New 308 AR Build - Bolt Catch Not Working Reliably

[bruss01]

*** UPDATE ! Problem solved! See post #25 below ***

I just built not one but TWO 308 AR's and both of them are displaying the same behavior. The rifles have been assembled but have not been out to be fired yet, this is all just manually cycling the action.

For the record, the receivers are Sun Devil 308 (DPMS pattern) matched billet upper/lowers with nickel/teflon coating, the bolt is the JP Enterprises Low Mass, and the bolt catch is a DPMS type 308-specific bolt catch.

If I pull the charging handle back slowly with an empty mag inserted, I can *sometimes* get the bolt catch to engage. Also I can sometimes coax this engagement with a mag removed, while pressing on the bolt release ( the small end of course). This engagement is not reliable, with the bolt going forward unexpectedly more often than not. The other rifle does not engage at all, just hesitates slightly as if it were going to catch, as the bolt proceeds to slam home.

Can anyone advise me on what/where to check to determine what is going on and how to remedy the issue? Thanks.

[bruss01]

The bolt catch:

DPMS Bolt Catch LR-308 Matte



BCG:
JPBC-4 .308 Low Mass Carrier and JPEB-308 JP EnhancedBolt™ with cam pin, firing pin and firing pin retainer.



Receiver:
Sun Devil 308 matched billet set, Devil Coat finish (nickel/teflon)

[THT]

When you manipulate the bolt catch manually, does it take a lot of effort? Maybe the catch is oversized or the slot in the receiver is undersized?

[bruss01]

The bolt catch appears to operate freely. It is neither tight (sticky) nor loose (jiggly/rattly).

[kalieracer]

What mag are you using?

[jhmc2000]

I built my 308 in a matched JD upper/lower with the same JP BCG. I never installed the stock bolt catch and just ran the phase 5 you see in the picture. After the install I had the same issue........The bolt just would not catch. The lower was restricting range of motion of the lever. So I bent the lever out and that corrected the bolt catching issue. I think the correct answer is call JP.

[crazychinaman]

Have you checked to see if the bolt catch spring is correct.?What mags are you using. ?Sometimes C/P mags come with weak springs,will not push the bolt catch up enough.Have you tried it with Magpul mags.I was having problems with my rifles also in the past.I switched to Magpul mags and have no problems at all.

[bruss01]

Answer questions -

LPK is a Rock River standard with the following exceptions:

Giesle trigger/hammer group
DPMS 308 bolt catch
Proprietary Sun Devil bolt catch retaining pin

Pretty sure there's nothing questionable about the spring - unless there's some kind of special spring for a 308, it's the standard one that came in the RR parts kit. Yes, the nubbin is there on both of them. Is there such a thing as an XP bolt catch spring? If so I might give that a try, see if it makes a diff.

I've tried it with both PMAGs and with D&H steel mags - no difference. Also no difference with the mag removed, manually engaging the bolt catch. Sometimes it works, sometimes not. Doesn't appear to depend on how hard you are pressing it with your thumb while pulling the charging handle.

[Thorax]

Proprietary Sun Devil bolt catch retaining pin

That right there definitely raises my eyebrows.
I suggest a set of calipers and measure the bejeezus out of it in comparison to a standard one.

[Mute]

Could be the spring. When I built a Mega MA Ten, I had a similar issue as you. I snipped about one coil from the bolt catch spring. Everything has been working fine since. Over 3000 rounds without a hitch.

[bruss01]

I am corresponding with Sun Devil on the issue. It seems they are very tight tolerance, and have received reports of some bolt catches not being proper dimensions to work with them. They suggested relieving the bolt catch to allow sufficient clearance to engage properly. The description of how to do so was unclear to me though. I have sent them a diagram indicating where I think the relief needs to occur.

[kblack583]

Is the buffer spring a stahndard AR15 spring or a 308 specific spring? Standard AR15 spring in a 308 will cause what you are experiencing.

