Vltor VST-1C Folding Front Sight Assembly Torque test

7/30/07 - I previewed Vltor's VST-1C Folding Front Sight Assembly on the M4K and on the midlength VIS build, back in January '07 and they're now available. I've since installed another on an M4 barrel with carbine gas system and I really like them.


The question of the security of non-pinned gas blocks has been brought up numerous times. Factory-pinned front sight blocks have taper pins, which secure the gas block/front sight assembly to the barrel. Aftermarket gas blocks in general come in clamp on, and set screwed types. This is because it is next to impossible for the end user to re-drill taper pin holes in a blank gas block to match ones already present on a barrel. While taper pins can be installed in a blank gas block and new barrel, this is a gunsmith-only installation requiring the right tools and equipment. Set screws work by 'digging' into the surface of the barrel when tightened, or pre-drilled depressions or steps machined into the barrel. Barrels with these are still relatively uncommon, but available. If you look at the cross section of a gas block as a ring, set screws push the ring away from the barrel at the set screw locations. Normally, they draw the gas block down, to form a tight seal over the gas port. However, you have reduced surface area of contact - and a knock or torque to the gas block powerful enough to dent the metal underneath the set screw can possibly cause the gas block to loosen up. For that reason, set screw gas blocks are probably better left to low profile ones protected under a rail or handguard, rather than exposed ones with front sight towers.


Clamp-style gas blocks/front sights like the PRI and the Vltor have a split-ring cross section - there is a gap at the bottom of the clamp with two clamping screws that tighten the gas block around the barrel. This ensures maximum surface area contact between the gas block and the barrel. While theoretically not as secure as a taper-pinned gas block, I wonder if concerns about the security of clamp-style gas blocks are for the most part exaggerated. The split-ring clamp configuration is a very old, extremely secure design used in countless applications. Motorcycle triple clamps are one example - which secure the front forks to the steering head of the frame. And these are on very smooth polished surfaces, not relatively rough surfaces like a parkerized or bead blasted barrel. The key, as with all mechanical things, is proper installation (correct hardware torque and a clean surface) to prevent failure.


Additional security - The Vltor VST-1C has small divots on the gas block body to the side of each clamp screw ends. After installing the screws with high-temp non-permanent loctite and torquing them, the ends of the screws are center-punched/staked, which drives some of the screw's metal into the divots. This prevents any rotation and loosening of the clamp screws due to vibration or heat cycling. They can be removed with tools but will need to be replaced.


Still, I was curious how much torque or force it'd actually take to move a clamp-on gas block.


VST-1C


At the range
Torque Test - Since the Vltor VST-1C has provisions for a QD sling swivel, some people have brought up concerns that attaching a sling to the gas block might cause it to rotate, thereby rendering the weapon inoperable. I asked the guys at Vltor if they were willing to perform a test that would quantify the force needed to move/rotate one of their gas blocks and they were open to it. An impact test would simulate a sharp knock on the gas block, such as banging it against a metal door when exiting a vehicle. However, an impact test requires a specialized instrumented setup which was unavailable and is much more complicated to set up as there are so many more variables (what the gas block hits, how fast is it moving, how much weight if behind it etc). So we decided on a torque test, which was measurable and relatively easy to setup. This would provide a number which some enterprising individual might use for further calculations (not me) if they wanted to. We also wondered the sight block would move before damage to the barrel or receiver would happen.


The test, as performed and written up by Vltor is as follows:


Vltor VST-1C Development Testing: Security of clamp mounting system.


The Vltor VST-1C folding front sight was tested to determine the amount of torque needed to cause the sight to rotate, when mounted to the barrel. Testing was unable to determine the torque measurement, as the upper receiver and barrel indexing pin failed before any sight movement.


To conduct the test, a Milspec A4 style upper receiver was assembled with a take-off M4 heavy barrel. A standard barrel nut was torqued three times to 30 f/lbs, then torqued to align the first gas tube notch after a torque wrench reading of 40 f/lbs, the gas tube was not installed.


The following comments correspond to the numbered photos below:


1, 2 - This upper receiver and barrel were clamped into a clamshell type holder.
3 - A production release Vltor VST-1C sight was taken apart, and the sight base portion was modified by welding a barrel wrench extension to it.
4, 5 - The standard fixed sight tower was removed from the barrel and the surface cleaned, the cuts for the original cross pins were not filled in. The modified Vltor VST sight base was installed and the clamp bolts torqued as per the installation instructions.
6 - A witness mark was made on the barrel and the front sight base.


7 - A torque wrench was used to turn try to turn the barrel in the same direction as tightening the barrel nut. There were four test torques conducted with increasing break point of torque. The results were:


30 ft-lbs  Some flexing of the barrel, no noticeable rotation.
50 ft-lbs  Noticeable flexing of the barrel, no noticeable rotation.
80 ft-lbs  Dramatic flexing of the barrel, some slight rotating of the barrel.
100 ft-lbs  Dramatic flexing of the barrel, permanent rotation of the barrel.


All torque readings were set on the wrench scale; actual torque values would be approximately 10% higher, due to the mechanical extension of the modified sight base.


8 - Testing was stopped after the fourth test (100 ft-lbs), due to obvious permanent damage to the upper receiver/barrel.
9 - Inspection showed that the front sight base had not moved.
10 - Removal of the barrel nut showed that the barrel index pin had been partially sheared, and that the indexing slot in the upper receiver was permanently deformed.
11 - Removal of the barrel and the index pin better shows the damage.
12 - Closeup of partially sheared barrel index pin after removal.


Conclusion - The test clearly shows that the Vltor VST-1C sight attachment will hold against rotational torque beyond that which will cause catastrophic failure of the barrel index pin and upper receiver index notch.


While the primary purpose of the barrel index pin is not to prevent rotation of the barrel, it is clear that the mounting system used with the Vltor VST-1C sight is clearly strong enough when properly installed to hold the sight in position against forces well beyond those that would cause damage to the base rifle. This may also be true of other manufacture's clamping sight mount system.


Due to the failure of the barrel index pin and damage to the receiver at about 110 ft-lbs (1320 in-lbs) (actual was more than indicated setting on the torque wrench due to the moment arm length), it's inconclusive at what point the gas block would have rotated on the barrel. The distance of the sling cup to the centerline of the barrel is 0.6". This means that a force of more than 2200 lbs would have to be exerted on the sling to obtain similar results. For those who were worried that attaching a sling to a properly installed Vltor VSDT-1C would cause it to rotate, I think that worry can be put to rest. As far as impact to the gas block from dropping the weapon or smashing it against a rock or hard object, it's going to take a hard hit to move it, if at all. For those who still want a pinned-on gas block, Vltor will be making a taper-pin version in the future which will have to be gunsmith-installed.