So, I'm building my first AR, an AR pistol. I ran into a problem installing the buffer tube to the lower receiver. The buffer tube I'm using is not a standard buffer tube, it's shorter and lighter which why I desire it for my lightweight pistol build. However, I think there isn't enough threading on it for a regular castle nut. Here are some pictures showing my problem. In the two pictures below, the standard castle nut is at its rearmost position, the buffer tube is screwed all the way into the lower receiver, as much forward as possible. But as you can see, the buffer tube is still not over the buffer retainer hole.


So, I removed the castle nut. I was then able to screw in the buffer tube more, to its proper position, just over the buffer retainer hole. Now, look at the threading after the end plate, it's thinner than the width of the castle nut.


Anyone has any suggestion on how to solve my problem?
One solution I can think of is to find an alternative thinner nut to replace the standard castle nut. Does such nut exist? What would be the dimension and threading I need? I don't even know where to look beyond homedepot.com
Another solution would be to ditch the end plate. Without the end plate, I was able to get the buffer tube to the right position and have the castle nut on there. But then, how do I cover the hole for the rear take down detent and spring?
Lastly, is it feasible to add more threading to the buffer tube? I have no idea who does such work and how much would it cost...
Any advice or comment are welcome. I really appreciate the help from the experts here. Thank you.

Without the end plate  all you have to do is put a set screw into the hole that the spring and detent fit in to capture it in there might have to remove a couple wraps of the spring but you'll have to measure it and figure it out..

Sand down the nut (not optimal)
sand down the face of the end plate where the nut seats

looks like you only need about a threads worth of advancement

if you want to get real crazy sand down the rear of the reciver

ACE rifles stocks makes a thin lock ring.

Thanks for the suggestion on the thin lock ring, didn’t that existed. It looks exactly like what I needed, so I’ll try this before other non-reversible modifications. A couple more questions though. Do I need to get two of these? It seems the website says you need two to interlock together? And since it’s round and smooth on the outside, how do I tighten it beyond using my fingers? Thanks.

It has a hole in it for the old style spanner wrenches.

The seller of the buffer tube should be able to tell you what they recommend. Perhaps they sell a recommended castle nut.

Contact the manufacturer for help.
There may be online instructions.

A more expensive option is to run a captive buffer/spring combo like JP Silent Capture spring. I like them and it eliminates the need for the buffer retaining spring/pin combo.

Good advice so far from others.

Thread the takedown detent hole with a 4-40 tap, you don't even need to redrill it as it's already the right size. Use a 4-40 X .125" allen head set screw, cut the coils off the spring the matches the length of the set screw. I do this mod to all my receivers, takes less than 5 minutes. The screws can be had at any Fastenal store, google'em there is one by you. Most likely they will simply give you a few, mine always does.

ETA: https://www.fastenal.com/products/details/1125024?term=set+screw&r=~%7Ccategoryl1:%22600000%20Fasteners%22%7C~%20~%7Ccategoryl2:%22600047%20Set%20Screws%22%7C~%20~%7Ccategoryl3:%22600049%20Socket%20Set%20Screws%22%7C~%20~%7Csattr01:%5E%22Imperial%20(Inch)%22$%7C~%20~%7Csattr05:%5E1/8%22$%7C~

I bet after you tighten it down it'll sit flush. Doesn't look like you need much

Here's a novel solution. Get rid of your POS receiver extension and buy a proper mil spec one.

Skip the lock nut and use blue or red loctite?

I ordered the Ace thin lock ring. But while waiting for it to arrive, I watched a bunch YouTube videos on tapping the takedown pin channel and I thought, hey that looks easy enough, that could be a good mod for my other AR-15 rifles as well. So I got a few 4-40 1/8” set screw and a tap today from my local hardware store. Then I realized my lower is a polymer lower (TN Arms). So I want to check with you guys here again, is tapping polymer same as tapping aluminum? Or is it easier? Harder? Thanks for any experience you can share.

It should work about the same.

so you are spending all this time and money on a lower that will probably break?

Yikes.  I hope you made a conscious decision to go polymer after reading the pros and cons.

I understand you're looking for a lightweight build, but IMO the place to save that weight is not the lower receiver.

On the buffer tube; are you sure you've got a gun tube and not an airsoft tube?  I can't imagine any reputable manufacturer wouldn't thread it enough to let you capture the buffer retention pin and leave enough room for a standard castle nut.

That extension has cut threads so it can be chucked in a lathe and threaded farther towards the butt.

Have you considered running it without the buffer retainer pin? Sure, it makes assembly, disassembly a little tricky, but it is not a required part for the function of the pistol/rifle.

Thanks for all the helpful advice and comments, I really appreciate it. This community has so much wealth of info here, god knows the amount of time I spent here searching and reading...  Here are some more info on why I'm doing what I'm doing.

First, the polymer lower. Yeah, I know all about the pros and cons of polymer. If I had a choice, I would not use a polymer lower. Heck, if I had my choice, I'd build a proper SBR. But the state I'm in, CA, is impossible for SBR and very restrictive on AR pistols. I can't just go out get a stripped lower and build an AR pistol like most of you guys here. A lower registered as handgun is rare here and I had to pay a premium for one, even if it's polymer. So, I have to work with what I got. Since the one advantage of a polymer lower is its lower weight, I thought I might as well build a lightweight AR pistol. This will be a range toy, something fun and not intended for hardcore "my life depends on it" use.

Second, the buffer tube. This buffer tube is legit. It's from Maxim Defense, part of the CQB Pistol assembly. But I was planning to just use the buffer tube for this lightweight build, as the other parts add a lot of weight - the sliding cheek weld and its housing total almost 1 lb. The housing for the sliding cheek weld also functions as an end plate and is designed such that you don't need a castle nut with it installed. But this housing weighs about 5 oz. The end plate and castle nut I got to hopefully replace it weigh less than 0.5 oz. So, it's a no-brainer for me to try to make it work. I'm going to try the set screw method. That would eliminate the need for the end plate, which means more weight loss...

are you using the JP captured buffer?

No, it currently has a standard weight buffer. But I do plan to play with the buffer weight once the pistol is operational. Together with a low mass BCG and an adjustable gas block, I hope to eventually find a good combination. I know the JP captured buffer system is highly recommended, but it's not cheap. I'll consider it if other cheaper option doesn't pan out first.

Did you even READ the OP’s post?

Here’s a novel idea, only post a reply when you have some helpful advice.

i was asking because with the JP you dont use a buffer retainer.

It is unlikely to run well with a standard spring/buffer. Exactly what buffer you require is a bit hard to predict with pistol length uppers.
I have an SBR with a 10" barrel which needed a H2 (JP Enterprises captive spring equivalent - 2 tungsten and one steel weights).

Building a pistol with a 7.5" barrel, I tried with a standard buffer - no go. So bought an JPE captive spring H2. Cycled ok, but wouldn't lock back. Ended up having to change the weights to make it an H1 to get it to work correctly (super, sub and sub+suppressor).

Without knowing more, I would say try the standard buffer. You may be lucky. Some people have been. If that doesn't work, don't screw around, get the JPE captive spring buffer in H2. You may want to order a spare steel weight at the same time in case you have to reduce it to H1 - it depends a LOT on your upper/barrel/gas system (and, apparently, the phase of the moon).