Chad at SOTAR says pinning gas blocks isn’t always a good idea. I pin mine for the same reason I use OCKS screws. Eliminating a potential point of failure seems like something you would want to do. Am I wrong?
Does he specify what type of pinning and why? I can understand not preferring having a taper pin pounded in vs a coil pin inserted, etc.
I prefer pinning, but a set screw that is dug in to a dimple isnt likely to go anywhere without other damage occurring.
I’ve read that BCM dimples the barrel and uses red Loctite on the setscrews.
Is this true? If so, has there been any reports of a factory installed GB coming loose?
Pinning is good. Set-screw and dimple w/red lock tite is also good.
For precision application id favor a dimple.
Aggressive torque on the set screws can create a measurable constriction in the bore. You need pin gauges accurate to basically .00005" to measure it, but you can. Obviously a .625" journal with a thinner wall will deform earlier than a .875", stainless barrels with lower hardness levels, etc… it all plays into things.
If you can feel the bore tightening up under a set screw, how do you think you can pound in a tapered wedge with a hammer without it having an impact on bore diameter?
Or go the opposite direction and use a hollow straight roll pin… so you’ve removed solid thermal mass under the gas journal (where heat is concentrated by the block) and now you’ve got a hollow spring pin in there instead… what’s that doing for harmonics?
The bottom line is any change in material/ pressure there is going to have an impact on performance, and localized force or inconsistency in material composition is not going to improve the precision the barrel is capable of.
For most applications this is entirely academic, but it’s not a bad idea to use different methods of assembly on precision rigs vs “trench brooms”.
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Just speaking theoretically, but would dimpling the barrel help transfer some of the radial load imposed by the set screw into an axial load, from the cup of the set screw pushing obliquely outwards against the walls of the dimple? Would this be something even worth considering when selecting the drill point angle?
Seems like that would depend on the fitment between the set screw and the dimple.
I look at the dimple or flat as being the best way to do it versus just the round barrel surface while kind of wondering if it might be over kill due to all the factors.
The entire circumference of the cup tip can dig in to even the original round barrel so combine that with not much about a gas block to give leverage to any hits and usually under a rail.
That being said, I stake the gas block screws instead of loctite.
I stake around the top of the set screws with a center punch AND usr red loctite.
I’m fine with pinning if its already set up for it
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GB could get so hot that fails thread lockers. The thermal cycle + mechanical shocks and vibrations would back out the set screws. In this sense, pinned is the best for mission critical weapons. I have no such need, so I prefer screws for ease of disassembly. I just routinely check the screws for tightness. Simple blue loctite seems to hold well the way I shoot. Cramp type GB should be much better but they are not very low-profile.
For dimple profile, I avoid the point type set screw, so that it would bear on a surface area instead of a point.
Not very fond of staking. If needed, I would use other options such as double screws.
-TL
That is exactly why I stake, I’m thinking heat could weaken the loctite.
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My gas blocks have 2 set screws. I don’t remember finding both loose. After applying blue loctite I haven’t found any come loose yet. I shoot slow though.
Double screw is to replace a single set screw with 2 shorter ones. They lock each other in the same threaded hole. It is almost like staking, just easier. With loctite added, I think it should work well, although I have yet to put it in real use.
I have been using loctite blue 243. Just looked up it’s specs. It is actually not bad at all. 40 lb-inch prevailing torque up to 180C.
There several grades of red. 272 is probably the toughest one. I wouldn’t think it is necessary for what I’m doing.
-TL
If you have a .75" bearing diameter on the barrel, the wall thickness for a .308 barrel is 0.221". How much force is required to deform the tube to an oval of approximately 0.0005" less than the original diameter? This can be calculated with the information provided and the modulus of elasticity (29,700,000 psi for 4150 steel).
It’s around 6,000 pounds.
A 10-32 screw, torqued to 45 in-lbs will produce a maximum of 1,500 pounds.
