure 0.483" wide so the crows foot would not slide onto the nut without modification.

So, am I being too critical? I welcome your thoughts on their specs.
One thing is for sure, it will not get any tighter.

BTW: My first post on this forum.

2

Pretty scary yet not surprising. I’ve installed a few of these for people (against all my recommendations) and no offense but many consider them near the absolute bottom of the pile. The ones I installed were no different. Absolute shit rigidity and tolerances that seemed all over the place. Tons of deflection in the rail itself.

That’s more than 10X the amount of play than I want in a precision rail/nut fitment and 15X more than some of the best rails on the market right now.

I’d never consider the Parallax rails for anything in this lifetime.

3

Send it back for a refund and try to find something else that works with your upper. If you can’t find a quality rail that will work with it, you may want to consider a different upper that will give you more rail options.

4

None taken, I only wish I had come across a post or review noting their lack of quality prior to purchasing.

Obviously they have been crossed off my list.

5

For Sure

Looking at a longer Wilson TRIM rail but I’m strongly considering the KAC URX 3.1. My issue with the URX is not so much with the rail, it is spending another $100+ for a wrench.

6

Parallax Tactical is a stand up company. I have ran some of their product on my AR hard and it has held up. I can imagine the bitter taste your experience has left but I would ask to speak to Joe the owner of the company. He will make it right, he’s very hands on. From time to time as you well all know companies screw thing up.

Good luck, and keep us updated. -Geo

7

Really! I have called them twice about this issue (Unfortunately I think I talked to the same person twice) and I was greeted with the same response both times, in summary, “Just tighten it down, it will be OK”. Uh, no it won’t.

I’ll call him, give him that opportunity and post the results.

8

You are bitching about 15 and 1/2 thousandths correct?? I think your worried about nothing. That is the thickness of 4 pieces of cheap paper. The thing is designed to clamp on. I’ll bet if you snug the bolts up to half the recommended torque it won’t move. GH

9

You might want to look at the KMR rail. The anti rotation tab is separate from the rail. You wouldn’t have to use it. GH

10

0.0155" of clearance to the smallest ID but it is 0.020" to largest ID. 0.020" across 2 inches (length of barrel nut) is an issue. You say 4 sheets of cheap paper, do the math, that’s 34 minutes of angle. At 100 yards that translates to a POI swing of 34". Try dialing in a laser or BU sights on the front end of that rail when it starts moving around. Your POI will never be the same.

Sorry, strongly disagree.

11

are you just speculating that it will move, or you 100% positive?

12

Are you suggesting the excess clearance will some how magically disappear, the fasteners will never loosen up and the aluminum & fasteners will never stretch under load? Take a look at how thin the material is where you fasten the rail to the barrel nut.
BTW: Did you read the reply posted by JChops?

13

I have already spoken to OP on the phone regarding the rail. The conclusion is that we will replace the rail to something with a smaller ID to his satisfaction. This post is just here to answer some questions and clear up some assumptions along the way.

  1. The barrel nut diameter is designed to accommodate the expected variance in extrusion inner dimension sizes. The ideal size of the barrel nuts are 1.346". The ideal ID of the extrusion is 1.35". This leaves a four thousands gap, ideal according to OP. (However this number varies widely across the industry. There is no “ideal” distance as long as the system used for mounting by the mfg takes this into account. Sometimes, the extrusion will be wider than our ideal tolerance, when we start a run we try to take this into account to produce rails with as close as possible rail height to receiver height to match. (I forgot to explain this to the OP in our phone call.)
    Now, why is that? Extrusions are hardly ever accurate. The standard aluminum extrusion association tolerance is something like five to eight thousands of diameter on ID. A diameter tolerance of one thou is impossible on a extrusion unless the ID was machined. (The OP later explained that he would like this ideal, but we do not machine the ID of the rail.)
    To correct that, all the critical dimensions are machined on the OD while holding on the ID, leaving tolerance as low as possible. Occasionally the extremes will allow a tight barrel nut. (This was the original response on Sunday, before I had a chance to read OP’s post.
    We’ve since redesigned our fixtures to locate even more precisely on the ID of the extrusion to bring variations down even more. (This is still true, we’ve reduced our TIR by 50%.)

