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View Full Version : BCM BCG for mid ar-15 doesn't always lock back after last round


protoolsnerd
05-01-2011, 2:40 AM
I've noticed that the BCM BCG i have is particularly strong. At times a bit hard to rack.

'Bout half the time the bolt doesn't lock back after the last round in a mag. It's happened with both pmags and sheet metal mags. I'm thinking it's that tough bolt, but i'm new to the ar scene.

Any suggestions? Anyone else with the same BCG that had this issue?

supersonic
05-01-2011, 6:08 AM
It's not the BCG causing this. Sounds like it is short-stroking. Is it possible you may have the wrong buffer spring in your receiver extension?

dieselpower
05-01-2011, 7:35 AM
^I disagree, since its not always happening. Its possible, just not probable.

what ammo are you using...EXACTLY what ammo?

Noobert
05-01-2011, 7:46 AM
Hard to rack? Is it lubed, and is it still hard to rack if you rack it without a magazine inserted?

r6raff
05-01-2011, 8:31 AM
Explain "hard to rack"

Is it just a bit resistant or do you need to pull on the CH with all you got? Can you take some pics of your BCG so we can see if there are any abnormal wear marks. Also most specifics on the Ammo, I would first say Mags, Short stroke due to ammo, short stroke due to incorrect spring/buffer combo, SS due to gas... but since you have trouble manually racking then it could be something else. Get that BCG wet, lube the inside of the upper receiver and see if that changes, it could be that something is out of spec and a bit to tight.

supersonic
05-01-2011, 8:58 AM
^I disagree, since its not always happening. Its possible, just not probable.

what ammo are you using...EXACTLY what ammo?

^ You may disagree, but your 'since it's not always happening' logic is flawed. I have fixed a few inexperienced guys' AR's with the same problem by simply replacing a rifle spring with the correct carbine equivalent. I have witnessed the short stroking myself on the range. With less-than-average power level 5.56 the carrier will do this virtually every time. However, ammo that is loaded to full power spec can (and will) overcome the force of the longer spring. Plus, you have to take into account the small - but significantly important - variances in powder charges in many ammo lots. And, in the lower-quality types, this number only gets larger and more erratic between lots. Been there, done that, bought the T-shirt.;)

EDIT - While my guess as to what is causing the problem may not, in the end, be the culprit, the resistance he is feeling during charging the weapon could absolutely be caused by the incorrect spring. In fact, that could be just about the only thing causing resistance, unless something is way out of spec, or other parts are in there that shouldn't be.

dieselpower
05-01-2011, 9:05 AM
^ You may disagree, but your 'since it's not always happening' logic is flawed. I have fixed a few inexperienced guys' AR's with the same problem by simply replacing a rifle spring with the correct carbine equivalent. I have witnessed the short stroking myself on the range. With less-than-average power level 5.56 the carrier will do this virtually every time. However, ammo that is loaded to full power spec can (and will) overcome the force of the longer spring. Plus, you have to take into account the small - but significantly important - variances in powder charges in many ammo lots. And, in the lower-quality types, this number only gets larger and more erratic between lots. Been there, done that, bought the T-shirt.;)



Oh I agree its possible, I just wouldn't go there yet. I would first look at the more probably causes of a intermittent FTL. That is underpowered cheap ammo. so why go spend money for no reason other than to allow a lock open with cheap ammo? Its not cost effective.

supersonic
05-01-2011, 9:32 AM
I've noticed that the BCM BCG i have is particularly strong. At times a bit hard to rack.

'Bout half the time the bolt doesn't lock back after the last round in a mag. It's happened with both pmags and sheet metal mags. I'm thinking it's that tough bolt, but i'm new to the ar scene.

Any suggestions? Anyone else with the same BCG that had this issue?

Bottom line is this: you need to check a few things - 1)take your rifle down and remove the buffer & spring. Check to make sure the buffer is of carbine pedigree & the spring is carbine-length. If everything checks out, try sliding your BCG through its range of motion through the upper; the extension; and through the assembled rifle once more. Make sure there is nothing on the lower that is riding against the bottom of the carrier. Next make certain the bolt catch spring is installed & is the correct part. Sometimes, the assembler inadvertently leaves out the catch spring, which causes the part to rock back and forth at random during recoil. During each firing cycle, you have a 50/50 chance that the catch will rock outboard and lock the carrier. These steps should expose the issue.

