Hello,
I just put two Varminter rifles together with the heavy 24” barrels. On one of them, I put a Magpul PRS lite and on the other a Magpul PRS Gen 3. The PRS lite is asking for a carbine length buffer and the Gen 3 a rifle length. I installed both as specified. I went to the range to test them on two occasions. The first time, I had no luck with any sort of cycling. I took them apart and checked what was recommended. The other trip, it was pretty much the same thing. I’m sure I’m under gassed because it always fails to eject or it fails to feed and or lock open. Everything else like the BCG, gas tube, and ammo checks out. I just ordered two Odin works adjustable buffers and will try to set the weights to function smoothly. Have any of you done this? With one rifle length buffer, I was planning on using the weights from the carbine buffer to adjust the rifle length since I can’t find an adjustable rifle length buffer anywhere and I’m hoping they’re interchangeable. Are there any tricks you can share? This is my first time assembling an AR and I was having a good time until this.
Thanks for the help.

ALIGN the gas blocks. Hint.............

Stick is correct, but didn't really provide much advice.

I use a drill bit to align the gas block and gas tube holes, I simply drop it down thru the set screw hole and install the roll pin.

Secondly, most gas block journals are designed for a pinned gas block/front sight and the corresponding handguard cap that is roughly .032" thick. I use a .032" feeler gauge to set that gap. I also dimple every barrel, except for nitrided barrels, too dang hard. Then I use Loctite 242 or 620 to bed and seal the gas block to barrel. This also seals the gas block to minimize any gas pressure loss.

Thirdly, I never use a standard H1 buffer or an adjustable gas block. I start with a H3 and I go up to a Slash's Heavy Buffer or move down to a H2 depending on ejection. I am anxious to try a new A5 style buffer kit, 308 carbine spring and a slightly longer and heavier buffer than the H3.

If I could find a clamp on style gas block that would fit under a slim handguard I would go that route.

I think you are gassed fine, I think your issue is the BCG cyclic speed. Too fast and it will fail to pick up the next round and lock back. The heavier buffers will slow down the BCG and greatly improve cycling.

Will work great if you want the GB upside down.

Wrong, how would you get the drill bit thru the gas block and into the gas tube with the barrel in the way?

There is some common sense involved




Then I dimple the barrel if it is not already dimpled using one of these jigs:

Good lord if anyone has problems lining up the gas block and gas tube, they shouldn’t be messing with mechanical objects.

Forums are a poor place to learn because everyone is an expert.

True, but it helps keep everything lined up and makes it easy to install the pin.

Mismatched part selection cause more problems than gas block to barrel alignment or gas leaks.

About the only things I change are barrels, handguards stocks and grips.

BCGs, buffers, buffer tubes, receivers, LPKs, triggers and muzzle brakes are generally the same, cartridge specific.

I agree.. Sorry to hear the OP is having issues with his new builds. They are "fun", but do require a little mechanical knowledge and mindset. I like his choice of stocks. I like and use the PRS quite regularly. I have some gen 2 PRS that require a rifle length buffer (or a spacer and carbine buffer) and some newer Gen 3 PRS stocks that I can use either or, depending on how I have the stock set up. Generally I just run a spikes tactical ST-T2 buffer in those (carbine length) for my rifle length gas systems with low pro gas blocks because I don't like bulky handguards. No need for an adjustable gas block either. That's just me though, as I like keeping it simple. My AR's run like sewing machines, without fail. Going back to the buffers, I have also used a captured spring system and those are smooth and quiet. One of my buddies has a pre ban Bushy that was noisy as hell, so we installed one of those and he loves it. Accuracy even improved a bit. I know I have a couple in some of my rifles, but they are not necessary. It also sounds like the op has more issues than just buffer weight. As was suggested, double check to be sure the gas block is lined up properly. This stuff isn't rocket science, but does require a little mechanical thought. Good luck with it..

With a gen 3 PRS, you use can use either buffer length. That all depends on what buffer tube you went with. Whether it is the rifle length or carbine buffer tube. Which did you use on your builds? Do not use a rifle length buffer in a carbine tube and don't use a carbine buffer in a rifle length tube! I realized you are a new member, but pictures of your build would be good/helpful. Here's an example of a gen 2 PRS:

Those require the use of a rifle length buffer only, or you can use a spacer and carbine buffer. This is similar to your set up and has a 24" barrel, but older PRS stock.

Here's an example of a rifle length buffer tube:

I just run an H2 buffer in this one and it works just fine...

