Before the last election, I put together a pistol lower and had a friend build an upper for me. I got the whole thing assembled, and took it to the range. It works great as a single-shot pistol. I did some tinkering with it trying to get it aligned on my own and then took it to the range. Same result. Life happened, it got jammed in the back of the safe and that was that. I know, I quit at the finish line.

Enter 2020.

Assuming that the problem is that the low profile gas block is misaligned, what's the best way to get this thing cycling? For $6, this interesting device seems like probably the way to go, but it was definitely time to at least ask for input:


https://hbindustries.net/store...lock-aligner-5-56mm/

I use a dimple drilled into the barrel for the gas block set screw. The gas port in the block is aligned with the rear set screw bore. Use a jig that indexes off the gas port in the barrel to put a drill point divot in the barrel on the opposite face. This gives the set screw a better surface to bite into and secure against, and also gives the screw a tapered alignment to ensure everything is in-line.

The alignment pins you linked to work, but they don't address the screw trying to tighten on a round surface, and in the case of nitrided barrels, an extremely hard surface that is difficult for the screw to bite into. A piece of raw spaghetti will work the same way as those pins, but there's a little more room for error (and leaving spaghetti in your gun if you manage to do that)

A dimple tool is more expensive, will only work with one size of gas block journal, and doesn't help for clamp-on or taper pin gas blocks, though.

https://slrrifleworks.com/750-dimpling-jig/

I have the SLR Dimpling Jig and have done the same thing as Rustpot.

quote:

Originally posted by Rustpot:
I use a dimple drilled into the barrel for the gas block set screw. The gas port in the block is aligned with the rear set screw bore. Use a jig that indexes off the gas port in the barrel to put a drill point divot in the barrel on the opposite face. This gives the set screw a better surface to bite into and secure against, and also gives the screw a tapered alignment to ensure everything is in-line.

The alignment pins you linked to work, but they don't address the screw trying to tighten on a round surface, and in the case of nitrided barrels, an extremely hard surface that is difficult for the screw to bite into. A piece of raw spaghetti will work the same way as those pins, but there's a little more room for error (and leaving spaghetti in your gun if you manage to do that)

A dimple tool is more expensive, will only work with one size of gas block journal, and doesn't help for clamp-on or taper pin gas blocks, though.

https://slrrifleworks.com/750-dimpling-jig/

Buy a $50 Teslong USB borescope from Amazon and you can use it to align the gas ports and inspect all your barrels properly for pitting, flaking, wear and cleanliness.

I was going to replace a FSB on a barrel I chopped the front sight off of, but after looking in the barrel with the borescope, I realize it's time for a new barrel. I didn't waste money on a new FSB or gas block.

Just my humble opinion.

Tony.

quote:

Originally posted by benny6:
Buy a $50 Teslong USB borescope from Amazon and you can use it to align the gas ports and inspect all your barrels properly for pitting, flaking, wear and cleanliness.

I was going to replace a FSB on a barrel I chopped the front sight off of, but after looking in the barrel with the borescope, I realize it's time for a new barrel. I didn't waste money on a new FSB or gas block.

Just my humble opinion.

Tony.

This is a good idea. The Teslong is a surprisingly good borescope for it's price. It can do this job and still maintain major usefulness in cleaning etc. I recently ordered extra mirrors to fit different bore sizes for mine. For just $20 or so they were a useful addition. I'll still use my Lyman borescope too because the rod has it's advantages but for the price it's kinda hard to beat the Teslong even considering where it came from!

quote:

Originally posted by benny6:
Buy a $50 Teslong USB borescope from Amazon and you can use it to align the gas ports and inspect all your barrels properly for pitting, flaking, wear and cleanliness.

Thanks, Tony. That actually makes a ton of sense to me, and something I never considered. I always just assumed a bore scope was too expensive a piece of kit to consider. $50 is definitely doable.

