Ar15 midlength jamming

[Dan Forrester]

Hi, I’m having a problem with a midlength upper and was hoping the forum could help me diagnose the problem?

I put together my first midlength upper a few months ago and am having problems with it. The build is a BCM pencil weight barrel and BCM bolt along with a Colt A2 upper with factory M4 feed ramps. All other parts to build the upper are Colt or BCM.

The first time shooting it I put it on an factory built LMT rifle lower with A2 receiver extension, buffer and spring. It was basically a single shot. It might cycle a shot or two but most of the time it would not pick up the next round. I was shooting federal lake city 55 grain ammo and using USGI OK industries magazines in excellent condition. I attributed this to the reduced power of the midlength along with the rifle buffer and spring.

Second time out with it I used a factory LMT car15 lower with an unmarked (I would assume standard weight) buffer that came with the lower from the factory. I only fired 20 or so rounds through it this time and it worked great. I was using the same Lake City 55 grain but this time a magpul gen 2 magazine.

Yesterday was my third time out and this time I was using UMC yellow box .223 along with the same magpul magazine and LMT Car15 lower. I was having the same problem as the first time with it failing to pick up the next round from the magazine along with one “bolt over base” jam.

So is the system not getting enough gas to push the bolt back far enough to pick up the next round? Meaning I need a lighter recoil spring.

Or is the bolt moving so fast that the magazine can’t push up the next round in time? Meaning I need a H1 or H2 buffer.

Thanks,

Dan

 

[mcb]

Where is it ejecting? If it's too much gas (ie fast bolt) ejection will be forward 1-2 o'clock. If it's not enough gas (ie slow bolt) the ejected brass will be coming back at you 4:30 to 6 o'clock.

I would bet on under gassed. Check that you don't have any major gas leaks at the gas block, gas key or gas rings and that your gas tube is in good shape and long enough. If your sure you have no leaks or other problems slowing the bolt the best solution is to increase the gas port size in the barrel.

Buffers and springs play a secondary role in bolt speed, gas port is the primer driver of bolt speed. Get enough gas (or turn the gas down if that is the case) first then use buffer weight and maybe spring rate to fine tune the system.

 

[BreechFace]

Can you weigh your BCG and buffer? Give those weights and it will help. Did you happen to measure your gas port on the barrel before putting the rifle together?

Some things to check:

1. You sure your gas block is correctly aligned forward and aft and side to side with the gas port?
2. As was said by @mcb check for any leaks around gas block and gas key, look for carbon spread from those areas.
3. Are you running the BCG wet?
4. Are you able to completely retract the BCG in to the buffer tube and set the bolt catch?
5. What happens when you load one round in a magazine and fire, does the bolt lock back?

To me, my money is that you are undergassed which can mean, undersized gas port, gas leakage, too heavy a buffer, spring and BCG.

Your "bolt over malfunction" most of the time is magazine related, but also can be from short stroking or out of spec or worn extractor spring. This is caused by a weak magazine spring not presenting the next round quick enough, or it could be your BCG is short stroking (undergassed and not completing it's cycle rearward thus not giving enough presentation time for the next round causing the bolt to ride the side of the next case out of the magazine rather than the case head.

 

[BreechFace]

One thing to make sure is that you are sure you have rifle length spring, standard rifle buffer and, A1 or A2 buffer tube. If you are combining anything out of a carbine setup into a rifle setup you will have issues.

If you are getting the weights I had requested you might get your rifle buffer spring out and measure it as well, it will help determine you have the correct parts.

Your rifle buffer should weigh about 5.0 - 5.2 ounces and the rifle buffer spring should be about 12-3/4" long and should have 41-42 coils.

 

[jak67429]

Also make sure the gas port is lined up with the gas block.

 

[FL-NC]

Check the simplest things first, which would be a gas problem. Is your block properly aligned? Gas rings passing a drop test? Gas key on BCG tight? If all of these check out, then try some different buffers that you KNOW are of different weights.