Thanks for the replies. Pictures are going to be hard for me to do. I’m not a total idiot, I have no trouble posting in the other forums I take part in, just here it seems to be trouble. Maybe its my age and understanding sizes in electronic format. While I am no expert, I do good with my hands and such and feel like I do the right things and look for answers when I don’t know something. I’m not afraid to ask in a forum either, chances are there will be a few people that will like to help. I have a few videos from Sinclair International on the ar 15, and have done all sorts of work to the others I own like triggers and barrels and all that connects to barrels. It seems like the only thing I never explored is buffers in all of the work I’ve done to these things. Since I can’t post pictures, at least not without sending them to someone that can, I will post the specs.

Lowers
Stag with Timney and one with a Timney Calvin elite. CMMG Ambi safety. Magpul enhanced trigger guard. Hogue grips. Standard pins and hardware from stag. One with a PRS lite and one Gen 3 PRS stocks. PRS lite has carbine length tube and buffer, Gen 3 has rifle length tube and buffer, both from Stag.

Uppers
Left hand stag 24” barrel with m loc hand guard. One has a black rain ordinance nickel boron and the other is a stag nickel boron BGG. Stag upper receivers. Ambi Charging handles from Raptor. I bought the uppers from Brownells and they were a unit with the barrel, hand guards, gas block, tube and upper receiver already built without the BCG or handle.

I also have never had a problem aligning the gas block/gas tube on an AR15 or AR10, but YMMV. It may be a gas issue, but this thread reminded me of a customer that brought 2 AR15s (5.56NATO) he recently assembled, and neither would cycle. I took his uppers off, and tried them one at a time on one of our lowers at the shop, and they ran fine. Turns out "Mr Assembler" ordered .308/AR10 Buffer springs & weights.....no bueno. Not saying that's the case here, but one more thing to check if it's not the gas part of the operation. -Tom T

Good description. It sounds like they did not install the gas block right. An easy way to check is get a piece of small vacuum line and put it on the end of the gas tube, then blow in to it and see if there is a blockage. That is a quick and simple way to see if anything is plugged or if it is not in alignment. It shouldn't be a buffer problem, especially if they were supplied by the manufacture. Yours sound a lot like the rifle I posted a pic of earlier. One of them is a left hand stag with 24" barrel. I was using a black rain ordinance BCG and Raptor as well. Good combo. Now, I don't have my lefty anymore. I see no need in having a left handed AR personally, but I'm assuming you bought a left hand BCG for both uppers?? I know it sounds like a dumb question, but just making sure..

Yes, definitely. I checked and triple checked when ordering and upon delivery.

Email pictures to:

xxx**Cleophusguildersleive@gmail.com**xxx

Everything between the asterisks and xxxs

I doubt a factory Stag would have gas block issues.

If you’ve got a rifle length receiver extension, use a rifle buffer and spring. You’re cheating yourself if you don’t.

24” barrels can have issues with carbine receiver extensions, buffers and springs. Different gas system than a rifle on a 20” due to 4” extra dwell time. Hard to say but I’d guess heavier might be better just nothing but guesses without seeing it. I’d start with the rifle lower, get it sorted out first with both uppers. That way you’re not chasing your tail with so many variables.

I suspect the BCG’s are your hiccup. But again, I’m just guessing from this point.

Here are the pictures sent by the OP, let him know and he can send me more for upload.

I still say an H3 buffer will fix the issues.

Well the rifle carrier weighs more than the H3 so let’s try to get the rifle running first.

Have you tried the BCG from the carbine in the rifle?

Yes. I tried both out in each rifle. Same results. I thought it might be the rubber o ring in the BCG extractor providing too much outward force thus keeping it from cycling 100%. Only minor improvement was made.

What ammo?

Federal American Eagle
Lake City
Herters
All 55 grain FMJ all factory.

I tried handloads of 69 grain Sierra HPBT match with TAC and BLC-2.

A gas block, is a gas block, is a gas block.
I don't understand why some folks have to overthink sonething so simple?
I'm on another site where there are "builders" that are physicists, aircraft engineers and I think, even some rocket scientists (claim to work for NASA).
They all use AGB's and are constantly boring open barrel gas ports to get more/less "gas".
Then they want to use Loctite to seal the gas block to the barrel!? WTH?
Get everything lined up and tight, and you won't need "no stinking Loctite"! ....to seal the gas block OR lock any screws in place! LOL !