If you are seeing accuracy changes after torquing your gas block, it is from some other source . . .
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Red Loc-tite will burn off after a couple hundred rounds, blue doesn’t stand a chance.
The ones I have used Red on were able to be easily removed cold.
I just make sure they are still tight, so far in my limited usage they have haven’t.
If I do get one backing out I’ll get some serrated ones and use Rocksett or Vibra-tite Hot lock.
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In my opinion… in my shop… only pinned will do. Set-screwed on just doesn’t cut it, I don’t care what thread locker is used, dimpled / not dimpled barrel… it’s not a positive retention. Worse yet are the clamp-on type where you are using two or three or four pinch bolts, where the bore of the unit is split and the pinch bolts clamp it on. And the worst of the worst in that category is the one with two separate caps, like the bottom end of a connecting rod. One commonly used one, each cap is held on with two 6-32 screws. On top of everything else that is barely secure about this one is that 6-32’s are prone both to breakage and to coming loose, 6-32 being actually a very coarse thread. Screws 101 here, the helix angle on a 6-32 is about 4.4°. For comparison, a larger screw with fewer threads per inch, you might think that’s a coarser thread. But let’s take 1/2-28, our muzzle thread, it has a very low helix angle of about 1.3°, and so, it is much less prone to coming loose. In my toolmaking days we almost never used 6-32 because the taps, too, are easily broken. Just going up from 6-32 to 8-32 would have increased the strength of the screws, and reduced their propensity for shaking loose, significantly. On the pinch-bolted and split-cap ones there is simply no way to pin them after the fact.
It’s kinda funny, I’m like most people, I will go to great lengths do do some little trick or mod that might give me a theoretical bump-up in accuracy, but in my mind if pinning a gas block costs me half an MOA, I’m still in. I don’t think it does but I confess I have not tested it, it just hasn’t seemed necessary.
The ultimate might be the HM Defense barrel where the dang thing is integral to the barrel, machined right out of barrel steel. I have one and that barrel has been very accurate. There must be some downsides but I got it, set it up, and have been running it for I dunno, 4-5 years now and have not messed with it further so if there are downsides I just haven’t run into them.
In my way of doing things, when I’m pinning a low profile gas block, I’m using a 1/8" X 3/4 split pin. When I install the pin I orient it with the split down, now here’s some OCD thinking, for drainage and also, it seems to me like most of the force exerted by a split pin is at 90° to the split so I figure theoretically, that force directed along the axis of the barrel might be better than towards the bore but you could argue it either way. I don’t believe there is enough force to do much more than hold the pin in place anyway. For years I have popped the hole in right at the edge of the barrel, so the pin is ½ into the barrel and ½ into the gas block bore. I have taken to biasing the pin a little further down, like another .025 or so, but again, I don’t think it makes any dif with anything.
If a 1/8” split pin seems weak, my thought is that since it’s a low-profile gas block, it doesn’t have the sight tower on it for leverage if it gets bumped by something. Plus, most are protected to some degree by the handguard.
Will set screws affect the bore? By trying real hard I have managed to do it, this was a .750 journal and they typical 10-32 set screws (~3° helix angle, you know you want to know that :-)). In a barrel that would pass a .218 or .219 gage pin, I don’t remember which, allowing it to drop through, I could get it to hesitate slightly at the journal by giving the screws a mighty overtightening. I doubt it’s an issue most of the time but it’s a possibility.
If you used a tapered in and really jammed it in there would that distort the barrel more? I guess in theory it could. I don’t think it’s ever a real problem but I by far prefer the split pin for ease of doing it.
Could a split pin or roll pin fail to hold after a six-mag mag dump from the heat? Probably. In theory. Don’t do that. I’ve never, ever seen one fail by trying to slide itself out, or in any other way. I safety wired one once, “JIC”. Not necessary.