I did a little quick googling and there's a review of a different borescope, but the guy uses it exactly how you recommend:
https://ultimatereloader.com/2...s-in-depth-overview/

I did a video on it last year. I was still new to it and I learned how to use it better since then. The big thing is focusing it the right way. Even with my image slightly out of focus, it still looks great.



Tony.

Well, I'm sold. Ordered one, should have it next Monday. Thanks, Tony.

The one time I installed a low pro gas block, I got and aerosol wand (like off a can of WD-40) bent it slightly, inserted it into the gas tube outlet in the block and worked it down until it was just shy of where the gas inlet of the block meets the barrel. Then, installed it on the barrel and worked the aerosol wand into the gas outlet of barrel. Locked it down and removed the wand. It worked really well for me, but maybe I got lucky. This was on a .300 Blackout CMMG barrel. I don't remember what the brand of the gas block was.

I have a carbine upper that originally had a regular FSB and the round plastic handguards on it. I had a friend remove the FSB and replace it with the low-pro block and install a free float handguard last year. I noticed the other day that the low-pro gas block is canted about 10 degrees to one side. It works fine, just looks a little "wrong". The low-pro block is a clamp-on style with 2 screws. I need to pull the handguard off and see if they've come loose. It may be that the gas tube is the only thing holding it in place.

I've changed out or instaled three or four gas blocks over the years without the benifit of an allignment tool. All the barrels were cut for the USGI taper pins, so I used those cut outs for allignment.

I have bought a couple barrels with low pro gas blocks pre-drilled for a roll pin. Also bought a barrel with preinstalled FSB.

Some barrel makers dimple the barrel opposite the gas port so it's easy to line up the gas block with one screw. Gas blocks need to have one set screw hole opposite the port in order to internally drill the port. The other screw is not dimpled because gas blocks vary on location of the other screw(s).

Clamp on gas blocks are a little more difficult. One trick is to plug the barrel on one end and hook the other end up to compressed air. Fiddle with the block until the flow is maximized.

Also remember that many barrels have the shoulder cut to allow space for a hand guard cap, so make sure you know if the gas block should be flush to the shoulder or spaced for the cap.

Some barrel makers don't do dimples and say they are unnecessary (BA for example). Most people prefer to have a dimple and ask the manufacturers for it so I don't see a reason not to have it. It's not going to affect accuracy or harm the structure integrity of the barrel.

I replaced two FSB with low profile gas blocks.....I just got the barrel in the vise with the opening at TDC, then slid the new block on it. My MK1 Mod0 eyeball worked just fine.

Every one I did in the service was the same way....

Well, the borescope arrived today. I removed the handguard and had a look with the borescope and at first, I couldn't make sense of what I was looking at. So I loosened the gas block and fiddled with the brightness until it started to focus a little better - I didn't read the instructions on focusing it . It didn't take long and I'm pretty sure it's as aligned as aligned gets.

I never saw the inside of the gas block and there's no removing it without taking off the forward comp and I'm not inclined to mess with all that, but it sure looks to me like the diameter of the hole in the gas block is exactly the diameter of the the gas port hole in the barrel. I was expecting it to be ever so slightly bigger and allow a little bit of room for error, but I guess not.





I've tried the methods in the preceding three posts and thought I had it aligned and after about the third trip to the range, I hadn't, and I gave up. I'll have to make some time to get to a range and see if it will cycle correctly now. If it doesn't, I may just drive it over to Rainier Arms and tell them "make it work" and be done with it. I talked to a guy there who quoted me $65 and a two week turn-around, so that isn't too bad.

I am glad to have the borescope, gas block issues aside. I had a look down a few barrels I thought were pretty clean. Suffice it to say, my next project is going to involve copper solvent and lots of patches.

Take the gas tube off and shine a light in the gas tube hole. You'll be able to see the light inside.

You can also screw the mirror on as far as it will go, which will set your focus point farther into the gas port.

Tony.

Here. I just did this with mine. I backlit it with a flashlight.



I removed the taper pins and rotated my gas block out of alignment. Here's what it looks like misaligned...