Mr_Monkeywrench
05-01-2011, 9:41 AM
Want something reliable? Get a Stag ;):p:D

Cokebottle
05-01-2011, 9:47 AM
Where is your ejected brass falling? It should be falling somewhere between 3:00 and 4:30.
If it is ejecting more behind you (4:30 to 6:00) then it is short stroking. This can be caused by the buffer being too heavy or the gas system being undergassed.
If you have the wrong buffer spring, that could cause the same problem, and that sounds like it might be the problem if it is "hard to rack."

But you did say that you are new to ARs. How many different rifles have you operated? My A2 is much easier to operate than my A4 carbine.

Shenaniguns
05-01-2011, 9:55 AM
Yeah that's not a BCG problem, you may have the incorrect or broken buffer and spring. I had a pin walk out on my buffer that made it difficult to rack, so I knocked it back in until I replaced it.

protoolsnerd
05-01-2011, 1:59 PM
Here are some specs of my gun and ammo

Lower - LMT sopmod buttstock, double stage trigger

Upper BCM mid 16" 1 in 7 twist 5.56

BCM BCG

Ammo: Mostly federal xm193 and some of my own reloads. It's not consistent at all. I've used pmags and sheet metal mags with no correlation . I had thought about short stroking but my brass is going exaclty where it should. That 3-4 oclock and not heaving it too far either . The brass isn't being chewed up . It's accurate and shoots consistently .

I have a decent amount of lube on it, but the bcg just seems to be tight. Even well lubed up it requires a decent amount of effort to rack back . I"ll get a pic of it and post it in a bit.
96246

96244

96245

96247

96249


So it seems that my buffer tube retention pin and spring don't want to stay in now, and there also looks like a bit of wear on it hmmmm well here's all the info i have .... hopefully we can figure this out .

Thanks everyone for your input

r6raff
05-01-2011, 2:10 PM
Here are some specs of my gun and ammo

Lower - LMT sopmod buttstock, double stage trigger

Upper BCM mid 16" 1 in 7 twist 5.56

BCM BCG

Ammo: Mostly federal xm193 and some of my own reloads. It's not consistent at all. I've used pmags and sheet metal mags with no correlation . I had thought about short stroking but my brass is going exaclty where it should. That 3-4 oclock and not heaving it too far either . The brass isn't being chewed up . It's accurate and shoots consistently .

I have a decent amount of lube on it, but the bcg just seems to be tight. Even well lubed up it requires a decent amount of effort to rack back . I"ll get a pic of it and post it in a bit.

How about with the upper removed from the lower, does the BCG slide in and out easy with a little play? if so, then I would have to agree its probably spring issue... that or every Mag you have is shat lol

Shenaniguns
05-01-2011, 2:17 PM
You still posted nothing about the buffer and spring, do you know how to remove and inspect it? If you have in incorrect one it will short stroke (H2, H3?) and possibly be difficult to pull back (Rifle length instead of carbine).

protoolsnerd
05-01-2011, 2:38 PM
It's not impossible to pull just very spry lets say. I can do it , and do it quickly , just seems to take more effort then others i've used. I'm not sure how to tell which spring came with the lmt lower. It's a mid length rifle . I also find that after locking the bolt forward i can feel the buffer tube spring rattle a bit. If that's all normal then i'm at a loss

protoolsnerd
05-01-2011, 2:46 PM
How about with the upper removed from the lower, does the BCG slide in and out easy with a little play? if so, then I would have to agree its probably spring issue... that or every Mag you have is shat lol

It seems to move in and out just fine with a little bit of play . The bolt has never stuck or failed to eject a sing cartridge. I think once or twice in around 500 rounds it failed to load the first round so i had to rerack once. Just in case i'm buying a new buffer tube retention pin and spring.