[kalieracer]

Most 308 lowers have a threaded bolt catch pin.. My MWS has one, The SR-25 / M110 has one and pretty much all the 80% 308s have one.

Originally Posted by Thorax

That right there definitely raises my eyebrows.
I suggest a set of calipers and measure the bejeezus out of it in comparison to a standard one.

[kalieracer]

Does it look like this?

http://www.midwayusa.com/product/110...tainless-steel

[bruss01]

Originally Posted by Thorax

That right there definitely raises my eyebrows.
I suggest a set of calipers and measure the bejeezus out of it in comparison to a standard one.

Most DPMS- pattern 308 AR's use a screw threaded retaining pin for the bolt catch. The Sun Devil receiver uses a roll pin like an AR-15, only it's a little bigger due to dimensional scaling up for the larger receiver and 308 bolt catch. It's not a bad system. there is also a little "trough" of sorts on the forward side of the pin hole. I am assuming this is so the pin can be driven completely through for removal. It's tough to envision driving it back... that would require a crooked pin punch.

Just waiting on confirmation from SD now that my impressions of where to file the bolt catch are correct.

[bruss01]

Originally Posted by kblack583

Is the buffer spring a stahndard AR15 spring or a 308 specific spring? Standard AR15 spring in a 308 will cause what you are experiencing.

I'm not aware of a separate spring for .308 AR's. Maybe you can clarify, or provide a link? Anyway, I do not think that a weaker or stronger spring in the bolt catch would make any difference when the catch is being engaged manually, such as when the mag well is empty. You're pressing on it with your thumb (or whatever) at that point with more than sufficient force to overcome any spring resistance. If the catch is not engaging at that point, it's not a spring issue, it's a clearance issue. I think I have this worked out in my mind, I'm just waiting on confirmation before I break out the pin punch and the file.

For the record... the bolt catch spring disengages the bolt catch. The magazine spring is what engages the bolt catch. Assuming the mag spring is strong enough to overpower the bolt catch spring (it should be, if it can lift the weight of a mag full of ammo) then if the bolt catch isn't engaging the bolt, then the catch isn't going high enough to engage the bolt. Considering it mechanically, I think there's only one place that kind of interference can occur, where I've indicated in the attached pic a couple posts up.

Found this about the operation of the bolt catch. Very illuminating.

http://www.calguns.net/calgunforum/s...d.php?t=446407

[kblack583]

Originally Posted by bruss01

I'm not aware of a separate spring for .308 AR's. Maybe you can clarify, or provide a link? Anyway, I do not think that a weaker or stronger spring in the bolt catch would make any difference when the catch is being engaged manually, such as when the mag well is empty. You're pressing on it with your thumb (or whatever) at that point with more than sufficient force to overcome any spring resistance. If the catch is not engaging at that point, it's not a spring issue, it's a clearance issue. I think I have this worked out in my mind, I'm just waiting on confirmation before I break out the pin punch and the file.

For the record... the bolt catch spring disengages the bolt catch. The magazine spring is what engages the bolt catch. Assuming the mag spring is strong enough to overpower the bolt catch spring (it should be, if it can lift the weight of a mag full of ammo) then if the bolt catch isn't engaging the bolt, then the catch isn't going high enough to engage the bolt. Considering it mechanically, I think there's only one place that kind of interference can occur, where I've indicated in the attached pic a couple posts up.

Put the file away!!!!! Do a quick search on 308 buffer springs and you'll see that they are different from AR15. An AR15 buffer spring in a 308 will not allow you gun to hold open on a empty magazine. Get the correct parts in there before you start to modify.
's

[bruss01]

Originally Posted by kblack583

Put the file away!!!!! Do a quick search on 308 buffer springs and you'll see that they are different from AR15. An AR15 buffer spring in a 308 will not allow you gun to hold open on a empty magazine. Get the correct parts in there before you start to modify.
's

We're not talking about a buffer spring. We're talking about the bolt catch spring.

Of course I know that the buffer spring is different. I am using a 308 buffer spring. I was born at night, but it wasn't LAST night. Sorry if I contributed to any confusion over that. More coffee, stat.