Speaking of ease of doing it, AR manufacturers love that, to the point of knowing it’s gonna fail when they use a straight pin but… it saves them from having to taper ream the holes and from having to source the right tapered pins. That’s gotta be what, 27 cents a gun they saved? I have seen a number of these in class where one-- or both-- pins are missing. At least one lost a pin in the box, in shipping. Dear manufacturers-- we’ll pay the damned 27 cents, just make the thing so it can hold itself together at least until the end of the first range day! Straight pins done right would be perfectly adequate but it’s harder to do right, that’s the issue here. Industry standards for getting a proper press fit on a 1/8" hardened and ground dowel pin call for a minimum of .0001 interference or “press fit”. That’s a tenth of a thou. Pretty easy to get that wrong by a tenth or two… of you do, it falls out. I think someone who is using straight pins is now knurling one end of the pins, that’s an upgrade assuming the pin is otherwise a decent fit.
It’s not just the gun that needs to be foolproof. The manufacturing process has to be harder to get wrong, too. Those guns that lost their pins? Whoever installed them had to have noticed they basically fell in. That was not a stupid or bad person, just someone who did not get adequate instruction at the plant.
If you’re only using set screws, I think dimpling is better than not dimpling, I mean at least it helps locate the GB, assuming the dimple is in the right place and that the GB is compatible with what whoever dimpled the barrel had in mind.
“Pin them. Pin them all,” sez me. Just my opinion.
–Edited after a half hour due to the inevitable typos.
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Sig GB pins are peened over like a rivet that has to removed to drive out.
For $220 BRD has a nifty jig for pinning.
Triarc and Geissele have pre-drilled GB’s for pinning that’s a cheaper option too.
Just need a drill press or a steady hand with a hand drill.
BRD has drill bits to make a base hole and a reamer for final size as well as coil pins.
Haven’t seen that on an AR.
Best I can recall, that was the gas block on 1990’s Ruger Mini 14 though.
This summer in Patrol Rifle we had a gun that was short cycling at best and otherwise simply “not” cycling. It had a Rock River handguard on it but the lower was not RRA so not sure, really, whose upper it was, possibly Frankenstein’s. The gas block set screws had come loose and it had shucked forward about .040. I think it was shucking back and forth but then got stuck forward in the carbon build-up ahead if it.
Someone had had the handguard off before and remounted it 90° off-- so the Pica rails were situated such that we could not access the set screws to re-tighten them. The handguard had to come off… and it takes a special wrench which, I guess maybe I should have it but… I don’t. Whoever had worked on it before had used the hammer and punch method in the very shallow wrench cuts so it was less of a big deal for us (me and my helper) to do the same, using an aluminum punch. We got the lock ring loose finally and with some heat we were able to get it apart. We removed the GB set screws, cleaned everything up, red Loctited them, tightened them down (the barrel was dimpled), and staked them with the automatic centerpunch. Got the handguard back on the right way, which, there is nothing about it that registers and locates it so you eyeball it and tighten the lock ring, which moves out of location… so it’s a trial and error thing, starting with the handguard out of location and making it go into location by tightening said lock ring. Bit of a pain and between us I’d say we had 1.5 man-hours into it.
At the end of which we re-mounted the guy’s laser, which obviously had lost its zero; also we had to relocate, I don’t remember exactly, but I think a light and BUIS front sight, also needing re-zero.
Lots of trouble for two friggin’ loose set screws!
I just checked and that handguard does not appear on the RRA site, good if it’s been dropped.
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Good point on helical angle. I will keep that in mind next time when I need more margin. There are finer pitches to choose from, although everything would be rarer.
#6 screws are indeed too weak. Fortunately all the GB set screws I have come across are #10. One ultra low-profile GB may have #8 no less.
I have seen an mod to turn A2 FSB set screw instead of taper pin. It has 6 screws total. It may not be as good as pinned, but with dimpling and thread locker it would get real close.
Except for one particular model, all red loctites have similar temperature rating as blue loctites. The difference are the torques specs.
-TL