 

[d2wing]

Hold on, what barrel length. If it is 16 inch and you are running a mid length gas system you may not have enough dwell time for enough gas to be available to work the system resulting in short stroking.. Dwell time is the time from when the bullet passes the gas port until it leaves the barrel. If on the other hand the bolt is moving too fast a heavy buffer can help.

 

[Shawn Dodson]

The Lake City M193 5.56 you were shooting is higher pressure than the UMC .223 Remington. Either your gas block is not properly aligned on the barrel or the gas port on your barrel is the proper diameter for shooting 5.56 with a carbine buffer system and does not function reliably with weaker .223 Remington ammo.

 

[mcb]

Hold on, what barrel length. If it is 16 inch and you are running a mid length gas system you may not have enough dwell time for enough gas to be available to work the system resulting in short stroking.. Dwell time is the time from when the bullet passes the gas port until it leaves the barrel. If on the other hand the bolt is moving too fast a heavy buffer can help.

A mid length gas system was design to give roughly the same dwell time on a 16 inch barrel as a carbine gas system on the 14.5 inch barrel. You can always beat a short dwell time with a larger gas port.

Somewhere on the internet is a video of 2-inch barrel cycling, yes the barrel was shorter than the cartridge and it still cycled.

I found it:

minimal dwell time.

 

[Skylerbone]

It sounds like your freshman effort may have misplaced the gas block. What block you used will make a difference but most require a gap between block and shoulder. If you slid the block down against the barrel shoulder then your barrel’s gas port will be partially blocked.

Shooting your A2 lower with its 5 oz buffer was enough to point out the problem whereas your carbine buffer at 3 oz was just forgiving enough that you believed it was working.

Grab your .025 feeler gauge and check for that gap.

 

[Dan Forrester]

Unfortunately I didn’t check where the brass was ejecting. I can make it back to the range in a few weeks and will let everyone know.

I had ADCO install the FSB with tapered pins so I would assume it was done correctly. The FSB was a new non F marked FSB. I would think the hand guard cap should give it enough spacing right? I don’t see any evidence of gas leaking around the block.

Also forgot to mention that the bolt was not locking back after the last round.

Gas key is tight and properly staked and the bolt was being run wet.

The rifle length spring has 42 coils and the buffer weighs 5.15 ounces.

The carbine buffer has 38 coils and weighs 2.95 ounces and the spring is 10.25” long.

I appreciate everyone’s help diagnosing this rifles problem! I guess the moral of the story for me at least is to stick with 20” rifles and 14.5” SBRs from now on. I have uploaded a picture of the FSB and the rifle.

Thanks,

Dan

 

[d2wing]

A mid length gas system was design to give roughly the same dwell time on a 16 inch barrel as a carbine gas system on the 14.5 inch barrel. You can always beat a short dwell time with a larger gas port.

Somewhere on the internet is a video of 2-inch barrel cycling, yes the barrel was shorter than the cartridge and it still cycled.

I found it:

minimal dwell time.

That's amazing to me.

 

[David Hoback]

Why would the moral be to stick with 20” & 14.5” rifles? That is a horrible moral! I’ve built more ARs than most. I’ve done 20”s, 18”s, 14.5“/14.7”s, 8.5”, 11”....and a hell of a lot of 16”s! And every single one functioned perfectly, with exception of a couple having minor short stroking issues. Those were easily remedied with opening up the gas port.

A 16” barrel with Midlength gas system is a better design & my preferred. The ONLY 16”/mid I had short stroking on was in 7.62x39. Actually very common for barrels in 7.62 to have an undersized gas port. In your opening post you forgot to mention the bolt was locking back...kind of a big piece of evidence, LOL Even still, I was convinced short stroking was your problem. If the place who installed your sight block has it slightly covering the port, nothing you can do about that...but opening up the port will rectify the issue either way. Pull the sight post, and measure the port size using a corresponding drill bit. Here’s a thread showing normal port sizes for different length/gas system/caliber of barrels. https://forum.308ar.com/topic/15037-barrel-gas-port-size308338260243-etc/

 

[Skylerbone]

measure the port size using a corresponding drill bit.

Pin gauges are what’s needed, not drill bits. I don’t see many undersized gas ports, in fact universally the opposite.