Shenaniguns
05-01-2011, 2:49 PM
You bought the rifle in pieces so most likely something is not matched correctly, did you buy the lower complete from LMT? Or someone else? Why don't you replace the buffer tube spring with a correct carbine model and H1 buffer which should fix the problem. Did you pull the buffer out to see if a pin is dragging? I do not understand what you mean by locking the BCG forward, do you mean when you can see the BCM logo in the ejection port or not? I still believe there is a problem related to your buffer tube and/or buffer.

protoolsnerd
05-01-2011, 2:58 PM
I"M leaning that way as well. But i did get the lower complete from a gunshop near me, but , they got it complete from LMT. Assuming they didn't switch out any parts it should have all matched. Now what is the difference in length between a carbine length buffer tube and a rifle length ?

And yes what i mean when i said locking the bolt forward is when you can see the BCM logo.

docsmileyface
05-01-2011, 3:06 PM
INCONCEIVABLE! BCM IS TIER 1! YOU SIR MUST BE A LIAR!

protoolsnerd
05-01-2011, 3:13 PM
Unfortunately sir i do believe I AM NOT!!! But i don't think it's about any of the BCM parts. I think it's more likely related to the buffer tube ....... so we may need to say nay nay to LMT....but they're a solid company as well. Not sure yet what's the issue.

I mean at long last it may be the mags , who knows?

dieselpower
05-01-2011, 3:21 PM
If the buffer retaining notch came out the buffer tube is out of position. The tube needs to lock over the top of it at the shoulder. The buffer tube is probably 1 turn less then it should be.

when the BCG is forward there should be no pressure on the buffer and spring. Its possible to hear a little rattle. Its no different than there not being an upper attached. the buffer detent holds the buffer in.

I am still betting on this being an ammo issue. Its a 16" mid gas, with a FA BCG and H buffer..if not an H2. Look of the face of the buffer..is there an "H" on it?

Your rifle doesn't like the cheap stuff. either don't buy the cheap ammo, or replace the buffer with a standard carbine buffer like Sonic was saying.

Myself I wouldn't really care. Its not that big a deal. This is NOT the first time BCM mid upper owners have reported this. The last guy after researching this issue (and finding others with this same problem) placed his upper on our forums for sale...LOL His reason was he wanted flawless function with cheap ammo..thats not going to happen anyway. But whatever. http://www.calguns.net/calgunforum/showthread.php?t=402217

I wouldn't care. Grab some 62gr and I bet this goes away, or replace the buffer with a lighter one, or go with a SA BCG and not a FA BCG.

ProlificARProspect
05-01-2011, 3:29 PM
How Did you get the buffer retention pin and spring out? Did you remove your castle nut and end plate on your buffer tube to take it out?

The buffer retention pin should always stay in place unless taking off/ untighting the castle nut and end plate.



I have a decent amount of lube on it, but the bcg just seems to be tight. Even well lubed up it requires a decent amount of effort to rack back . I"ll get a pic of it and post it in a bit.
96246

96244

96245

96247

96249


So it seems that my buffer tube retention pin and spring don't want to stay in now, and there also looks like a bit of wear on it hmmmm well here's all the info i have .... hopefully we can figure this out .

Thanks everyone for your input

dieselpower
05-01-2011, 3:41 PM
How Did you get the buffer retention pin and spring out? Did you remove your castle nut and end plate on your buffer tube to take it out?

The buffer retention pin should always stay in place unless taking off/ untighting the castle nut and end plate.

yeah, I saw that too. I told him the tube was 1 turn too loose.

supersonic
05-01-2011, 3:46 PM
Judging from your pics, it looks like the buffer detent has some unusual wear on it. Is your receiver extension loose? If not, it may have been locked down before being seated correctly on the detent. If it won't 'stay down,' then that is most likely part of your problem right there. The carrier is striking it (hence the wear seen in the pic) making the BCG feel sluggish as it is operated. How about a pic of the bottom of your carrier? Also, as I said before, check to make sure the bolt catch spring is behind the detent. A loose buffer detent wouldn't keep your BCG from locking open. In fact, it wouldn't allow your bolt to go into battery if it becomes wedged in the "right" position during firing. Once you have done this troubleshooting, the only thing left to look at is ammo.

