Another Front Band Question

4137 Views 41 Replies 13 Participants Last post by M1AallTheWay, Jan 26, 2011

Discussion Starter · Jan 23, 2011

I did not want to steal the other guy’s thread, so I am asking this in a separate post.

I apologize for perhaps my ignorance, for I know there have seen many threads on “front band tension” but I still an unclear.

I have absolutely NO front band movement between the front band and the stock ferrule. No matter how I press (pressing upper band and lower hand guard together at the stock ferrule), there is no movement at all; the two are ROCK SOLID tight against each other. Again, ABSOLUTELY no movement up or down or side to side.

I have shimmed the gas cylinder so the front band has no movement on the gas cylinder, but the top of the stock ferrule sits against the lower bends of the stock ferrule (where the ends are bent to hold the upper hand guard in). There just is no place for there to be ANY movement, so that is where my confusion lies.

Reading posts (and http://m14tfl.com/upload/showthread.php?t=54400), it seems I should have pressure where if I squeeze the upper band and the stock ferrule together, there should be some movement with pressure. I must be missing something here, for the front band does not seem to have any gap within it where movement of the stock ferrule is possible. If I shim the stock channel, I’m not sure it that will really do anything.

In the link it says "Place a stand off spacer in the barrel channel midpoint between the rod guide and end of cyl." . I’m not real clear on what a "stand off spacer is (or what it would be made out of). I'm also not clear on how to do that, because I sure don’t want anything rubbing against the operating rod as it moves. It seems any shim would need to go under where the OP operating guide rod is, or under the gas cylinder itself, but not between the two. If so, what material would you recommend to shim?

Where do I have a hole in my head? (Please don't say all over :O) )

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M1AallTheWay

· #11 · Jan 24, 2011

GDavis said:
...It just seems that the front lower stock ferrule sits snug in the front band, squeezed between the lower band lip (lower area) and the bent “tabs” (on each side) that holds the upper foreend in the barrel band. The bottom “half” where the stock ferrule is fitted into, is tight. I just don't see how pressure down on the front band against the lower stock CAN move anywhere...

On a good day, there isn't a lot of room between the tabs on the front band and the top ends of the ferrule's "U" they sit near. Indeed, one of the steps in unitizing the gas cylinder is to bend those tabs up and that moves them up and away from the ferrule, providing more room for ferrule/front band lip movement. You're correct in thinking that until there's clearance, the rest of the discussion is moot. In comparing three front bands, two USGI and one SAI, I noticed two things. First, the tabs all seemed bent just a little differently and sometimes with the two sides different, and that could eat up what little clearance there is. Second, the lip is sometimes horizontal and sometimes angled slightly down. On the latter, it acts as a kind of ramp, so the closer the ferrule is to the back face of the front band, the further up the ramp it is, and thus higher and closer to the tabs. It's also possible that the ferrules are at the long end of their tolerance, but with three stocks acting the same way, I'd look at the front band first. (I vaguely remember Gus Fisher commenting that the holes for the barrel and GC aren't always exactly where they should be either.)

Also, it's possible that you do have tension there but without clearance to allow movement, the only way to infer it is that locking up the trigger guard pulls the heel down on the stock with some reluctance.

TheTinMan said:
...The trigger group clamps down immediately behind the receiver legs. In effect, the receiver-stock contact point forward of the trigger group serves as a fulcrum to put pressure on the front band / stock ferrule contact point when the trigger group is locked in place and pulling downward on the rear of the receiver. If there's no gap between the stock and the rear portion of the receiver, the pressure created by closing the trigger group can't put additional pressure on the front band//stock ferrule...

I've described this as a kind of see-saw, with your shims the fulcrum and the barrel moving up in front (and taking the front band lip/ferrule contact point with it) as the heel is pulled down in back by the trigger group, but your description is the way it works. If the heel is already down, the see-saw doesn't work, so the shims raise the heel, giving the see-saw room to rock.

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M1AallTheWay

· #17 · Jan 24, 2011

GDavis said:
...Also, if all this is about so the ferrule returns to the same position in the front band after each shot, why did they not build in a groove in the front band and a ridge in the stock ferrule so that the two would be locked together?

Well, actually they do, it's just kind of wide. The tension we're talking about serves at least a couple of purposes. When you shoot, the barrel will whip around (these high speed videos make the point well). Especially in rapid or automatic fire, the barrel is still in motion at the next shot, which screws up accuracy, so the tension on the barrel tames the movement. Secondly, because of the stuff hanging off the barrel, it can't truly be floated like a bolt gun or gas impingement AR. The curve of the front band lip and the curve of the ferrule match, and with that tension applied, the two curves are pulled together since when they align, the tension is at a minimum. IF the receiver is secured in the stock by bedding or a very tight fit, neither will move relative to the other, so the ferrule end of the stock always presents the same position for the front band lip (and the barrel attached to it) to return to, and it's the tension which provides the encouragement to do that.

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M1AallTheWay

· #19 · Jan 24, 2011

GDavis said:
Understood, and makes sense.

But- I still question that there has to be a gap so when you press the front band and the lower stock together, you feel the pressure and see some movement. Would not a tight fit of the stock ferrule and the front band do the same thing (dampen any vibrations)(assuming there is upward pressure)?...

