Help me end my Low mass Vs full mass dilemma

[HoodRich]

Ive read through a lot of post and searched different forums but I still get mixed opinions on the best BCG for my setup. I am building a SBR as follows:

Lilja 12.5 barrel
JP low pro adjustable block (carbine gas tube)
JP SCS H2
JP low mass or JP full mass with bolt
Will run factory loads depending on accuracy results.

Will be used for predator hunting, and overall just a target/fun-Gun

Build will be suppressed when my paperwork comes back witch is going to be a while, so I will be running it non suppressed for probably a year unless I do a form1 built suppressor.

So I sent JP an email and called them. The guy I talked to on the phone Sayed I could run a low mass no problem and the guy that emailed me back Sayed I should run full mass. Ive also read on hear of people doing it both ways but each ways gets mixed opinions....The safe bet sounds like full mass but If I can drop a little weight with smoother, lighter recoil I would like to do that if possible. Any real world advice on this would be appreciated. thanks

[CJW]

A5B will chime in when he reads this, he is our low mass BCG expert.

[A5BLASTER]

Since your running a heavier buffer system with the scs h2 it would be a waist of money to put a lowmass bcg in your weapon.

Now if you was running a standard weight or a lowmass carbine buffer and spring then it would be worth putting in a lowmass bcg.

Since your going to suppress this weapon in the future I would just go with the full weight bcg so the added weight will help eat up gas pressure along with the adjustable gas block.

It's a system and best if you go to either full lowmass system or go full weight with adjustable gas. Going middle ground isn't really worth it.

For what it is my pistol and sbr run on adjustable gas and lowmass bcg and standard carbine buffer and spring, both have carbine gas length, I don't own a can but if I did, I would go with the heaviest bcg and heaviest buffer and strongest buffer spring I could run with adjustable gas.

[HoodRich]

Since your running a heavier buffer system with the scs h2 it would be a waist of money to put a lowmass bcg in your weapon.

Now if you was running a standard weight or a lowmass carbine buffer and spring then it would be worth putting in a lowmass bcg.

Since your going to suppress this weapon in the future I would just go with the full weight bcg so the added weight will help eat up gas pressure along with the adjustable gas block.

It's a system and best if you go to either full lowmass system or go full weight with adjustable gas. Going middle ground isn't really worth it.

For what it is my pistol and sbr run on adjustable gas and lowmass bcg and standard carbine buffer and spring, both have carbine gas length, I don't own a can but if I did, I would go with the heaviest bcg and heaviest buffer and strongest buffer spring I could run with adjustable gas.

I could change a weight on my SCS and make it standard weight. Would that be worth going with the Low mass. I did H2 from the recommendation of JP.

[grayfox]

You would be just trading one weight for another, it's all physics, the bcg and the buffer all move together against the spring tension, conservation of momentum. Conservation of $$ would be a better goal here... a low mass carrier would be less interchangeable and probably more $$ but it's up to you.
If you want to do something different then use the adjustable bolt carrier (aka bootleg carrier) in place of the adjustable GB and any weight differences in the buffer. I had one along with carbine buffer and spring, worked ok but I just didn't like the shorter barrels.
The up-side to the adj bcg is you can do it on the fly with a tweaker screwdriver, since the adjustment screw is accessible from the port opening. The up-side to the adj GB is it has more fine tuning available to you, the bootleg has 4 positions. Lrrpf52 uses the bootleg a lot and likes it over the adj GB's.

[Bigs28]

I have basically the same build as you. Jp adjustable gas block, jp scs h2, 12" suppressed barrel. I use full mass carrier and it runs flawlessly.

[A5BLASTER]

I could change a weight on my SCS and make it standard weight. Would that be worth going with the Low mass. I did H2 from the recommendation of JP.

You could do that. But down the road when you put the can on your very likely to have issues and have to drop in a full weight bcg and replace the tungsten weight and make the scs a h2 weight again.

When I tested my pistol with a friends 6.5 ultra, I had to change from standard carbine buffer/spring and lowmass bcg to a JP extra power spring a h2 buffer and a full weight bcg to get it to run properly with adjustable gas.

That was the first and last time any of my weapons have had a can on it, just makes the gun run too dirty for my likeing and I'm not willing too deal with the heavier bcg and buffer system.

[LRRPF52]

My biggest concern with a CLGS 12.5" barrel suppressed is keeping the bolt locked as long as possible.

A full mass carrier helps accomplish that, in conjunction with the optimum gas port size, action spring weight, and buffer weight.

If I was configuring a competition AR15 chambered in .223 Wylde with a RLGS or ILGS and I wanted my split times to come down, I would use the JP SCS with a JP Lo-Mass carrier and adjustable gas tuned to one or two specific loads for competition.

For 6.5 Grendel, I would feel better about a longer gas system length used in conjunction with a Low-Mass carrier and adjustable gas, but I would want adjustable gas on the carrier instead of up-front at the block. Right now, I'm unaware of any company making both a lightweight/lo-mass, adjustable gas carrier.

[Wanderson]

[HoodRich]

I went with the Full mass... But ill always wonder how it would run with a Low mass . Guess ill have to buy one down the road to try out. Thanks for all the info guys.

[js8588]

My biggest concern with a CLGS 12.5" barrel suppressed is keeping the bolt locked as long as possible.

A full mass carrier helps accomplish that, in conjunction with the optimum gas port size, action spring weight, and buffer weight.

If I was configuring a competition AR15 chambered in .223 Wylde with a RLGS or ILGS and I wanted my split times to come down, I would use the JP SCS with a JP Lo-Mass carrier and adjustable gas tuned to one or two specific loads for competition.

For 6.5 Grendel, I would feel better about a longer gas system length used in conjunction with a Low-Mass carrier and adjustable gas, but I would want adjustable gas on the carrier instead of up-front at the block. Right now, I'm unaware of any company making both a lightweight/lo-mass, adjustable gas carrier.

Low mass adjustable carrier

https://rubbercityarmory.com/product/6-5mm-grendel-titanium-black-ti-complete-bcg-m16-profile-adjustable-gas-key/

[Klem]

Sure, the bolt won't unlock at peak pressure and damage the receiver, but it still can unlock 'safe' but too early. Spitting in your face if you are a left-hander, stressing and dragging the bolt against too much pressure, which in turn robs the system of momentum to fully cycle, the extractor claw can strip or jump over rims.

Unlocking and going backwards is only half of the cycling, you need mass going forwards into battery. If you go a light BCG then it might not have enough mass going forward to, as you say, strip the next round in a mag with a stiff spring, or heavy bullets (like a full mag of heavy 300 Blackout), or go into battery if the gun is clogged with dirt.

Always full-mass/full-auto (more weight) BCGs for me, and make sure the system is generously gassed so this can be tuned at the port with an AGB. Frankly, I can't see a situation where a low mass carrier is better than a normal carrier, given you will have to compensate in some other way to ensure reliability. Maybe Blackout which as a calibre is undergassed when subsonic, but then there's the stripping of the next round to worry about.