Where are my AR guys at?

[Rooster41]

Are any of you good with figuring out problems? I have built a few AR's and consider myself knowledgeable enough to figure out problems, but this one has me stumped. I have an issue with my bolt catch not catching the bolt. The upper is a 10.5" 300 blackout that was purchased from KM Tactical. I believe it was one of their own builds. The lower was purchased as a completed product from Palmetto State. The BCG is a full auto BCG, nothing special about it. I have ruled out problems with the upper and BCG. I put the BCG on a different upper and it worked as it should. I also put the upper on a different lower and everything worked as it should.

When the bcg is pulled all of the way back, the bolt is still sticking out and the bolt catch is unable to lock it open. I did some reading online and one person suggested cutting a couple of loops off of the buffer spring. I did that and it did not work. I am not sure I want to cut off anymore, and quite possibly I think another buffer spring is in my future. This is the only AR Pistol I have, so I cannot compare it to another pistol. For what its worth I do not shoot this suppressed, not that would have anything to do with the problems I am experiencing.

When I first started shooting this about this time last year I did not have any problems with it. I took it deer hunting and fired one shot, which I missed. When I got back to the truck, I found out I had been walking around with an empty chamber all day. The second time I took this hunting I again spent the day with an empty chamber. I tried loading it at the truck and noticed the bolt would not come back far enough to take up the round out of the mag. I am using 10 round PMAG for hunting. I also had the same problem with metal mags and 20 & 30 round PMAG. I don't believe this to be a mag issue. I will get some pictures up later to try and visualize my problems.

Thanks in advance.

[The_Shadow]

Be sure the bolt is properly lubed to include the cam pin as it could be adding some friction to the cycling!

[Bimmer]

First, go straight to M4Carbine.net and ask there...  (I think that whoever told you to cut the spring was an idiot.)

My guess:  Your buffer is too long for your pistol lower. 

Aren't pistol receiver extensions and buffers shorter than carbine, which are shorter than rifle-length? 

Try this:  Remove the spring.  Now see if your buffer and BCG both fit in the extension.  I bet they don't. 

[sqlbullet]

Carbine and pistol have the same length buffer.  Only rifle is longer.  But mechanical obstruction certianly seems like the most obvious answer.

I would start with no spring or buffer and see if it will lock open.  Have a plan for how you are going to get the bcg back forward before you do this.  (finally a use for the forward assist :-) )

If that works, I would grab a buffer from a different carbine or pistol lower where it does work and try that in this lower.

I would guess the plastic cap on the buffer is out of spec.

[Mike D]

Have you removed the buffer and spring since you noticed the problem?

I would compare that buffer with another known carbine buffer. You are definitely getting a mechanical stoppage and either you are getting excessive carrier tilt (not likely) or your buffer is too long.


I suppose as a long shot your buffer tube diameter could be out of spec and not letting it travel far enough rearward.

Sent from my iPhone using Tapatalk Pro

[sqlbullet]

And, let us know what you find.  Inquiring minds want to know.

[tunnug]

I don't have any experience with pistols only rifles but I do know that in certain situations the screw at the rear of the buffer tube can be too long and keep the buffer from going back far enough, usually when taking out the extension to use a shorter stock.
Do pistols use any screw at the rear of the buffer tube? Could that be the problem?

[sqlbullet]

Nope.  Pistol and carbine buffers don't have that fixture at all.

[Rooster41]

OK. So I have not played with this since I posted it. I had received the parts I needed to upgrade the DMPS Oracle AR10 from its original configuration to a free float handguard, low profile as block, VG6 Muzzle break and magpul stock and grip. Also, for the pistol, I ordered a new buffer spring for carbine since I blindly followed a suggestion on another forum and cut some of the spring off. Instantly regretted doing that when I heard the metal hit the floor. But after cutting off a couple of coils it still did not go back far enough to lock the bolt open.  The buffer tube/spring came as a kit from PSA and is the KAK Shockwave.

[Bimmer]

The buffer tube/spring came as a kit from PSA and is the KAK Shockwave.

Hmmm...  Dunno about that. 

I'm still guessing there some obstruction or something that's preventing your buffer from going all the way to the rear of your buffer tube. 

It shouldn't take you more than about 3 minutes to (1) remove the rear take-down pin and separate the rear of the upper from the lower, and (2) remove the buffer and spring, and (3) see whether the buffer and BCG fit in the buffer tube. 

[cwlongshot]

Im assembling two uppers... might possible
Morph into three...

357 Sig. 450 Bushmaster. Maybe a 6.5Grendel or a 350 Legend as well. Probably not both. (All four)

CW

[Rooster41]

I finally got his figured out. The BCG was too long. I put the BCG in a different 300 BO rifle and 223 Rifle and the bolt locked open. But when comparing the 3 BCG's side by side the one I had in the pistol was longer. So I put one of the shorter BCG in the pistol and now everything is working properly. Kinda weird, but it works again. Thanks everyone for the tips and suggestions. 

[sqlbullet]

Anything other than 6 and 11/16" in length for the bolt carrier is out of spec.

I would be contacting the seller of that BCG.