This is aligned as I could get it, which is where the taper pins are centered. This is a 20 year old barrel of unknown manufacturer.



I also took off the locking ring from the borescope and screwed on the mirror as far as I could get it on to shift the focus farther into the hole.

Tony.

quote:

Originally posted by P220 Smudge:
Well, the borescope arrived today. I removed the handguard and had a look with the borescope and at first, I couldn't make sense of what I was looking at. So I loosened the gas block and fiddled with the brightness until it started to focus a little better - I didn't read the instructions on focusing it . It didn't take long and I'm pretty sure it's as aligned as aligned gets.

I never saw the inside of the gas block and there's no removing it without taking off the forward comp and I'm not inclined to mess with all that, but it sure looks to me like the diameter of the hole in the gas block is exactly the diameter of the the gas port hole in the barrel. I was expecting it to be ever so slightly bigger and allow a little bit of room for error, but I guess not.

I've tried the methods in the preceding three posts and thought I had it aligned and after about the third trip to the range, I hadn't, and I gave up. I'll have to make some time to get to a range and see if it will cycle correctly now. If it doesn't, I may just drive it over to Rainier Arms and tell them "make it work" and be done with it. I talked to a guy there who quoted me $65 and a two week turn-around, so that isn't too bad.

I am glad to have the borescope, gas block issues aside. I had a look down a few barrels I thought were pretty clean. Suffice it to say, my next project is going to involve copper solvent and lots of patches.

Yes, your gas port is aligned. Mark that off the list.

This is what an armorer will check to diagnose the gas system further;

1. Is the gas tube installed correctly? It's not impossible to install them upside down, or have the gas port drilled in the wrong spot. Or not drilled at all. Things get missed or overlooked.

2. Is there excessive leakage from the gas block, or gas tube area? You've shot the rifle, it should show some blowby if there's a leak issue.

3. Check the carrier key fit to the gas tube. The gas tube should interface snugly into the gas key to form a gas seal. I would also check at this time to make sure the gas tube is aligned in the upper - strip the bolt/firing pin/cam pin from the carrier and drop the carrier in with the upper receiver held vertically. The carrier should slide neatly over the gas tube and clang against the barrel extension. The gas tube may need some minor tweaking (bending) to make the fit proper, and this will extend gas tube life from eliminating excessive wear from poor alignment.

4. Check the carrier gas path. Some .080" weed eater line works great as it has the right diameter and stiffness to make the bend. With the carrier still field stripped, stick the line into the gas key and get it to pop through to where you can see it in the bolt's bore. It can be a little tricky to get it to want to go through, but obstructions in this area can happen and will cause the system to fail.

5. Check the gas key for fit to the carrier. This is a bolted interface and can fail. Usually if the gun is a single shot and this is the problem, the gas key can wiggle just using your fingers. But there's potential it's still tight and just misaligned, mis-fit, or otherwise leaking. Check for leaks emanating from the gas key. If you want to check the key further, look for rear impact marks from where it could impact the lower receiver, which tends to break gas key screws. Applying 20-30in-lb of reverse torque to the gas key screws checks to make sure they're still in there and doing their job, if they're broken or otherwise improperly installed they'll break loose with the light attempt at removal.

6. Check the bolt gas ring fit to the carrier. Easiest test is with an assembled carrier, pull the bolt to the forward position and attempt to stand the carrier on the bolt face. If the gas rings hold the weight of the carrier you're in good shape. If the carrier drops your gas rings could be worn, or the fit of the bore could be off.

7. Check the bolt tail fit to the carrier. An excessively large carrier bore, or small bolt tail, can allow gas to blow by before enough pressure is built up in the carrier piston chamber. Typically a lot of blowby can be seen, and fouling on the hammer/firing pin is excessive compared to a properly fitting bolt/carrier.

Thank you, Tony and Rustpot. I'll pull it apart later this evening and try all those things out.

