the subject of the clamp-style gas block/sight assemblies and how susceptible they are to movement due to constant pressure from a sling or impact has been brought up before.
it’s a no brainer that a taper-pinned gas block is the most secure and damage to other parts of the rifle, including the front sight will occur first before the gas block moves on a barrel.
i talked to the guys at vltor, and asked if they’d be willing to perform a test to see how much it actually takes to move a properly installed clamp-on gas block, like their VST-C1 or a PRI. they were interested in the results as well, and happily agreed. an impact test would have been too complicated to set up and quantify with so many variables, so we decided to go with a simple torque test. username and password are both ‘mm’ to view larger photos.
http://www.militarymorons.com/weapons/ar.optics2.html#torque
That seems very conclusive. I have often wondered how the PRI units would handle a similar test. Thanks for sharing this!
Yes, thanks for setting that up and sharing.
I was concerned about how secure the PRI flip-up front sight/gas block would be with the clamp-on bolts, so I asked my gunsmith; he pointed out that similar bolt-ups are used on the forks of motocross motorcycles. That was proof enough for me.
Good stuff and should put peoples minds at ease.
C4
MM,
Thanks very much for sharing this. Very informative.
Stephen
Interesting…
I am more curious about frontal impacts howevever – since I have seen two PRI’s come out of alignment from that.
hey kev, i agree - i’d be more concerned with an impact than a constant torque or pressure, but that’s a tougher test to set up.
i can understand your concern, based on your personal experience with the PRIs - do you have an idea if their movements were caused by impact or related to incorrect (over or under) torque, or the bolts loosening up after time, or some other factor?
here are a couple of points about the vltor that differs from the PRI:
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the PRI flip-up sight locks in the ‘up’ position. if hit from the front, the shock is transferred through the locking mechanism to the gas block. the vltor sight does not lock up - it’s detent loaded. an impact from the front or at an angle to the sight itself will fold it. as for side and rear impacts to the sight, the sight will bend before the gas block moves. some people like that, some don’t, but that’s why it was designed that way - to minimize the possibility of gas block movement from an impact to the sight.
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the vltor gas block has two divots beside the clamp bolts. the ends of the bolts are center punched (bolt metal is moved into the divots) which stakes the bolts, in addition to using blue loctite after the gas block is torqued. the staking prevents the bolts from backing out and loosening up from normal use, and losing torque.
vltor is not claiming that the clamp-on gas block is as secure as a pinned one - but they’ve tried to ensure that their gas block is as secure as the clamp design allows, and that might be enough under most circumstances (for a civvie like me who doesn’t want to send the barrel out for pinning). time and usage in the field will tell.
but if for people like you whose lives depend on your weapons, and the additional peace of mind of a pinned gas block is desired, then their pinned-on version should address that.
Cool testing. Personally I would never trust a non pinned block with my life if given the choice.
Pinned blocks cannot move laterally or fore or aft (unless both pins are sheared, if that happens you’re probably already SOL). This is why for custom builds that I build I just ‘shave’ the FSBs into low profile gas blocks and refinish to fit under LaRue rails and DD Lite Rails. If a customer brings me a barrel that didn’t have a FSB I use a VLTOR gas block and dimple one of the set screws and make sure the rail or FF tube covers the gas block.
Perhaps this is because most people use these on free-float barrels, which do not use the regular handguard cap. This leaves a gap between the front of the seating shoulder and the gear of the gas block; this gap is a potential failure point, as it gives space for an impact to move the gas block back.
I’m having an upper built with a PRI gas block, and my gunsmith is putting a shim behind the PRI to fill the gap.
True – as for MM’s comment we also had a PRI shift at the Benning Sniper comp one year - the single shot Cdn’s suppressed SPR gun…
After that one I had barrels for FF’s profiled not to need the gap for the handguard cap, and pinned…
I know that the sights where not locked up when the impacts took place.
I’m also bringing .mil gun issues into this – but any AR type gun I get I ensure it will be able to do duty work too.
i actually brought up shims/washers to eric when the test was done. first, i asked why not make the vltor gas block a bit longer so it butts up against the step, and eric said that (as kevin had done) some barrel manufacturers are making barrels with the step moved forward so there’s no gap, so the gas block is automatically centered above the gas port and also prevents movement of the gas block rearward if struck. if he made the vltor block longer, it wouldn’t be compatible with those, so a shim is the correct solution. if they hear that it’s needed with theirs, they’ll look into it. i just think it should be supplied anyways so the gas block hole is centered on the port automatically.
Years ago I had a (I beleive his first) 11.5 Colt barrel done with a GG&G flip front via Wes Grant – he used a shim to position it for the pinning - and then the shim was removed.
For 99% of users I am sure the crossbolt method works – I’m just a paranoid.
I bought a box of 0.032" thick 0.75" ID shims.
I use these to account for the missing handguard cap when installing gas blocks.
When pinning, I also remove the shim like was mentioned in the post above.
Note the PRI picture I posted above also shows the gap from the shim…
Why do you remove the shim, Randall?
By the way, I’m having the PRI put on a Sabre 18" barrel. My gunsmith said the usual gap runs about .030", but mine is nearly at .060"…so there is some variability on this, depending on manufacturer.
A few years ago I put a PRI flip up (clamp style) on a DPMS M4 cut barrel and it was .058" (IIRC). I think there was some variables at work here. Possibly the handguard cap edge was too far back on the barrel (I didn’t measure it to the barrel extension), or the PRI gas port was machined too far forward in the gas block and/or the gas port was too far forward on the barrel. Or it was a combination of the ‘stacking’ tolerances everything being on the edges of the allowable tolerances.
I do the measurements and shim as required to center the ports.
I remove the shim(s) after pinning because it’s ugly to have a stainless shim on a black barrel.
On stainless barrels, I would leave them in as they don’t stick out like a sore thumb.
Any chance Vltor could be convinced to perform a follow up trial and test the clamp on style gas block for impact resistance? I realize an in depth analysis setup would not be possible, but they could smack the gas block into a few solid objects or hit the gas block with a hammer(afterall it is already wrecked) to get a subjective evaluation of its resistance to impact.
After reading the test, and the comparison to the motorcycle triple tree application, I am convinced the Vltor clamp style gas block (if properly assembled, loctited and staked) would be able to take a lot of abuse. Motorcycle triple trees are subjected to a lot of forces constantly. I can’t think of the last time I have seen a triple tree come loose (dirtbikes and streetbikes). I know I have trusted the setup at 170+ MPH. Not an exact apples to apples comparison, but the fundamental design concept is the same. Anything mechanical can always fail, but this test and topic has definitely given me some food for thought. I may have to get one just to see if Vltor won’t do some subjective impact tests.
they might, if you contribute a barrel for the test.