Never tried either before, but the main goal, aside from obvious things like reliability, is light weight.

Thinking of using a light BCG and adjustable gas block, but this is not my wheelhouse. I know I'd like a gas block that I can adjust without yanking the HG.

It will be run suppressed full time. Will be a range toy at least to start.

Still not sure what barrel length I'll go with, but at least 10.5".

Any input for me? TIA

I’ve done it. Works well.

Most of my builds have had a lightweight focus, including a lightweight BCG in nearly all of them. My favorite setup has been to avoid an adjustable gas block and get a BRT EZTune gas tube setup specifically for your build. Takes out the guess work and keeps it simple.

My favorite build to date used a JP LMOS carrier and their SCS along with the BRT. Not the lightest assembly out there as you can get ultra light buffers and reduced power springs, but it was gassed enough to be reliable but still smoother than any other rifle I'd put together.

That gas tube would make my life easier if it works.

Unsure of what BCGs to consider, though. JP is out just because they are $$$$.

I will be running the Geissele Super 42 setup

I've used several different lightweight BCGs. Some cheaper ones that are just milled out standard BCGs, and they work but aren't as smooth as I'd like. Don't blame you on the price though. I was dreading it as I'm starting a new build, but I got a complete JP LMOS BCG for the price of just the carrier by itself.

My only suggestion for the 42 is to go with the lightest buffer they have, you don't need to go heavy since you'll be able to tune the system going with a AGB or the EZTune gas tube. Ideally, to make the most of the lightweight carrier and adjustable gas, you'll want to get a lighter buffer and a reduced power spring.

What are some of the other light BCGs you have had success with?

So now I have to spend more $ on a new buffer & spring? LOL
I already have the Super 42 setup and it has always worked on every build I have used it on. I think I have a plain old H1 buffer in my parts bin. Worst case, it doesn't work then I try the H2.

Ar15discounts.com.  Usually "unadvertised" $240 when in stock.
$240 isn't much for the best.  Most other sites sell them for about $100 more.  I just picked up another Full Mass from them for about the same price.  Good place to buy from.

The other lightweight one I've used was the RTB lightweight carrier (85$ on RightToBear's site) and the Sharps Low Mass (270$ on Sharps site). I just found that the LMOS was smoother than the others.

You are more than welcome to use whatever buffer you want, I'm just saying that using a light BCG and AGB is only two parts of the system that also involves the buffer and spring.

Standard BCG's have always worked well for me, are cheaper by far and plentiful.

I had one adjustable gas block that installed on a Colt 16" H-bar because the case rims were getting damaged by the extractor yanking the case out while it was still under pressure.

A heavier buffer helped just as much. I ended up going back to the original front sight tower.

A lot of items in the AR marketplace are solutions looking for a problem. If your rifle is running fine, don't try to "fix" it. I would try a heavier buffer first, before getting an adjustable gas block. IMO, going with a lighter BCG is a step in the wrong direction suppressed.  We had to get a heavier buffer for my brother's DD Mk18 after we installed a suppressor, I think we ended up with an H3.

I prefer longer barrels than 10.3/10.5", a 12.5" or 13.7" is plenty short. The only reason I have my DD Mk18 is for clone reasons.

I’ve currently got a 6 ARC running an RCA Ti BCG, Unrivaled Technologies Deadblow Competition buffer, and an Aero adjustable gas block. With a can, it’s only got slightly more recoil than my 5.56 guns, which is pretty good for roughly 20% more powder and double the bullet grainage.

First, you really need to lay out what it is you’re trying to do.  Are you looking to solve a problem or enhance certain attributes of the gun with the swap to a LW carrier?  Bottom line, reducing reciprocating mass can have a detrimental effect on the reliable operating envelope of the gun, especially under less than ideal operating conditions (dirty, cold weather, etc).  Trimming a little bit of weight likely won’t have a noticeable effect, but if you start getting extreme, it can happen.

You also need to look at how you’re accomplishing this.  A JP LMOS (non-aluminum) carrier is roughly 2oz lighter than their full mass carrier.  It’s roughly 2oz - 2.25oz lighter than a milspec FA carrier.  You can drop 1.7oz from the reciprocating system by simply swapping from an H2 to a carbine buffer.  You can drop even more if you start removing weights from the buffer. If you don’t want to buy a new buffer, you can always just disassemble the buffer and swap out the tungsten weights for steel.

End of the day, if weight savings is the primary goal, it’s certainly going to be cheaper to swap a buffer than an entire carrier, so I’d maybe look at that first.  I wouldn’t swap the carrier and leave the H2 because you’re essentially sacrificing nearly all the weight savings you buy with the carrier upgrade.

Did you read my post? I'll be building a new AR from scratch. This is not a case of changing something I already have.

Thank you.

Oh yes, I know, did not mean to come off as saying otherwise.

Trial & error. I can try the H1 first and if the gun is not reliable, put the H2 in. Was just saying I planned to use the Super 42 H2 is because I already have it and it has never failed me in anything.

So what exactly are you trying to accomplish with a LW carrier if using an H2 buffer is on the table?  A standard steel LW carrier with an H2 buffer leaves you with essentially the same amount of reciprocating mass as a standard FA or SA BCG with a standard carbine buffer.  Seems fairly pointless, although it’s entirely possible I’m missing something.

I say this as a guy who has done the whole low reciprocating mass thing to minimize recoil.  This was with an LMOS carrier, gutted carbine buffer (no weights inside), and an AGB.  It was stupidly soft shooting when tuned to a specific light accuracy load.

As someone who uses AGBs and lightweight internals extensively for race guns there’s no way in hell I’d use them for anything real world.

This is just a range toy & I want to see how light I can make the gun without spending a fortune

This 100%

If that’s the goal, it seems a bit counterintuitive to keep a heavy buffer in the gun.

I already stated I'll try H1 first.