ProlificARProspect
05-01-2011, 3:48 PM
yeah, I saw that too. I told him the tube was 1 turn too loose.
This could be the problem, OP fix this first then see if you still have resistance from your BCG when charging/racking the BCG back.

Could be the buffer rationing pin failed and the BCG was out of the buffer tube and not in line when racking the BCG back.

protoolsnerd
05-01-2011, 4:07 PM
Wow... well i did take the castle nut off to install a sling plate....i must not have screwed it in enough when i was reassembling . I'm going to get a new buffer retention pin and spring just in case and try all of the above.

I know the xm193 isn't the best ammo but it's not like i'm shooting wolf or tula. As my reloads go , i use fairly decent materials. Win 748, LC brass, CCI primers, hornady bullets, and it happened with them as well. Is xm193 that crappy? What are my options for cheap'ish ammo then? Mind you this is a cqb gun, not a long range tack driver

dieselpower
05-01-2011, 4:23 PM
Wow... well i did take the castle nut off to install a sling plate....i must not have screwed it in enough when i was reassembling . I'm going to get a new buffer retention pin and spring just in case and try all of the above.

I know the xm193 isn't the best ammo but it's not like I'm shooting wolf or tula. As my reloads go , i use fairly decent materials. Win 748, LC brass, CCI primers, hornady bullets, and it happened with them as well. Is xm193 that crappy? What are my options for cheap'ish ammo then? Mind you this is a cqb gun, not a long range tack driver

The issue is in the timing. A FA BCG and a stiff carbine spring, and a H buffer don't necessarily work great with a mid gas tap-off and xm193. It should be ok, but depending on there not being a leak, it could just be a stiff system. As long as you are not having feed issues and extraction issues, I wouldn't loose sleep over it.

thats just me. Once again, is there an H on that buffer face? sonic could be right on target here.

you could replace the full-auto BCG with a semi-auto BCG and that tiny bit less mass could be all it takes to clear the bolt catch and get it to lock every time. It could be a bur in the gas port. it could be a tweak in the gas tube... it may just go away after a few hundred rounds...

Shenaniguns
05-01-2011, 4:41 PM
The issue is in the timing. A FA BCG and a stiff carbine spring, and a H buffer don't necessarily work great with a mid gas tap-off and xm193. It should be ok, but depending on there not being a leak, it could just be a stiff system. As long as you are not having feed issues and extraction issues, I wouldn't loose sleep over it.

thats just me. Once again, is there an H on that buffer face? sonic could be right on target here.

you could replace the full-auto BCG with a semi-auto BCG and that tiny bit less mass could be all it takes to clear the bolt catch and get it to lock every time. It could be a bur in the gas port. it could be a tweak in the gas tube... it may just go away after a few hundred rounds...


What? That setup is made for each other, XM193 is 5.56 and works great with said setup or else every reputable manufacturer of middy's (BCM, DD, Noveske etc...) would not build there rifles that way. My old Noveske 16" middy/H1/FA BCG buffer had no malfunctions with underpowered .223 to 5.56 though I never tried Tula or Wolf.

protoolsnerd
05-01-2011, 5:18 PM
Yeah luckily no feed issues , really no other issues at all. Other then sometimes not locking back after firing the final round in the mag ... it's damn near flawless

BobbyZ
05-01-2011, 6:02 PM
Gotta be a BCG mishap, probly just missing a stag upper.

supersonic
05-01-2011, 6:03 PM
Yeah luckily no feed issues , really no other issues at all. Other then sometimes not locking back after firing the final round in the mag ... it's damn near flawless

If your rifle is put together correctly, M193 should function perfectly every time the thing goes bang. Your BCG should always lock back after the last round in the magazine. Did you ever double check the bolt catch to see that there is spring pressure behind it? I sent you a few important questions via PM, but got no response. FYI, if you post something like this up asking for help, you should really make an effort to answer all the questions put to you (both in PM's and on the thread). It's just common reciprocal courtesy.;)

JayBeeJay
05-01-2011, 6:07 PM
My BCM 16" middy eats everything I feed it literally. Got abot 1500 rds. through the pipe and have used Wolf and Tula with no fails.

r6raff
05-01-2011, 7:58 PM
Gotta be a BCG mishap, probly just missing a stag upper.