As for the barrel but, it was purposely designed at 1.23“ flats.
Why? This is the average of the width of standard GI wrenches. We’ve since given up on having end users use existing wrenches and bbl nuts are using 1.25" flats. The undersize is not a mistake.
We’ve since redesigned our BBL nuts to use 1.25" flats as there was too much variation from armorer’s wrench to wrench. Our drawing shows a width of .5" on the newest design. The old design had a small width, so OP has a legacy BBL nut.

Another few things from the phone call with OP and other answers to some questions:

  1. We changed our BBL nut threads purposely to 10-24. We’ve had reports of stripped screws from the 10-28 design from end users over torquing the BBL nut screws to the point of stripping.
  2. The method of measurement by the OP is based on the bbl nut and rail being concentric. It’s an easy assumption to make. Here is a drawing to illustrate: rJKM004.jpg
    By measuring this way, the variance is smaller.
  3. The OP commented on the size of the screw shelf. This is done on purpose to make sure the rail can “clamp” onto the barrel nut. It has been simulated in Solidworks for strength. It is plenty strong with a large safety factor.
  4. All rails do this to some extent from their design, e.g. BCM KMR (top screw clamp), Troy bottom clamp series (deflection of over ~.08"!), most bottom clamp rails (Samson, etc.)
  5. The rail, screws and barrel nut will not leverage/stretch the actual bolts and barrel nut to the point of deflection to the extreme OP has theorized. However I will mention that ALL free float rails deflect to one extent or another, just not due to the mounting system that we use. This is inherent to all free float rails.
  6. The rail is machined with the assumption that the bottom screws are BDC (bottom dead center) in the actual attached barrel nut, and the corresponding rail height is adjusted to match the reference upper receivers we actually have on hand. Theoretical is awesome, but real life gets in the way.
  7. The reason we give out torque specs and with the torque pattern is to make sure users: 1, Tighten the screws to the correct torque (over and under) without stripping 2, Ensure enough pressure it placed onto the six mating surfaces to properly engage the barrel nuts for a mechanical fit 3, Ensure the screws do not back out and balance themselves. This way the rail and barrel nut will properly position themselves and eliminate as much variance as possible. (A better answer to “just torque it and it will fit.”)

I’ve agreed to cherry pick and send OP the best fitting bbl nut and rail we have in inventory. I hope this post clears up some misunderstandings about our design. I would also like to publicly apologize to the OP about having a hard time getting someone on the phone to answer his technical question on the rails. We are a full service gun store, and apparently he called into our retail parts department. The retail clerks do not have any knowledge of what the OP was asking for (as it relates to a technical matter that our mfg dept handles), I will issue a memo to them to forward any calls of that nature to me or to take a message and I will call personally.

Thank you,
Joseph Chen

14

OP i would not even consider trying to work anything out with parallax while expecting good customer service. Save your head ache and use that rail for target practice and spend the little bit more money to get something from a reputable company. I tried to like these guys but every time I have tried to do business that involved anything more than a simple purchase it has been a nightmare.

15

Parallax agreed to make it right with the OP, why dont we wait until he updates us before bashing a company that (seems) to be doing the right thing and making the customer happy.
FYI I have no affiliation with Parallax or the OP and I do not own any products from parallax. I just respect that they agreed to do what they can to make it right.