The gap is there because the barrel is going to whip regardless and this way it has some breathing room. More important, the curve of the front band lip means that if the barrel is to move sideways during its whipping, it's got to follow that curve, thereby temporarily increasing the tension (if the stock and its ferrule stay put, as the ferrule rides up the curved side of the lip, the barrel is bent slightly down [the stock will likely bend up a bit too, but either way, the tension increases]). That self-centering couldn't work if the parts were locked together. And a side benefit is that by squeezing, you can feel the tension if there's room to squeeze.

BTW, what I think Art was describing was a way to allow the barrel to establish its rest position and then let the front band adjust itself to that before locking the front band and the GC together. The self-centering works best if there's no sideways tension caused by a misalignment of the ferrule and front band lip when both are at rest without interference from the other.

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M1AallTheWay

· #27 · Jan 25, 2011

GDavis said:
...What do you guys shim with?

You'll find a discussion here dealing with shimming the area that'll increase tension as well as giving a snugger fit of the receiver in the stock, along with results reported by an impartial experimenter. I'd suggest reading through the thread since the sizing of the part of the shim that provides the tension is different from what's shown in the first pictures.

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M1AallTheWay

· #29 · Jan 25, 2011

bunsen27 said:
...I've gather from reading this thread and others that one issue may be the fit in the stock. I've relieved the stock somewhat in the front where the gas cylinder was rubbing. Any help or suggestions would be greatly appreciated. I can post pictures of specific areas if that would help. Thanks.

Improvement in accuracy comes from a combination of many modifications working together until the inherent accuracy of your rifle is reached (recognizing that sometimes you just can't get there from hereGI4). The fit in the stock is important because keeping the receiver and stock tight helps the front end of the stock to always present the same position for the barrel to return to. Bedding is the recognized way to do that, but check out the link in post #27, which presents another technique that has proven to work for some TFL members.

One simple way to check for that tight fit is to grasp the GC with your fingers, with your palm resting on the stock and ferrule. While watching the heel as it rests on the stock (focus on wood grain or paint imperfections), push the ferrule area sideways with your palm. If the fit of the receiver in the stock is not tight, you'll likely see the heel move over the stock. That's what bedding is intended to eliminate.

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M1AallTheWay

· #32 · Jan 25, 2011

bunsen27 said:
Thanks for the feedback. What would the status of the TG be for this test? Locked in or just cammed over?

As I mentioned previously the TG on this rifle locks up extremely tight so I'd be surprised if the heel was able to move with the TG locked in...

The test would be done with the TG locked up. (BTW, an easier way to lock up is to use something like this to pull the heel down on the stock before you close the trigger guard.) The basic idea is to prevent the receiver from rotating sideways in the stock, even a little bit. Unless it's rear or double lugged, the only parts of the receiver that are in contact with the inside surface of the stock are the outer faces of the receiver legs and the section just forward of the legs. If the fit there isn't extremely tight, a small amount of rotation is possible (which is what the test of movement of the heel over the stock is designed to disclose). And if there's a little rotation at the receiver, imagine how much displacement there is way down at the end of the stock/barrel contact point (front band lip/ferrule). The tension intended to guide the barrel back to it's rest point on the ferrule would be guiding it to a different place every time, not a good thing.

Even if the heel is really tight against the top of the stock, if there's some slop between the legs or the section forward and the stock, you can still get a bit of sideways movement there and a lot at the ferrule.

Using shims to raise the heel even further off the stock would make locking up even harder or maybe impossible. But using the "L" shaped shims just inside the stock would tighten the fit without tightening the lockup. In any case, thinking of that tension as part of the "see-saw" motion has the ferrule end of the stock being pulled down (creating tension) as the back end of the stock is pulled up to meet the heel. Since that's what's happening to you, I bet you've got the tension, just not the clearance so it can be demonstrated or operational.

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M1AallTheWay

· #34 · Jan 25, 2011

GDavis said:
Great info (and a lot of reading). It talks about aluminum and steel not being good together, and "paper" (match box) can retain moisture, what is your thought on "plastic" shims? My thinking is that you would be able to heat to a point where they are pliable / shapeable and would not retain moisture. Not sure of examples of plastic items than could be used though.

I'd be a bit concerned that a plastic which could be softened enough by heat to be workable might be softened on a hot day by a hot receiver.

I was surprised to learn about the potential problem mixing steel and aluminum, but I've been using the combination for over four years now and haven't seen any ill effects at all. YMMV.

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M1AallTheWay

· #37 · Jan 25, 2011

GDavis said:
...One post says the problem with aluminum and steel is with moisture between the two. Have you poped up out of a lake/pond/river lately and surprised your target? That may cause a problem :O)

Can't say that I've ever gone swimming with my M1A, but if I had to, I suspect my troubles would extend well beyond a dissimilar metal interaction and would move a lot faster, too.GI4

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M1AallTheWay

· #42 · Jan 26, 2011

bunsen27 said:
I think the light bulb just lit over my head reading your explanation and re-reading the shimming thread. I may even have comprehension. Thanks for the detailed explanation.

Glad to help. The key idea is that it takes very little displacement at the muzzle to change the POI significantly, e.g., for an 18" barrel, about 0.006" movement at the muzzle changes POI about 1 MOA. So anything that can encourage the muzzle to be in the same place relative to the receiver after each shot without otherwise interfering with functioning is a good thing. Note that with iron sights, when the muzzle moves, the front sight does too, but with a receiver mounted scope, the scope points where the receiver points.

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