"Vertical fore-grip torque" issue on ARs...


Shooting at the big range in heaven
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Jul 30, 2005
Metrowest, MA & Points South and West
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Source found on "vertical fore-grip torque" issue on ARs...

I found the source of the rumor going around of vertical foregrips causing torque, which in turn, caused bolt-locking lugs to break.

It was in the June, 2006 Shooting Illustrated magazine in the section where readers may query them at: [email protected].


“ M4 Overload


I have heard a significant number of M$ Carbines being used by our troops in Iraq and Afghanistan have been breaking their bolt-locking lugs. Why is this happening? The M4 is based on the M16 system and has been around for decades without this problem.

- Jan Plaster, via email


Unfortunately, many M4 Carbines have experienced broken locking lugs in recent action overseas. Initially it was thought to be a manufacturing problem, but further investigation has found the real culprit.

The M4 has been fielded to many units with an integral-rail-handguard system that allows the mounting of a wide variety of accessories like laser designators, flashlights, red-dot sights and vertical fore-grips. Some have turned a light, handy weapon into a veritable Christmas tree of accessories, with the accompanying bulk and weight. All of this extra weight can cause the barrel and barrel extension, into which the bolt locks, to droop in the receiver slightly.

A particular offender is the popular vertical fore-grip. Shooters typically pull this grip solidly to the rear with substantial leverage that loads the barrel and barrel extension in the receiver even more. As the barrel and barrel extension yield to this pressure, the locking lug seats in the barrel extension are no longer vertical. When the locking lugs go into battery, only the top lugs are engaging. The bottom lugs are not touching the lug seats at all. Even the top lugs are only engaging at the end of the lug. Those lugs are receiving all the rearward thrust on the bolt, and that thrust is well beyond their design and capability. The result is the lugs are failing. “


Here are some source links to this topic:


Thanks for this info. I have never been a fan of vertical foregrips but I never dreamed that they might be bad for your rifle.

P.S. Those links at the bottom are broken.
Davidk said:
Wouldn't a free float rail solve that problem?

Until you start bending the rail tube and causing problems in other areas.

The problem (from my reading of the article) is the manner and direction in which the force of the shooters hold on the rifle is transmitted to the various components. On a regular forearm the shooter is supporting the rifle and the force is parallel to the axis of the barrel/forearm grip. But with the vertical grip the force generated by the shooters support hand is perpendicular to the same axis which produces a much more pronounced bending movement. The harder you grip the rifle and the vertical grip, the more force you apply in this bending movement.

Perhaps a free float rail would work better by isolating the barrel and bolt components but how would this arrangement stand up to combat conditions where you might have a literal death grip on your rifle?
This seems more appropriate in the Firearms, Equipment and Ammo forum, so I am moving it there.
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