Bedding AR receiver

[LRRPF52]

I know guys who have used all kinds of different epoxies, JB Weld, aviation-grade stuff, thermo-fit, press fit, over-sized extensions turned down to interference fit or thermo fit.

If you are going to turn down an oversized barreled extension, make sure you establish center with the right tooling first. The referenced video above is not exactly what I would want to see, although it was on a 300 BLK, so who cares.

There's a $2100 chuck and some other 5 and 6 jaw chucks designed for truing and blue-printing actions. There aren't a lot of machinists who are competent anymore at spinning a pipe on center, in my experience dealing with them. The ones that know what they are doing are gold in this industry.

[CastorTroy]

who sells an undersized upper?

[Keep The Change]

You are right LRRPF52, you should really ensure you are dead center and squared. I'm rebuilding a lathe and I fretting about getting it all level and the stock going back in and being squared and true. It's going to be quite a task in making all the checks and measurements as I'm setting it up.

Turning something short like an extension is one thing, but turning a long barrel on square is another task altogether.

What should I do for fit between upper and lower receivers? Should I just use a wedge or do any sort of bedding or shimming?

Or is this really even worth it?

[LRRPF52]

For receivers, I just look for certain forge codes and manufacturers. Keeping in mind that the TDP calls for a rather loose fit so a soldier can easily push out the takedown pin in the field with a cartridge, a lot of manufacturers for the civilian sector prefer to have tight-fitting receivers due to consumer demand.

Several different tricks have been used to tighten them up by some of the big names in the industry, but many will wear out with use. One company wedges the TD pin lug into the pocket with a U-shaped rear wall of the pocket moved forward a little using a 6061 set, and they wear out quickly and loosen up.

You can bed an upper to a lower, both in the rear lug and front, using releasing agent and glass bedding. People have done it, but the parts market is so full of great options, there is no need to mess around with that.

[Keep The Change]

What about the expanding rear take down pin made by JP?

That could potentially deal with a vertical slop like similar to an accu-wedge.

Now what about a lateral slop between upper lugs and lower lug pockets?
What would be a solution for that?

Maybe some kind of shimming process?
I don't want to have to weld and machine. I guess your first solution of a U-shaped pocket address this.
I was more thinking about a shim adhered to the lugs, but I guess where would still be an issue and even more difficult to change out than the solution you mentioned.

[LRRPF52]

There are so many quality billet sets on the market now, I wouldn't even mess with it. PF's billet set has to be pulled apart with force even with no pins, it's so tight. Mega's and Seekins are very well fit also.

A lot of the forged sets even have a tension adjustment screw in the rear.

AA makes a very nice forged receiver set with little adjustments that pull the receivers together, but are still within spec to fit across the parts market. AA also has a nice billet upper side charger kit complete with BCG.

I personally recommend a quality billet set if you're using a heavy, bull barrel that is long, because of the whip flex it will induce on the upper.

[Keep The Change]

I'm too far along to swap to a billet set. Lower is Seekins Forged and is built and I'm about to put the barrel on my WMA 7075 billet side charger. I guess I could go with a WMA billet lower but I wouldn't be assured a tight fit since they won't be matched.

My barrel is a 24", but not a bull barrel but it is an inch diam till the gas block. I chose the WMA over the Gibbz because they are the exact same side charge design, but the WMA is 7075 versus 6061, which is big difference in strength.

I haven't seen AA's billet side charger. I'll have to look at that.