AR-15 Lower with "No Internal Block"?

H

Higgins

New member
What does it mean if an AR-15 lower receiver is being sold with the notation that is has "no internal block"?

Does this mean the lower is not suitable for building an AR-15 around?

Thanks.
 
In early AR15s, you could drop in an auto-sear and with relatively little work make it selective fire (presuming you followed all NFA laws). Colt then began placing a large block in the lower that would prevent you from doing so. IIRC, that block preceded their change from small pin to large pin. I think Colt is the only one that ever put the block in.

You can build an AR15 on a lower with or without the block.

M1911
 
Not just a DIAS

That's a drop-in auto sear, but also a little device called a "lightning link" could also be installed. The block plus the open-back bolt carrier prevented the user from installing such a device.

In response to your question, a "no-block" lower is still a legal semi-auto ONLY. Just drilling the auto sear hole will be enough to net you in the Fed's stick wicket.

A DIAS requires a hammer and disconnector swap/removal. If selective fire is wanted, then the correct selector/safety lever with the correct open-back trigger and long-tail disconnect is required.

Other makers of AR's followed suite by narrowing the gap between the receiver walls over the safety lever in an attempt to block installation of a DIAS. Since the receiver forging is solid in this area, all that was done was just less machining.
 
Thanks for the additional info, KeithJ. I have no interest in cobbling together a full auto AR. Neither do I want to violate any laws, nor do I have any need/interest in a full auto .223.

I just didn't know what was meant by the term "no internal block," but you and M1911 have cleared it up. I just wanted to make sure the lower I was looking at wasn't an incomplete lower, requiring additional machining, or an older design that's not useable with standard uppers, etc... Thanks.
 
Since no one would ever think of doing anything illegal, I have no idea why those lowers with no block bring more money than later ones.

Jim
 
Unblocked receivers with small pins use commercial parts. You can get a Jewell trigger with big pins and a few 3rd party uppers are available for the take-down screw but you still are stuck using a lightweight bolt carrier. This light carrier might not function with as wide a range of ammo as a "closed-backed" carrier due to weight.
 
Unblocked receivers also had the channel milled slightly smaller than the M-16 receiver. So, even if the M-16 auto sear was procured, it wouldn't fit.
 
Only some of the later non-Colts have a narrowed milling back of the trigger. Even though this requires less metal removal, its a more difficult operation especially with a manual machine. Its function is along the same lines as the block.
 
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This here is one variety of the Colt Block. POS makes lower receiver work annoying sometimes :mad: Bushmaster,Oly and others use the "narrow receiver cutout" style.
 
i had a buddy mill one out of one of mine because i was under the impression that it had to come out to put in a Jewell trigger(Wrong). That thing is made from some bad ass metal--he said it was harder than woodpecker lips--had to use a carbide bit to get it out..by milling it out the pin on the exterior still looks the same.....Dick
 
You can use an AR15 bolt carrier in a blocked lower

Keith J,

You can use any AR15 bolt carrier in a Colt lower with either type of block installed. You can't use an M16 carrier. I'm not sure exactly how much metal you would have to mill off an M16 carrier for it to work, I do know from personal experience that you can take a full circle Colt AR15 bolt carrier (one that is closed on the back end) and use it with a sear blocked lower. I've never tried an Olympic AR15 carrier (only milled at the rear far enough back not to trip the auto sear in an M16) so I'm not certain how far back you would have to cut one, if you were "neutering" an M16 carrier for legal use in an AR15 or clone.

I'm also not aware of ANY reliability problems using the new Colt carriers that are half moon shaped at the rear insted of enclosed. In fact my Colt R6920 LE Carbine came with one of these new bolt carriers, but the lower has standard size trigger and hammer pin holes and no sear block.

Jeff
 
See pages 57-58 in Zedicker's book "The Competitive AR15"... I tried to fit a closed-back carrier in a Sporter once and it wouldn't function correctly.
 
What problems did you encounter?

Keith J,

What didn't function right? It worked fine for me. The carrier was from an SP1, just closed at the rear, not much metal on the bottom towards the front.

Jeff
 
Sporter blocks are larger and require fully open bottoms. The need to protect us from Lightning Links required the carrier to be completely open, never mind the fact one can pick up a spare carrier for a few dollars.

The blocked model Poodleshooter posted looks like a standard carrier would work.
 
Keith,

I never tried it on one with the big rounded block. A guy posted a method to the AR15 list awhile back to remove the sear block. (It's totally legal BTW, it's a Colts thing, not required by law.)

I don't remember the entire method and I'm not on my home computer so I can't post the actual instructions, but this is the basic method.

You drill a hole in the top of the sear block behind the blind pin. Then thread the hole and fill with oil. Place a screw in the oil filled hole and tighten. The hydraulic pressure forces the blind pin out and you can remove the sear block.

Jeff
 
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