[Mute]

Before you start filing, try the catch without the spring and see if the catch moves up high enough to engage the bolt without binding. If it does, then your catch isn't the problem. If it doesn't go up high enough, then you'll need find what is preventing it from moving fully into the proper place to lock the bolt back.

[bruss01]

I relieved the rear of the catch (lower portion) and it now works fine (at least on one of them). Took me a few attempts of trial/error. The catch is HARD steel. I tried filing it manually but it was just too hard. Ended up using a stone dremel wheel and boy howdy the sparks did fly. It was probably only a 16th of an inch, if that. But it made all the difference for that gun. Still working on the other one... maybe just a wee bit more?

[freedom-lover]

Two whole threads on this in the gunsmithing forum

[crazychinaman]

Originally Posted by bruss01

I relieved the rear of the catch (lower portion) and it now works fine (at least on one of them). Took me a few attempts of trial/error. The catch is HARD steel. I tried filing it manually but it was just too hard. Ended up using a stone dremel wheel and boy howdy the sparks did fly. It was probably only a 16th of an inch, if that. But it made all the difference for that gun. Still working on the other one... maybe just a wee bit more?

I am glad to hear you got it some what working.Did Sun Devil say use and DPMS .308 bolt stop or an Armalite bolt stop.?Or a AR-15 bolt stop.

[bruss01]

Sun Devil claims these are DPMS-pattern receivers but there are some diversions from the DPMS standard. There was nothing with the receiver saying not to use a DPMS 308 bolt catch, so that was what I used.

[bruss01]

Originally Posted by freedom-lover

Two whole threads on this in the gunsmithing forum

I just did a search in the gunsmithing forum for 308 bolt catch. Three threads came up. None of them were about the issue I'm having, or the solution I'm implementing. If you meant a specific thread, that addresses this issue, please direct me to a link and I'll take a look.

[bruss01]

HOLD THE PHONE!

Apparently I spoke too soon.

The test that convinced me the issue had resolved was apparently one performed after too long a day, and too little sleep. The bolt catch was working perfectly in the gun - without being anchored by the roll pin!

After I actually inserted the pin, the failures resumed. No matter how much material I removed from the bottom/behind of the bolt catch, I was still reaching a "hard stop" in travel well short of where it would engage the bolt.

Finally, a light dawned. It occured to me that part of the bolt catch might be bumping it's head on part of the upper, restricting travel. In a "eureka" moment, I decided to install the bolt catch, put a mag in the mag well, and anchor the upper to the lower using just the pivot pin. Then gradually close the upper down onto the lower, watching the bolt catch for any sign of movement.

As I lowered the upper gradually, finally there came the moment just prior to closing, when the bolt catch moved... YES! So I broke out a flashlight to get a good look at where it could possibly be touching. It appeared to be the beveled portion of the catch as it descends towards the release lever (see pic below, RED arrow). Using a dremel cut-off wheel, I relieved this portion of the catch in an arc. At first I just took a bit off the sharpness of that edge, but it still wasn't even close to fitting. More dramatic sculpting was required, so I took a fairly sizeable arc out of it.

The bolt catch is now installed (with the roll pin this time!) and works perfectly! I am reporting my findings to Sun Devil as well, in the interest of helping them provide good customer support to those who come after me.

[bruss01]

Just a follow up note: Sun Devil has acknowledged this to be a clearance issue with their .308 upper, and has also acknowledged an issue with their .308 upper not properly fitting a standard .308 upper receiver vise block. They are correcting the bolt catch clearance issue for all future receivers they will produce. They will also be introducing a Sun Devil - specific upper receiver vise block.

For the record, I am an enthusiastic Sun Devil customer and will buy more product from them in the future. But Sun Devil has conceded that there are some differences between their product and what would be considered a "pure" DPMS-pattern gun. This should not be a problem for any future customers, PROVIDED the customer is aware going in, and prepared to accept some minor differences.