Quoted:
I use a DPMS panther claw. other people prefer the plastic clamshell ones that go around the outside the receiver.  I don't see why you would need an aluminum one

I'm not sure how the action block would interfere with a free float tube

Quoted:
I use a DPMS panther claw. other people prefer the plastic clamshell ones that go around the outside the receiver.  I don't see why you would need an aluminum one

I'm not sure how the action block would interfere with a free float tube

Quoted:
The aluminum barrel "V" blocks are for installing flash hiders.
Use a Panther claw type jig or the clamshell type receiver block for installing the barrel.
With the barrel blocks, the receiver can rotate and the indexing pin on the barrel can torque out the notch in the reciever, messing up the barrel alignment.  
In extreme cases the receiver can crack.

I use all 3 types for different applications.

I prefer the clamshell type for barrel installation.  
With the Panther claw type, you are transferring a lot of the torque to the receiver pin holes, and have the possibility if twisting or cracking the receiver.  
Especially if you are at the max side of the torque limits.  If you have a torque wrench at 80 pounds, there is an extra couple of inches of moment arm from the barrel nut to the receiver pin holes that will make the torque at the pin holes even greater.

I have also, in extremis, used the two wood blocks with a flat top receiver.  
I didn't feel comfortable the whole time, and kept expecting it to crack.
I have seen pics on this site of someone using that method and crushiing the bottom edge of the receiver.

The proper block is inexpensive insurance.

Quoted:
The aluminum barrel "V" blocks are for installing flash hiders.

Originally posted by Slash
Barrel blocks are only appropriate for doing work on muzzle devices, never for re-barreling. The correct tool for the job is a good set of receiver blocks, available from ArmaLite:

Barrel blocks put rotational stress on the alignment pin and the slot in the receiver, which were not designed to be exposed to this type of force. I've seen receivers destroyed by WECSOGers using the wrong tool for the job.

Originally posted by 13-F-COLT
what i have seen happen many, many, times are uppers with cracked index pin slots,
some were unsalvageable, because barrel blocks were used and the upper took the
rotational stress from torquing. something it was not designed to do.

the "which vise blocks to use" issue has been beat to death so many times. regardless
of whats in a manual, it has been proven that it is better to clamp the upper.

Originally posted by TaylorWSO
Vice blocks are the best, the barrel will hang on the upper receiver when you remove the barrel blocks, so I don't see the concern of it hanging it on their when using receiver blocks.

The DoD has many stupid ass regs/guidance that I would never put my personal stuff through. That TM/TO also recommends using a pry bar to hold the FSB if the barrel twists in the barrel block.

There will be torque from the nut, it can either be:

1. On the threads, through the receiver, to the receiver blocks, OR

2. On the threads, to the index pin, through the barrel to the barrel blocks. This route will have all force concentrated on a small pin pressing against the aluminum receiver. Bad news IMO.

Originally posted by Slash
The .mil is not known for always providing the best tools (or training) for the job.

This issue is quite simple if you think about it. When tightening a nut and bolt you use two wrenches, right? One on the nut and another on the bolt. If you try to tighten a nut without a wrench on the bolt head what happens? The bolt turns. If you don't use a receiver block, all of the torque you put on the barrel nut is transmitted to the alignment notch via the indexing pin.

With the proper receiver blocks (as shown in my photos above) there is ZERO torque on the pivot/takedown lugs. The pin-style blocks are useless for anything but cleaning/maintenance IMO.

There is less stress on the barrel-receiver interface with a new barrel hanging off it in a receiver block than there is when a fully assembled rifle is fired from a bipod. I'll wager that a VFG multiplies that stress several times. Do you think that ArmaLite would manufacture and sell receiver block if assembling an upper with them was really a hazard?

Again, I have personally seen receivers damaged by people who used barrel blocks and 'field expedient' receiver blocks, AKA chunks of wood. I've never seen or even heard of a receiver that was 'elongated' with properly designed receiver blocks. Good tools can be expensive, but the right tool for the job is priceless.

Quoted:
I just use 2 pieces of polycarbonate that I cut from a cutting board form wally world....held up for 2 AR builds and doesn't hurt the wallet