Re-reading my post, I worded some of it poorly to the point where it reads almost like I found that the gas block was perfectly aligned when I first took a look at it. This was not the case. In fact, I have no idea whether it was or not because I wasn't sure what I was doing with the borescope before I loosened the block and started trying to get it aligned. I assumed it wasn't, which wasn't proper troubleshooting and I know it. There was quite a bit of fumbling with the brightness settings before I got it to the level of clarity the pictures show, which isn't great at all, but it was good enough that I could see the gas block port and get it aligned. All I could see of the gas port prior to that was a dark hole. I did some more trial and error adjusting with the borescope after my post last night and got a much better look at it, and it sure looks about dead-on.

General information about the upper: It's a new gov't (I believe) profile 10.5", a new linear/forward comp, new Diamondhead VRS-T rail, a lightly used upper receiver, and a new BCG with a hard chromed carrier. Basically, everything but the rail is a mish-mash of take off or extra parts that a friend put together for me, so it is a bit of an odd combo. He assembled it, and has built a number of AR's, but things can and do happen. I don't want to do anything too permanent to the barrel because I'd like to replace it with something lighter eventually, but for now, it will have to do. Other than the receiver itself, everything else is new and has maybe ten rounds through it. So it's definitely not parts wear, none of this stuff is even close to broken in yet. I'll definitely still check all the other things tonight, though.

Oh. And I guess I should clarify it’s a pistol build on a new “other” lower with a pistol buffer tube. Again, all new parts.

Thank you guys for the info. It is very much appreciated.

Tony, I wanted to try your flashlight trick with the gas tube, but there's simply not enough room between where the gas block rests and the back of the forward comp to get the gas tube out of the receiver. However, I did get the Teslong pretty well focused (with a mirror almost all the way at the end before tightening the locking collar) and got a pretty good look at the gas port and it all looks like one machined shaft with no overlap, so the alignment seems good to me.

Ok, working through Rustpot's list in order.

1. Seems correct. Everything I'm seeing picture-wise on a search engine lines up with what I have in front of me.

2. This is interesting to me. I put some Remoil on a paper towel and wiped down the barrel fore and aft of the gas block and came away with fouling. There was some on the gas tube right by the gas block as well. The block itself seems to fit pretty snugly on the barrel, but this is an area I will pay attention to when I clean it after the next range trip.

3. Passed this check with a loud clang.

4. Didn't have weed eater line to do this, but a piece of floss went through without any issue.

5. Gas key seems pretty solidly unitized to the carrier. No play, no issues I can discern. No impact marks at the rear of the carrier, the whole thing looks brand new dull hard chrome, the bearing surfaces have only the slightest sheen to them. I don't seem to have an allen wrench in either my SAE or metric sets that fits the carrier key screws, but they do seem to be staked pretty solidly.

6. Passed this check with ease.

7. Other than checking for fouling, I can't think of a way to verify the fit. There didn't seem to be an inordinate of it on the firing pin, and there was almost nothing at all on the hammer.

The only other thing I would have wanted to check is to feed something through the gas tube itself to check for obstructions, but it seems basically brand new. In one of my last attempts to get the block aligned, I took a section of shrink tubing and closed it over the end of the gas tube inside the receiver end and opened the other end of the shrink tube enough to get a compressor nozzle into and it was definitely allowing airflow, so I'm going to guess it's probably ok.

So where I'm at now is everything looks correct. Had I been more patient with the bore scope on the first attempt and actually gotten a good look at what was going on with the gas block alignment in the first place, I would have been able to determine if that was where the fault lay. I didn't, so I'm going to have to assume it was off and I've corrected it now. The only thing left to do is apply some oil and grease to the places that need it and see if it will cycle. If it doesn't, back to the drawing board.

Oh, and when I take it to the range, I'm going to take my other AR that runs like a top. If there's still any issues with the pistol cycling, I figure I'll swap out the BCG's and see what happens. Whether it works after that or doesn't, it should still help narrow down exactly where the problem is happening.