As my late f-i-l used to say, "Cleverly tight!".

Interesting Martin. I am for damn sure not a Nasa engineer, and I don't use agb either. No need to. Have a cousin and ex brother in law that worked for Nasa, yad yada yada.. Had an engineer ask what I did for a living in a pizzed off manner once. He was a strange dude, but cool once you got to know him: I guess he didn't like the fact my hand loads were always in the single digit SD's. He thought since he was an aeronautical engineer, his chidt should have been better than mine. It wasn't. Surprising huh??? I use loctite on the gas block set screws. I also "dimple" the barrel, if it is not already done. I don't use a fn jig for that either. No need to. This is all interesting. Hopefully the OP gets it figured out.

Measure the length of the buffer tubes internally and springs. There were some proprietary lengths out there at one time, I ended up with a rifle cheap that wouldn't run. Turns out that the carbine buffer tube was 3/8" short.

I was building my first AR10. When it came to the stock/buffer tube assembly, I figured, "Hey, a buffer tube is a buffer tube."
The rifle (.308) would load and fire, but WOULD NOT function.
Yes sir, I had installed an AR15 buffer tube in an AR10 rifle.
As my kids would later explain to me, "One thing is not like the other....!" LOL!

That was just part of my lesson on "AR15 parts are generic. AR10 parts are proprietary."

Carbine: 7” tube length
10 3/4” spring
38 coils on spring

Rifle: 9 5/8” tube length
12 1/2” spring
45 coils on spring

If you are satisfied the gas blocks are aligned properly, but thinking your uppers are undergassed, take the carbine lower, pop out the buffer, and with a hammer and small punch tap out the roll pin that is holding on the rubber piece. Remove the rubber piece, and remove one of the three weights inside the buffer. Reassemble the rubber piece and roll pin onto the buffer body. Test fire. Better? There you go. No better? You can try taking a second weight out and test firing.

If you're undergassed, lightening the buffer may get you running. At that point you can either run a lighter buffer or see about opening your barrel's gas port. Another way to test a carbine lower is with a Sprinco yellow carbine spring. If severely undergassed, you may have to run the lighter buffer and the Sprinco yellow spring together.

Sometimes the gas port in the barrel is not exactly where it should be. Occasionally getting the gas block properly aligned with the port in the barrel means you can't have the gas block square against the barrel shoulder, or even have the gas block precisely at 12 o'clock.

Pull the gas tube out, loosen the set screws on the gas block, and use a flashlight to check that the holes are mating up. Or pull the gas block and measure everything with calipers.

That said...if all your buffers and tubes and springs are correct...

...and your gas block hole is perfectly lined up with the gas port on the barrel...

And the gun still doesn't cycle...

Then the gas port in the barrel isn't big enough. It is pretty easy to drill them out larger. Just stick an old cleaning rod into the barrel to protect the barrel and run a slightly larger drill bit in, the smallest step up you can, might be 5 or 10 thousandths. The more barrel you have in front of the gas port, the more this increase in gas port size will be magnified.

I’d try your uppers on a known good lower. If they still don’t work, send them back and let Stag fix them under warranty.

OP wrote “I bought the uppers from Brownells and they were a unit with the barrel, hand guards, gas block, tube and upper receiver already built without the BCG or handle.”

Contrary to the title of this thread it looks like it is turned into a gas block discussion. Either have a gas block it's real open and buffer it was spring and weight or get an adjustable gas block.
As far as buffer weights go there's a variety of springs to be played with. But you can usually do all by ordering the tungsten weights disassembling your buffer and changing weights in and out and get the correct combination. You can buy three tungsten buffers usually through Amazon or such for around well used to be under 30 bucks but nowadays it might be that much.

Good suggestion. That's what I'd do too. Seems strange that things are not working with either of them, unless they were both put together wrong by Brownells, or there is a similar defect. Figuring out if there is a blockage in the gas block or barrel is pretty easy to do without having to take schidt apart too. I've personally had a left hand stag 24" Upper and it ran smooth as silk. No issues at all and I tried the upper on different lowers, with the same brand (black rain ordinance). Still ran flawlessly.. Dwell time did not affect anything even with a lower I tested that had a carbine buffer. The rifle length gas system on that upper was very smooth and soft shooting.

What does your brass look like? Deep scarring from the extractor? Is it ejecting at all? Dirty carbon ring on the neck and shoulder area?

I still think you have a cyclic issue, bolt speed is so fast that it's not giving the bolt catch or next round time to move before it's already headed the other direction.