16

That’s what they told me via email when they jacked up my new cerakote job when I had gunsmith work done by them when their own barrel from black hole was having feed issues. After his own troubleshooting and telling me they fixed it after two weeks, I went to pick it up. I made them show me it was fixed. Turns out same issue was there. Nice astonishing look on your face joe like it was even test fired like he said. I had twist his arm to swap the barrel with a new one. Two weeks later after that it was finally ready to be picked up. They also scraped up my new gas block that they suggested it get realigned and ended up replacing it with their own in which I had to pay for. After joe told me via email that they would credit me the cost of the gas block because I could no longer return it (which I made clear I would do), he then told me when I finally got him on the phone that he would not do that and if I wanted it recerakoted I needed to get it there that afternoon. It was 2pm. Thanks for letting me know. They never would respond to my emails after telling me they would make it better. When I called and talked to joe even after several emails, he pretended he had no clue who I was. Parallax can suck it

17

So I emailed Joseph on the 28th after he posted his response and pointed out a few a “assumptions” of his own. He replied to my e-mail saying he would address these points the following day which he obviously did not. Here’s what I addressed to him in e-mail. BTW: The complete rail package has been returned to Parallax, to Joseph’s attention. Delivery scheduled to arrive on 5/5/2014 per UPS.

A) I received an out of spec rail
B) I never stated that .004" was an Ideal clearance. I only stated my findings relative to competitive product. In my e-mail to you I stated .001" clearance max and in conversation I stated the ideal clearance would be ZERO, at which point fasteners simply become retention and anti-rotation devices.

This is your issue because you choose to use an as extruded ID. If you machined your ID this wouldn’t be an issue.

You’re the one making assumptions. Here’s the CAD math based on your “actual” illustration. When we spoke on the phone I was specific about clearance quoted at the 3 & 9 o’clock positions.
As you can see the clearance or variance as you put it (at the 3 & 9 o’clock ) doesn’t get smaller because the barrel nut is now sitting lower than the largest ID of the oval shaped rail. Your still >0.015" of clearance (.0077" x 2) at the clamping positions and 0.0185" of clearance at the 11 o’clock position + another 0.002" at 5 o’clock for a total of 0.0205" of clearance across the larger ID.

Your comments in point 5 is very misleading and simply not true. Your point about the screw shelf thickness in your point #3 proves that. You state the thickness of the screw shelf is made to yield (with 30 in. lbs, your torque spec) for effective clamping force.
But you go onto to say that the rail will not move due to leverage or external forces. If the shelf is made to yield for clamping force (30 in. lbs), it will yield to external forces and deflect. That’s not theory, that’s real and you won’t need strain gauge to measure it.

I never said the barrel nut will leverage or stretch. Please read again carefully, I focused in on the 6 attaching points and the material. Fasteners as you know are nothing but clamps made to stretch, that’s how they maintain clamping force.

18

He gets the rail on Monday so he has next week to make good on his word.

19

Mr. Parker,

We appreciate that you are obviously passionate about free float rails and engineering. I am having my staff draft a response to your statements with some data from our testing analysis. Hopefully this will put this issue to rest. Please note this will take some time.

In addition, we’ve reviewed our drawings for the barrel nut and the extruded ID of the rail and made some calls to our extrusion house and BBL nut lathe guys in order to try to get the tolerance range down as close as possible without having the interference fit issues we’ve had in the past (too tight.) We do appreciate you pointing what you think is right/correct, however please note that in every design there are reasons behind why things are engineered certain ways. In our phone call, it seemed to me that you felt that there was only one “right” way to do something, we respectfully disagree. I will check on the status of your return when we re-open tomorrow.

20

UPS Tracking Information
Delivered On: 05/06/2014 1:14 P.M. Delivered To: SAN DIEGO, CA, US Signed By:CHEN ANDY

San Diego, CA, United States 05/06/2014 1:14 P.M. Delivered
05/06/2014 6:00 A.M. Out For Delivery
05/06/2014 4:59 A.M. Destination Scan
San Diego, CA, United States 05/05/2014 9:08 A.M. The receiving business was closed. / Your delivery has been rescheduled for the next business day.
05/05/2014 3:50 A.M. Out For Delivery