I L O L'ed so hard I all most p e e'd myself

I kid, stags are gtg, still wouldn't trade my BCM for a Stag. But my next econo build will most likely be a Stag, a lightweight KISS

r6raff
05-01-2011, 8:05 PM
My BCM 16" middy eats everything I feed it literally. Got abot 1500 rds. through the pipe and have used Wolf and Tula with no fails.


Same 4-5k rounds down the pipe with practically zero malfunctions. A few light primer strikes that took 2 taps to fire (thats because I lightened the trigger myself) and a FTE that was caused by my brass catcher.

Its eaten everything from Tula/Wolf, Federal .223 (walmart), xm193, Fiochi .223, Tap .556, BlackHills red and blue box etc...

OP, you mentioned that you had added the ambi asap plate right, did this issue occur prior to that modification? If it started after then thats a very good place to start.

Occams Razor... its usually the stupid easy thing

wikidklown
05-01-2011, 8:16 PM
The buffer tube might be a little short or the carbine spring might be a lil too thick when compressed. If you pull the CH all the way back, does the bolt clear the bolt stop? If it doesn't then the first sentence may be true. If it clears, does it barely clear or does it have some play? All my AR's have at lease 1/8 to about 1/4 inch of play. My 2cents.

protoolsnerd
05-01-2011, 8:22 PM
Really i'm not sure what it is. Bolt goes all the way clear, feeds great, ejects great , it's really just that tiny problem that the bold doesn't lock back probably about half the time when getting to the final round in a mag. I'm waiting on my parts to be shipped to me from midway and i'll be trying everything to fix this, prob won't get to the range until the weekend

Cokebottle
05-01-2011, 8:31 PM
Does the bolt lock back when you manually operate the charging handle with the empty mag inserted immediately after it fails to lock back?

protoolsnerd
05-01-2011, 8:49 PM
Does the bolt lock back when you manually operate the charging handle with the empty mag inserted immediately after it fails to lock back?

Yep, locks back fine manually every time.

protoolsnerd
05-02-2011, 2:44 AM
You and your picking of nits!!! I'm not sure if it's half exactly but it's probably around there!!!

And , yes there is spring tension behind the bolt release . Currently my lower is awaiting a new buffer retention spring and pin, but i did screw it in in one more revolution and i see why the pin popped out. Hopefully that will fix everything . If not, the investigation shall continue

r6raff
05-02-2011, 9:19 AM
Did you ever say whether you had this issue before installing the sling plate?

Vel Mori
05-02-2011, 11:59 AM
FWIW:

I had a similar issue with my BCM middy upper when I got it. I originally built it with a standard carbine buffer but found that replacing it with a H1 buffer resolved my timing issues.

Vel

gemini1
05-02-2011, 2:29 PM
Wow... well i did take the castle nut off to install a sling plate....i must not have screwed it in enough when i was reassembling . I'm going to get a new buffer retention pin and spring just in case and try all of the above.

I know the xm193 isn't the best ammo but it's not like i'm shooting wolf or tula. As my reloads go , i use fairly decent materials. Win 748, LC brass, CCI primers, hornady bullets, and it happened with them as well. Is xm193 that crappy? What are my options for cheap'ish ammo then? Mind you this is a cqb gun, not a long range tack driver

There's the problem.

hnoppenberger
05-02-2011, 3:51 PM
never was a fan of the mid length gas system. sounds like if everything is installed correctly then your oversprung.

post results.

protoolsnerd
05-02-2011, 5:42 PM
Ya know , i'm fairly sure i did have this issue then as well. Oh and no H on the buffer tube . I cleaned out the bcg yesterday, waiting on a new retention spring and pin. We shall see.