What do you prefer to do a barrel change? A Geissele reaction rod where the tool mates with the barrel extension or a clam shell where the tool holds on the upper receiver. I heard about people shearing indexing pins off the barrel ext when using a reaction rod. I can see that where some torque is transferred to the pin. I read that John Holliger suggested greasing the barrel ext shoulder where it contacts the barrel nut to decrease the risk of the pin shearing. I never did that but I use clam shell tools.

I've used both methods.  I like the rod better than the clam shell deal.  I also have the magwell block that clamps in a vice.

They all work fine as long as the torque is not excessive.  Plenty of grease for the barrel and nut threads will avoid having to use a 5 foot lever to loosen it.

One benefit of the rod and magwell vice block is not having to remove all the accessories (optics) from the upper rail, which can keep your optic from losing zero.  That assumes your optic isn't mounted above or forward of the barrel nut.   I think most clam shell products need the optics removed.

Also, the rod lets you slide the upper on vertically, horizontally or any angle in between.  You can easily rotate the upper to get a better angle on screws, gas blocks, gas tubes, roll pins, sights and so on.

As for potential damage, the magwell block always struck me as risky.  I know the aluminum upper is strong, but what if you want to work on polymer parts?  What if the part has the start of a crack and you are just stressing it more?  Seems like the steel of the barrel extension makes the most sense when using potentially significant force in a twisting direction.  As for clam shells, clamping anything around the outside of the firearm is just asking for gouges and wear on the finish.  Besides the aesthetics, the finish is what keeps metal from becoming rusted metal! 

If I had to choose one, I'd go with the rod.  It's not only a great tool, but it's hefty enough to use for SD in a pinch, too!   

I bought the Botach substitute for 1/5 the price of Geissele version at the time.

KZ Barrel Spline Socket Rod For AR15/M4 Upper 5.56mm Receivers

https://botach.com/kz-barrel-spline-socket-rod-for-ar15-m4-upper-5-56mm-receivers/#tab-reviews

What you want is the Midwest urr. It's not just a round reaction rod, it has a raised piece that fits tight in the charging handle channel and distributing some load to the receiver. It's what was recommended to me by one of the top ar15 specific gunsmiths in the country who trains armorers for top companies like solgw.
It's $90-100 and perpetually out of stock like vltor a5 kits. Bigdaddyunlimited is $70 when in stock but u need A stupid monthly membership. U can do a cheap trial month just to buy it and maybe grab some other deals.It's what I plan to do.

A quality clamshell is actually better than a reaction rod. As you mentioned the index pin takes all the torque.
Ncstar and magpul bev block are the best budget options. ncstar is better for uppers but bev block also has a built in lower receiver block so that adds value. stiff delrin nomar lower blocks are superior to the bev block though. Less flex.

I already have a clamshell for ar15 and ar10,reaction rod for ar 10, and a bev block and I still plan to get the Midwest urr when In stock for cheap. Gonna sell the bev block and ar10 reaction rod. Keeping clamshell for dedicated .22 and 9mms etc. Since those don't have locking lugs

I've used the magpul block.  Works ok.  Doesn't support the upper Receiver very much. It does engage the barrel extension like the reaction rod. 

Also used that white machines plastic one that accepts the pins and fills the hollow of the upper.  See jaynick above. That one seemed to most overall useful, and provided the most support, IMO. 

If one was removing a tough barrel nut I'd think a reaction rod type would be be better to break it loose.   

Never tried the clam shell type. 

Midwest URR
"I don't recommend the Geissele Reaction Rod or the Kley Zion Clone. If you use a Torque
Wrench, the Reaction Rod type devices are fine for Upper Receiver assembly, but if you plan on
tearing down AR Uppers or changing Rails you really don't want the Reaction Rod. The
Reaction Rod is NOT good for trying to remove seized or over torqued Barrel Nuts. Think about
how the Reaction Rod works. It holds the Barrel in a static position but does NOT lock the
Upper Receiver in place. If you have a stuck/seized Barrel Nut, the Upper Receiver will try to
spin with the Barrel Nut. What keeps the Upper Receiver from spinning? The Barrel Index Pin
does, and it will break off if you add too much force. Seen this countless times. This is why the
Midwest URR is best tool for the job, it has a sail that locks into the charging handle slot, and
teeth that lock into the barrel extension. These two features prevents the Upper Receiver and
Barrel Extension from spinning. The Midwest URR is a gauge of sorts as well. If your upper is
twisted, or the Barrel Extension is not timed properly, the tool won't go into the upper properly. I
have a video showing this issue from a few weeks ago. Clamshell Upper Receiver Blocks are
great for removing a stuck/seized Barrel Nut, but the Clamshell Blocks won't work on some billet
Uppers Receivers, and you can't use heat on the Barrel Nut without melting the Clamshell
Blocks."

Thanks for the product highlight jaynick, I never knew something like the URR existed. Will try it once in stock.

Went looking for the Midwest URR and everyone is out of stock. Do they not manufacture anymore?