I built a 358 Winchester upper that did this very thing and a few others. Everyone kept telling me to put an adjustable gas block on it. I read everything on the issue aside from the adjustable gas block band-aid. Then I found an article detailing adjusting the buffer weight to cure the issue. I ordered several H2 and H3 buffers and a few of Slash's Heavy Buffer kits and the H3 solved it. The Slash's kit made it a whole different animal, cutting recoil and improving the quality of the ejected brass. I was getting the little ding from the deflector, with the Slash's kit, that went away as well. Slowing dwell time will improve chamber seal and stop that dirty carbon ring on the neck and shoulder as well.

If you are having extraction issues, remove the double spring and any rubber under there, they are band-aids for poor extractor quality. I use NB BCGs, but keep Tromix extractors and spring kits at the ready, the 375 and 458 Socom will educate you quickly on real extractor quality.

Brass doesn’t show any signs of stress. It will sometimes eject and sometimes not. When it doesn’t, it will sometimes feed the spent casing back into the chamber. Sometimes it stops just before being locked in battery. I tried the magazine test for hold open, and it never worked. The gas blocks I checked a few times and ran cleaner and compressed air through the tubes to make sure there were no blockages of assembly grease or other fragments. I have seen online where someone drills the port slightly larger, but while I’m capable of that, my luck it will not be the issue and now problems compounded. I have two other AR rifles, carbine and rifle that work great, so I will try switching out the lower for the troubled uppers and see what that does. I don’t know why I didn’t think about that before, it’s so simple. I should have the adjustable buffers from Odin Works today and will try that out if that’s the issue. I’m patient so I will take the time and do it right. Someone mentioned the Silent Captured Springs and that has me very interested after looking around at those. I might have to explore that for down the road.

They sell fully assembled, quality ARs most places and in my experience they're really great.

Just tossing this out there, could this be a lube issue, action not operating smoothly?


Gun is drier than phuu.uck?

Just got my first AR and have never messed w em but from my understanding they prefer to run wet.

Not likely. That NIB BCG he has is pretty slick. I could run mine pretty dry and it ran like a sewing machine.. Hopefully when he tries his known good lowers all problems will go away. If that is the case, then I'd look at Trigger spring in backwards etc..

Just confirming, you are running .223 ammo WITH a .223 bolt. I did that recently. I put my 6.8 bolt in the bolt carrier and inserted it in my .223 AR. It would push the round into the chamber but obviously, it would not extract. The case just laid there inside of the chamber or on top of the magazine. I had this stupid look on my face for several minutes until I looked at the bolt face. Duhh!! I had to drive all the way home and get the right bolt and then back to the range. 10 short miles round trip.

Anyway, check the bolt face to make sure you have the right bolt.

kwg

Have you tried removing the spring but with the buffer in place and making sure that the rifle will fully cycle by hand and lock back with an empty magazine in place?

You can do this with the upper removed, you need at minimum 5/16" but no more than 3/8" between the bolt catch and bolt face when at its furthest rearward travel.

Wondering if the op has got this sorted out yet? Tried a proven lower on the uppers?

I have. Sorry for the late response. I did try it on proven lowers and they didn’t work. Same issues as before. It turns out there were a few made with slightly smaller gas ports and they got passed everyone. Long story short, I sent them back to the manufacturer and they will quickly repair and send them back. They were very helpful. I appreciate all the help from all y’all. My next thing with these may be the silent captured spring buffer. Those look and sound like a good thing.

That is good to hear. I use the silent captured spring system on a couple rifles. They are kind of cool. Needed? No, but they do reduce recoil slightly, said to help with accuracy, are quiet as hell (great for those thin A2 stocks where you hear the "TWANG" of the buffer and spring). I like the Amaspec stealth recoil spring system because it works well and is way less $ than a JP and does the same job. Keep in mind that you will need to buy or make a spacer for your rifle length system, but they just drop right in the carbine buffer tube. I remove the detent that keeps the buffer in as well. The new Gen 3 system looks good and less than $100:

Armaspec stealth recoil spring system. GEN 3

I was dinkin w my one MSR and it was horribly sproingy.
Never was before. Yanked buffer n spring and re inserted.
Ring was gone

That rig spits brass to 1 oclock 16" carbine length gas.
My other rig spits em at 3 to 4 oclock.

Have not compared buffers. Different manufacturers.

Am wondering if I should go w tunable buffers and flat wound springs.