Mounting things more to the rear or front of the rail slot?

AR/M4 AR Technical Discussion
1

This past weekend I let someone shoot my M4 as a way of introducing that person to the weapon.

The person wanted to shoot the rifle with the carry handle sight, so I reluctantly un-assed the Aimpoint T1 and LMT BUIS (I plan to put the Aimpoint on a QD mount so it was coming off in a few days anyway).

I mounted the carry handle all the way forward on the rail (flush with the front of the upper) and tightened the thumb nuts down as far as I could by hand, and then with a partial turn using a quarter. I know everything was tighter than Dick’s hatband when I finished.

At the end of the range session, I noticed that the carry handle had backed off just a tad. Clearly, it was still mounted and still as tight as I made it, but the carry handle had obviously shifted rearward maybe 1/32 or 1/64 of an inch, just enough to expose a tiny bit of the front edge of the upper receiver. In other words, I mounted the carry handle flush with the front of the upper, and it backed off just a hair.

I could swear I remember reading recommendations that a mount be situated as far forward as possible because the final recoil impulse has a forward momentum and this should keep the mount from shifting. Clearly this did not happen.

I also have performed a couple of searches on this forum and I’m either mistaken about what I read or my search criteria just didn’t pull it up.

It’s kind of a moot point, because I’ll be going back to the Aimpoint anyway when I get the QD mount, but now my curiosity is up. And I figure the same physics are happening to the Aimpoint’s mount.

Should I loosen and re-tighten the carry handle mounted all the way forward and shoot it again to allow things to “seat” better?

Or has it shifted to its natural position (still good and tight), and I should just leave it alone?

2

Strange. Is it possible your handle got bumped? Or maybe it was the return stroke of your bolt throwing it in the opposite direction? H2 or H3 buffer?

Just an amateurs speculation.

3

IIRC - Pat MacNamera broached this subject in the class that I attended, and his recommendations were (as I said, IIRC) to pull the attachment to the rear of the rail slots before torquing down. He did explain the why, but I have since lost that info.

Having said that, I have not noticed any movement on my Aimpoint PRO, and I did no more than fitting it on the rail (neither full forward or full rearward in the slots) and tightened the knob the recommended 3 clicks.

4

My bad memory also remembers F2S saying to tighten it all the way forward. Hopefully he will come by and correct me if I’m wrong.

Sent from my DROIDX

5

I used to notice the same thing with carry handle sights. It may happen with others but it is more obvious with the carry handles i guess. The only thing I could come up with was that it was happening when dropping the bolt on a empty chamber. I suppose that would give the gun a sharp jolt to the front and all accessories due to weight and inertia or some crap like that would travel in the opposite direction. The opposite would be true when shooting the gun due to the forces from the muzzle proppeling the weapon rearward. I can only guess that the force of the bolt going home is slowed by picking up the round from the mag and chambering it. Perhaps this is enough to not cause shifting that you see when dropping the bolt on an empty chamber. Again, this is me guessing what may be going on.

Question is, was the bolt “dropped” on an empty chamber when you were done shooting before noticing the sight?

6

I’ve heard this for a years now. Kyle Defoor talks about pushing it forward as well. I’ve been doing this with all my optics and irons and have never had one shift.

Now, the Aimpoint 3x twist mount base, no matter how tight I get it, shifts eventually.

7

Move it back forward. This just happens sometimes. I run a lot of carry handles, and every so often I’ll get a shifter like this.

8

So if it shifts to the rear and then stays there what difference does it make?

9

The normal way I have been taught to mount anything to a recoiling firearm is to ensure that it is seated forward.

That being said, drift can and does happen. Setting aside a looseness issue, if the mount is tight, where it stops moving is the right place for it, though in my experience a properly tightened mount that is mounted forward into the lug won’t move. The problem with this is that “properly tightened” can be a hard one to nail down, and (much like barrel torque) the user should be mounting optics with a torque device unless he has reached the level of experience that allows him to go by feel. To further annoy the user, more than a few mount manufacturers tell the user to mount with insufficient tension. The carry handle knob type tensioners blow. I have seen numerous (as in, literally, too many to count) “professional” users mount them with the recommended level of torque, only to have them shoot loose, or shoot off. Further, they need to be loctited, not just dry-mounted. This applies to the Trijicon TA51 mount as well.

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10

I am very interested to hear Pat’s reasoning for this.

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11

You should list it on the Equipment Exchange immediately.

12

LOL! I knew that was coming.

To summarize:

I have never let the bolt drop on an empty chamber except for a bad magazine that did it on its own several months ago.

I was using a carbine buffer in a 14.5" (pinned to 16") with very mild PMC bronze 55-grain .223 (has reliability problems with an H buffer).

I tend to agree that for whatever reason if the carry handle shifts to a different position and stays, then leave it there. It’s probably achieved its own “fit”.

I would like to know more about this.

I personally hate loctite on anything because I keep changing my mind. :smiley:

As for “what is tight enough” I think I achieve “snug” but not “overtorqued” most of the time (except for a Daniel Defense light mount where I stripped the screw threads but we won’t talk about that). :smiley:

13

In the interests of full disclosure I am required to report the following.

I decided early this evening to unscrew the carry handle and reattach it fully forward as I did originally.

Understand that both thumb nuts (knobs) were beyond hand-tight. I could not budge them with just my fingers.