Sent from my SM-G970U using Tapatalk

I have used the Brownells rod.  Not exerting any torque on the thin receiver shell seems like a no brain issue.  As mentioned, loading the rifle onto the rod vertical really helps the monkey humping the basketball of the other way.  Especially when timing the gas tube.

I needed a tool in AR-10 size and the -15 size does a fair job of locking the barrel extension, but is not optimum unless you are in a pinch.  I anti-seize the barrel nut and extension so should never see excessive break-away torque loads.

Its high in demand. People usually backorder them and seems like a 2-3 month wait from what ive been seeing.

Went looking for the Midwest URR and everyone is out of stock. Do they not manufacture anymore?

Sent from my SM-G970U using Tapatalk

Midwest URR
"I don't recommend the Geissele Reaction Rod or the Kley Zion Clone. If you use a Torque
Wrench, the Reaction Rod type devices are fine for Upper Receiver assembly, but if you plan on
tearing down AR Uppers or changing Rails you really don't want the Reaction Rod. The
Reaction Rod is NOT good for trying to remove seized or over torqued Barrel Nuts. Think about
how the Reaction Rod works. It holds the Barrel in a static position but does NOT lock the
Upper Receiver in place. If you have a stuck/seized Barrel Nut, the Upper Receiver will try to
spin with the Barrel Nut. What keeps the Upper Receiver from spinning? The Barrel Index Pin
does, and it will break off if you add too much force. Seen this countless times. This is why the
Midwest URR is best tool for the job, it has a sail that locks into the charging handle slot, and
teeth that lock into the barrel extension. These two features prevents the Upper Receiver and
Barrel Extension from spinning. The Midwest URR is a gauge of sorts as well. If your upper is
twisted, or the Barrel Extension is not timed properly, the tool won't go into the upper properly. I
have a video showing this issue from a few weeks ago. Clamshell Upper Receiver Blocks are
great for removing a stuck/seized Barrel Nut, but the Clamshell Blocks won't work on some billet
Uppers Receivers, and you can't use heat on the Barrel Nut without melting the Clamshell
Blocks."

Went looking for the Midwest URR and everyone is out of stock. Do they not manufacture anymore?

Its high in demand. People usually backorder them and seems like a 2-3 month wait from what ive been seeing.

Found this one as very similar
$55
https://vtsupply-com.3dcartstores.com/Tiger-Rock-Upper-Receiver-Tool-AR15-Rod-223-556-AR-15-AR10-308_p_4506.html

I'm no trainned gunsmith, but I have built many ARs in my day. More than my financial adviser (wife) would agree to! But I have used both the reaction rod and receiver blocks. Most my builds have been from the ground up so I've rarely had to deal with anything being over-torqued.  Except if I work on something already built. Anyway... over the years I've acquired both tools and I use the receiver block for barrel nuts, and the reaction rod for torquing muzzle devices. This way the index pin does not get any stress. Not a cheap way of going about things, but the gun scene is anything but that...

Interesting. i didnt see that one before. Wheeler also came out with a knockoff midwest urr this past month. Unfortunately its made of aluminum and is mediocre quality. The gunsmith i follow online tested the wheeler already. Ill send him the link to the tiger rock. He literally buys every tool available and even sells some of his own modified tools and gauges.

Tiger rock tool is probably junk though. Id save another 30 for a known quality tool. The locking lugs look poorly machined and undersized for solid lockup.
Tiger rock sells chinese complete buffer kits for 9.99, not sure if all their stuff is chinese.
I ordered one with my primary arms order last year for a friends ultra budget build. The tube was nicer than anderson but the buffer and spring were made of poor materials. Crazy how a sprinco spring cost  $20 while their entire kit is $10 with end plate and castle nut too.

Found this one as very similar
$55
https://vtsupply-com.3dcartstores.com/Tiger-Rock-Upper-Receiver-Tool-AR15-Rod-223-556-AR-15-AR10-308_p_4506.html

Went looking for the Midwest URR and everyone is out of stock. Do they not manufacture anymore?

Sent from my SM-G970U using Tapatalk

Just happened to check out Brownells a minute ago and "(in stock)"!

Clamshell for upper receiver and proper barrel nut wrench for the type of barrel nut is the way to go. Reaction rod is not good. The barrel nut on the AR should be torqued on with 30-45 ft lbs of torque tops  and that is with the gas tube hole aligned. Anti-seize should be used on the barrel nut threads to make removal later easier. Trick is to cover upper receiver with 1 layer of painters blue tape then have a filler inserted into the upper to prevent crushing and then you just snug the clamshell down into the bench vice. You should be doing a mock fitting of parts first to make sure everything goes together with proper fit before doing final assembly. I usually start with 30 lbs on barrel nut and then I check gas tube alignment to see if I need barrel shims. I've seen many AR's where both the barrel nut and the flash hiders were torqued on using way too much torque. Did you know you can distort the bore at the muzzle by using too much torque on the flash hider ?

If anyone is interested