Bolt Carrier Slipping off Catch

[DocLarsen]

Hey guys,

I just finished the assembly of my lower assembly and while testing things I engaged the bolt stop. It held for maybe 10 seconds and then the carrier slammed forward.  Thinking it was operator error, I tried it several more times with the exact same result.  I noticed that someone took a little off the bolt stop so that it could pivot more, thereby allowing the bolt stop to extend higher.  Is this a good way to proceed or is this something that I did wrong?  It is a pretty simple system, so I can't imagine what I could have done wrong.  The pivot pin is screwed all the way in, could I have used the wrong return spring and it is overpowering the push of the carrier and disengaging the stop? (I did have an extra spring; It is one like the take down and pivot detent springs.)  Not enough push from a wrong buffer spring to cause enough friction on the bolt stop?

BTW, I got the Bill Springfields complete lower kit with the trigger work done: 4lb pull and "speed hammer".  Trigger feels soooo nice and crisp! 

Any ideas?

Thanks,

DocLarsen

edit for clarification - DL

[TacBlade]

I am not sure exactly what you mean about the modification to the bolt stop?  I would say replace a part that has been modified for some reason, as I see no need to modify something like that?

[DocLarsen]

I think I've already figured it out.  My butt stock's buffer tube is not deep enough and this doesn't allow my bolt to come back far enough for the bolt stop to get in front of the bolt and stop it.  It was trying to grab that very tiny lip on the carrier just below where the bolt enters it.

What I am going to have to do is get an buffer tube extension to put in front of the butt stocks buffer tube, which is a commercial length AR-15 tube and is not long enough to accept the .308 longer throw on its own. I am sort of surmising here, so does that make sense to the more experienced here?

Thanks,

DocLarsen

[TacBlade]

It should work the way it is.  Maybe your buffer is to long and won't let the bolt carrier come back far enough.

[DocLarsen]

TacBlade,

You are half right.  The buffer was too long, so I shortened it, but once I got it to the proper theoretical length it was still no go.  The problem is that the spring is wrong too and I am now getting coil bind at just under 1/2 inch from where I need to be.  Bottom line, I have to get DPMS part nos: 308-CS-10A and 308-CS-10B.  This is a carbine length buffer and spring for the .308.  Midway has them for $25 for the buffer and $5.00 for the spring. I am going to fill the buffer I shortened with lead and I'll have it for a spare.

Thanks,

DocLarsen

[survivalshop]

Do you have a mag. in the rifle & the bolt stop will not hold ? If you don't have a mag. in , it is normal for it not to hold .

The spring pressure from the mag. spring on the mag. follower is what operates & holds the bolt stop up to hold the BCG open.

[DocLarsen]

Survivalshop,

Are you saying that if you manually depress the bolt hold button on the side of the receiver and let the BCG slide forward after pulling the charging handle all the way back, that it is normal for the bolt stop to not hole the BCG open?  If that is what you are saying then it goes against Brownell's first step of "Function Checking at the Bench": "1. Lock back the carrier, inspect the magazine well and chamber to ensure that the rifle is actually unloaded."

While the mag follower should cause the bolt to hold open following the last round fired, I really don't think that is the only way the bolt can be locked open. Virtually every semi-automatic gun that I have ever handled had a mechanism to lock the bolt open and then release it. 

Am I missing something?

DocLarsen

[survivalshop]

If there is not a magazine in the rifle , the bolt stop has no holding power , sure it is part of the function test , just to see if it mechanically can hold the bolt open , but as said with out the mag. in there you have nothing to hold back the heavy spring pressure of the buffer spring .

If it holds momentarely , its working . The mag. is what keeps it in possession to hold the BCG open .

All you are basically doing with the function test is to see if the bolt stop goes up enough to hold the BCG .

The 308 BCG &  the 308 buffer spring have a lot more mass & spring pressure than its little brother .

If you have other AR's, try the same test , with out a mag. in an empty rifle , manually engage the bolt stop and slap the rifle stock or some thing & you will see the BCG go back in battery.

The spring on the bolt stop is there to to reset the the bolt stop ,back into its normal position.,so with out the spring loaded mag. follower to hold it up , the BS spring is pressuring the BS to go back in its normal position also .

So that gives three components forcing the BCG back into battery with out a mag. to hold the stop up ,BCG mass,buffer spring & the bolt stop return spring , you asking a lot of just the friction interference of that little bit of bolt face, that the bolt stop has a hold of, to hold it open ,with out the mag in place

If the bolt stop is not holding at all , then you have some thing not right. The BS has to cover a certain amount of the bolt face for it to work properly . If you are using an AR 15 BS , than you may want to look into the larger one made by DPMS ,just for the 308 BCG.

[TacBlade]

Yes!!!!  What he said!!!

[DocLarsen]

survivalshop,

While waiting for the buffer and spring to get here, I nibbled away at the spring until it was short enough to allow the bolt stop to come up in front of the bolt. Specifically, the lugs are against the bolt stop.  Of course, now the spring is not quite strong enough to completely close the bolt, but it allows me to test the functionality.  I did your test and found that you are correct, but it does take a pretty good bounce of the butt stock to bounce the carrier enough to allow the bolt stop to retract and the bolt to go into battery, just as you said.  Put in the empty mag and, again, just as you said.  I took it a bit further and place a couple of rounds in the mag and cycled them through to make sure the bolt would catch on its own when the mag emptied; it worked as advertised.