OP something is reducing gas in your system. Assuming the FSB isn’t canted, next step on my checklist would be checking gas rings, then carrier key.

 

[mcb]

Pin gauges are what’s needed, not drill bits. I don’t see many undersized gas ports, in fact universally the opposite.

OP something is reducing gas in your system. Assuming the FSB isn’t canted, next step on my checklist would be checking gas rings, then carrier key.

A good set of number drills would be sufficient for both measuring and increasing the port size. Between .06 and .012 the biggest jump in size is .0045 inch. Plenty of resolution for measuring and stepping up the gas port size in an under-gassed gun if that turns out to be the OP problem.

 

[David Hoback]

Pin gauges are what’s needed, not drill bits. I don’t see many undersized gas ports, in fact universally the opposite.

OP something is reducing gas in your system. Assuming the FSB isn’t canted, next step on my checklist would be checking gas rings, then carrier key.

Yes, I’m aware... And you are correct. However, most people don’t even know what pin gauges are, much less have them on hand...and it doesn’t make much sense to purchase a set for one build. If at all! Hell, I AM a Gunsmith/Gun builder, and I don’t own a set! Mainly because I have a TON of drill bits in SAE, Metric & Wire Gauge. And any time I’ve needed to measure a hole, I’ve had the corresponding bits.

Port holes are fairly standard..(on quality barrels anyway). Can check with a 5/64”(.0781”), & #46(.0810”). Most anyone has a 5/64” bit & a #46 costs a few bucks.

If I were the OP I’d just pick up a #46 in anticipation. Because I bet his port is on the small side.

 

[BreechFace]

Without being there to further diagnose. I bet the port is on the small size.

For future reference for anyone building an AR, measure the gas port size before proceeding further into the build!

16” Mid-length gas on a 556 is an improvement over carbine gas when the port is sized right.

 

[Skylerbone]

most people don’t even know what pin gauges are, much less have them on hand...and it doesn’t make much sense to purchase a set for one build. If at all! Hell, I AM a Gunsmith/Gun builder, and I don’t own a set!

Your replies to, frankly darn near every thread tell me you are no more a gunsmith than you are a metallurgist. If one does not know that pin gauges exist then one should not set about monkeying with a firearm’s gas port. That MIGHT be dangerous.

Your chest thumping, ego driven self-flattery are in no way sound advice and though I have not “built” more than most (drilling holes in a jig) the nearly dozen I have assembled were never undersized. Isn’t “common”, almost unheard of. Feel free to list off the names of manufacturers whose barrels you’ve found to have undersized ports.

I may be censured for this post but advising people to have at a firearm with improper tools will some day cost someone an eye, a limb, or worse. There is a vast difference between casually checking port size with a drill bit (and most folks own junk that isn’t true to size) and advising someone to chuck that bit up and get 'er fixed. I should hope you don’t participate in the reloading section.

 

[David Hoback]

Your replies to, frankly darn near every thread tell me you are no more a gunsmith than you are a metallurgist. If one does not know that pin gauges exist then one should not set about monkeying with a firearm’s gas port. That MIGHT be dangerous.

Your chest thumping, ego driven self-flattery are in no way sound advice and though I have not “built” more than most (drilling holes in a jig) the nearly dozen I have assembled were never undersized. Isn’t “common”, almost unheard of. Feel free to list off the names of manufacturers whose barrels you’ve found to have undersized ports.

I may be censured for this post but advising people to have at a firearm with improper tools will some day cost someone an eye, a limb, or worse. There is a vast difference between casually checking port size with a drill bit (and most folks own junk that isn’t true to size) and advising someone to chuck that bit up and get 'er fixed. I should hope you don’t participate in the reloading section.

I’m sorry you feel that way. I was agreeing with you in fact....in the first sentence of my post. I wasn’t impugning what you said...not sure why you are so angry.

I do what I do. Of course you can feel free to disagree. This is the US... it’s “OK” if we don’t agree. Doesn’t mean you need to attempt to discredit someone because you feel threatened in some way. I certainly wouldn’t make outlandish allegations in regards to what you do...just saying. It’s a shame you felt the need.