As I unscrewed them with the edge of a quarter, I noticed that even though the rear thumb nut required a bit of effort, the front one broke with minimal torque. I emphasize again that both thumb nuts were beyond hand tight, but the front thumb nut was noticeably easier to “break” than the rear when I attempted to loosen it with the edge of a quarter.

I moved the carry handle all the way forward as it was originally, and retightened both thumb nuts with the edge of a quarter to the point where my fingers literally did not have the strength to go any further even with the coin.

Unfortunately, with ammo prices lately, I may not go to the range again anytime soon to verify that the “shift” in the carry handle was due to inadequate torque on the front thumb nut originally, but that’s my thinking.

I have to allow for the possiblity that the recoil impulse has something to do with it, and here’s why.

My 14.5" carbine with pinned flash hider and carbine buffer has a noticeably stronger recoil impulse than my 16" carbine with “H” buffer.

That could be a factor, too.

It’s kind of funny, but when I shoot full power 5.56 ammo in the 14.5 carbine and the H buffer, the recoil impulse is less than using mild .223 in the same carbine with a standard carbine buffer.

As Ripley would say, “Believe it, or not.”

So, my take?

Until I get another range session or some enterprising member decides to experiment with my info, I’m thinking that the front thumb nut was just barely not torqued enough to allow movement.

The other likely possibility is that if you mount an upper with recoil characteristics that suggest an "H’ buffer with 5.56 ammo, you may experience loosening of your flat-top accessories when switching to .223 ammo and a standard carbine buffer.

If anyone is able to offer any other analysis, I’d love to hear it.

14

Remember that Loctite 242 or 222MS are easily overcome with standard handtools and no heat needed. It’s no way permanent, but does keep bolts and nuts in place.

15

The whole “1/4” to “1/2 turn past finger tight” is where the problem is. Despite what the makers of those mounts want you to believe, it isn’t enough. How dare I say this? Because I have seen many many many mounts tightened as advertised come loose. It isn’t the gun (I’ve dealt with this issue on guns that recoil a hell of a lot more than any 5.56 gun), it’s the mounting.

So what to do with these troublesome mounts?

First, get some blue loctite.
Unscrew your knobs as far as they will go.
Hit the threads with non-chlorinated brake cleaner and let dry.
Place a generous dab of loctite at the termination of the threads where the threads pass through the angled tensioning plate.
Pressing forward on the rear of the sight base, tighten knobs by hand until tight.
Grab a pair of pliars/leatherman/BFS (big fucking screwdriver).
Tighten at least 1/2 turn further than you were able to do by hand.
Personally, I go 1/2 turn further, and then continue on until the slot is either perfectly horizontal or vertical.
Take a paint pen and make a witness mark that crosses both the knob and the tensioning plate (this is a way to quickly check to see if the mount loosened).
Let loctite cure for at least 48 hours.
Go to range and shoot. After 100 rounds or so, check for drift/looseness, check witness marks, and try to tighten mount with firm handtool pressure.

I have been doing it exactly this way for several years, and I have never had an issue since.

16

Just to echo what was mentioned all optics magnified or red dot get a push forward in the slot before tightening. Again, general rule is that thumb nuts require more tightening than advertised.

17

Well, somebody call Myth Busters and have them do an episode on the matter (full forward or rearward in the slots) to settle the issue.

F2S probably has the ultimate answer - make sure the damn thing is tight and won’t come loose regardless of location.

18

I agree with everything, but I sub Vibra Tite instead of Blue Loctite.

It’s designed to pre treat fasteners, and is set up in about 5-10 minutes depending on the humidity and temp.

I got hooked on this stuff after getting the blister pack that comes with GearSector Light mounts.

19

Quite! Boils down to, when folks jam out a manual or instructional material, and want to do it in such a way as something technical like an actual torque spec is defined, some noodnik will pipe up with something like “Well, is this user issued or have access to a torque wrench…?”

Liability concerns are broached, arguments ensue, fisticuffs break out, Hatfield-McCoy type feuds go on for generations, because nobody wants to publish 2-3 different ways to achieve the same end under the guise of not confusing the reader…the whole “The Way vs. The WAYS (plural)” partisanship thing.

Ultimately, the hand-wringing worryworts seem to win out more often than they truly deserve, and we get crapy, soupy compromise instructions like “1/4 to 1/2 turn past blah blah…” So, something deliberately undefined, based upon something that varies from one person to another pretty significantly. Do some technical writing and curriculum development, you’ll end up sitting in these meetings and get equally amazed/mortified at how entrenched the Poindexters are.

Anyhow…pushing forward on the rail is what makes sense from a sheer physics standpoint, seems to work in 9.99 out of 10 iterations. I’ve heard of folks doing the rearward push, but I wouldn’t dismiss it without getting the “why/how” explained to me. Ultimately, if the rail is in spec and the interface is made to fit the rail, it should stay and the whole pushing-forward thing is mostly the same kind of insurance that staking is (effective, takes but a moment, virtually fool-proof, short of being performed by the legendary Sufficiently Talented Fool…)

I’m not a big fan of having to shim a rail, but when the conventional wisdom ain’t cutting it, doing something unconventional is hard to argue, especially if it uses commonly-available stuff and is easily reversible: If the rail the thing’s mounted to is JUST out of spec enough for one of the mounting interfaces to not engage as it should, putting a layer of tinfoil down might provide what’s needed to get it to mate as it should.

20

FWIW, The Troy sight I just installed… directions indicate, push forward and use blue thread locker.