So, when the proper buffer and spring arrive, I should be in business.  I am a little concerned though because DPMS told me that the .308 carbine buffer only weighs 0.238lb (3.808oz). This may be just a tad light for a mid-length gas system. What do you guys think??  I've also just ordered a PRI muzzle brake.  I figure that if I have any problems, I'll take a que from Tubbs and get the total weight up closer to the 5.4 oz that Slash recommends by fabricating something like the CWS. I am not as concerned with recoil as I am with slamming the BCG around too hard or giving it enough momentum to get complete and full close & lockup of the bolt lugs. But keeping a good sight picture or, at least, minimizing the time to reacquire one after a shot is a nice thing too.

Thanks,

DocLarsen

[latewatcher]

this might seem silly, but what about the heavier carbine buffers for pistol caliber conversions. they are heavy and would add the weight that a .308 carbine buffer does not have. thoughts?

[imschur]

Theres info here

http://308ar.com/forum/index.php/topic,662.msg4478.html#msg4478

[survivalshop]

Doc

What receiver extension are you using ? I see you say Commercial , so I would assume it is a carbine receiver extension ,am I right ?

I only say that because you have a Mil Spec & commercial receiver extensions for a collapsible stock configurations.

If you are using a rifle stock , you would need a rifle ,receiver extension (buffer tube )

Now if you have a rifle buffer and or spring or any combo other than the right spring & buffer combination , you will have problems .

I am running a 16" barrel , with a rifle length gas system & a DPMS carbine buffer & spring & have had no cycling problems at all . Just some mags won't push the bolt stop up far enough on last round (PMags have all worked ) Just certain DPMS & Cproducts will not operate it all the time .

At first no mag would hold the BCG back on last round. Yet these same mags. all work in my 20" rifle with DPMS upper/lower .

You will like the PRI brake ,I have one on my 20" rifle.

[DocLarsen]

What I have is a commercial tube (internal dimensions=7" x 1" ) with a 6 position carbine butt stock made for an AR-15.  The new DPMS buffer and spring arrived today. I installed them and everything works as it should.  Well, at least everything works as it should short of actually firing the gun and having it cycle and/or catch the bolt after the last round. But, the spring/bolt travels far enough to allow the bolt catch to work properly. The bolt fully closes and locks from either pulling and releasing the charging handle or from releasing it off of the bolt catch with the button.  I'll post the specs on the spring and buffer in the Buffer Tube, Buffer and Buffer Spring topic, but here is the info on the parts:

DPMS LR-308 Carbine Buffer Spring - Midway #: 813595 / DPMS #: 308-CS-10A

Relaxed Length - 11 3/8"

Compressed Length - 3 1/4"

28 coils (inclusive of both ends)

spring rate - ≈10lbs/in (tested over 2" travel from rest on buffer retainer pin)

DPMS LR-308 Carbine Buffer - Midway #: 232006  DPMS #: 308-CS-10B

Length - 2.938"

Flange Diameter - 0.969"

Barrel Diameter - 0.686"

weight - 3.808 oz.

Thanks everyone for you input and help!

DocLarsen

[98Z5V]

What I have is a commercial tube (internal dimensions=7" x 1" )

Doc, internal dimensions don't determine commercial vs. mil on the tubes - it's the external diameter of the tube that makes it one or the other.  Mil tubes are 1.14" O.D. and Commercial tubes are 1.17" O.D.  Mil and Commercial stocks are sized (internally) to fit that external tube diameter of the carbine receiver extension.

Internal dimensions will all be the same, because the same parts have to work inside them.  The difference in the diameters doesn't necessarily equal strength - you'd think the larger one would be stronger, but it's not.  It's in the threading.  The 1.14" Mil RE has external cut threads (stronger), and the 1.17" Commercial RE has internal cut threads (not as strong as external threads).  Wall thickness doesn't really come into play here, as much as threading does.  It's a "bolt strength" thing with the threading differences - I'm really talking about bolts, like hardware, and not like firearms bolts.  No joke, brother.  <thumbsup>

EDIT - there's a Magpul chart/pic that shows the differences, but my hotel internet isn't cooperating with me right now - someone will post it, no doubt. 

[survivalshop]

Doc, internal dimensions don't determine commercial vs. mil on the tubes - it's the external diameter of the tube that makes it one or the other.  Mil tubes are 1.14" O.D. and Commercial tubes are 1.17" O.D.  Mil and Commercial stocks are sized (internally) to fit that external tube diameter of the carbine receiver extension.

Internal dimensions will all be the same, because the same parts have to work inside them.  The difference in the diameters doesn't necessarily equal strength - you'd think the larger one would be stronger, but it's not.  It's in the threading.  The 1.14" Mil RE has external cut threads (stronger), and the 1.17" Commercial RE has internal cut threads (not as strong as external threads).  Wall thickness doesn't really come into play here, as much as threading does.  It's a "bolt strength" thing with the threading differences - I'm really talking about bolts, like hardware, and not like firearms bolts.  No joke, brother.  <thumbsup>

EDIT - there's a Magpul chart/pic that shows the differences, but my hotel internet isn't cooperating with me right now - someone will post it, no doubt.

You can start a new thread on which is stronger or not .

Glad to see you got her working Doc. Now get out the & put lead down range. Photo's please.