But then, I am quite familiar with “your“ kind of attitude.

 

[David Hoback]

Without being there to further diagnose. I bet the port is on the small size.

For future reference for anyone building an AR, measure the gas port size before proceeding further into the build!

16” Mid-length gas on a 556 is an improvement over carbine gas when the port is sized right.

My thinking exactly my friend. Same conclusion I had reached. However, I was personally attacked for thinking the way I have..

I guess my 20 years of trigger jobs, repairs, upgrades..., 15 years of building pistols & rifles, 10 years of studying/researching metals/elements/compositions & 8 years making custom knives & machining....JUST AREN'T GOOD ENOUGH FOR SOME!

Oh...but I forgot.. I’m also 100% disabled, completely blind in my left eye & only have one arm/hand to use. Does any that mean anything?

 

[earlthegoat2]

[QUOTE="Skylerbone, post: 11671797, member:Feel free to list off the names of manufacturers whose barrels you’ve found to have undersized ports. [/QUOTE]

I agree with everything in this thread you have said.

This is for the sake of facts not just me being contrary. I do have an example. It is not my own personal experience.

Buffalo Outdoors did some vids on Bear Creek Arsenal uppers in 450 Bushmaster and 458 Socom. With the 458 Socom upper, he was having cycling problems and figured it was the gas port. He opened it up and it seemed to fix the problem.



Also, somebody played directly into my sig line.

 

[Skylerbone]

“My thinking exactly my friend.”


And so both of you gentlemen are in agreement, one of you being a Gunsmith, that rather than checking the simplest and most likely items (gas rings and carrier key in that order) the metallurgist in you is suggesting the OP unpin the FSB and check port size with a drill bit FIRST?

Skip right over easy and go to that and don’t bother with perhaps a little compressed air through the tube? Can you see where your self-defined expertise is lacking? I’m not attacking you, you’re attacking any position that is not your own and claiming victory by virtue of label.

Could the gas port be undersized? Sure. Most likely cause of the OP’s problem? No. A very distant possibility in my lay opinion.

If you have beef with my attitude you should take a hard look inward. 15 years ago I was in my 30s and had adjusted dozens of triggers by then. I was reshaping stocks, bedding actions, and rebluing. Do I claim to be a gunsmith for these efforts? No. And whether you’re disabled or not has zero to do with experience unless you’re implying the loss of an eye and limb is somehow related to your ‘smiffin.

More to the point whenever someone questions you, your immediate response is defensive, offended, and followed by your vague list of qualifications. Circumlocution seems your only direction of travel.

 

[David Hoback]

Awesome! Whatever you say chief!

B-Bye!

Edit: Oh, for the record.. I agree(once again), that the easy items, gas rings, gas key, etc.. SHOULD be looked at first. The OP built his AR with new, high quality parts. I wouldn’t underestimate his knowledge, to think he has not already checked the obvious. But, if one of these were overlooked & end up being the problem, I apologize. But I doubt it. If the OP reports back what the fix was, we shall see if others can easily admit being wrong.

 

[IdaD]

Hold on, what barrel length. If it is 16 inch and you are running a mid length gas system you may not have enough dwell time for enough gas to be available to work the system resulting in short stroking.. Dwell time is the time from when the bullet passes the gas port until it leaves the barrel. If on the other hand the bolt is moving too fast a heavy buffer can help.

I have a midlength gas system on an AR with a 16" barrel and haven't had any issues with it, and as far as I've ever heard there's nothing inherently wrong with that combo. In fact most of what I've seen would be that it's better than a carbine length system in a 16" barrel, but I'll be the first to admit I'm no expert when it comes to ARs.

 

[David Hoback]

I have a midlength gas system on an AR with a 16" barrel and haven't had any issues with it, and as far as I've ever heard there's nothing inherently wrong with that combo. In fact most of what I've seen would be that it's better than a carbine length system in a 16" barrel, but I'll be the first to admit I'm no expert when it comes to ARs.

Your thinking is correct. A 16” barrel IS better matched with a midlength gas system.