Tuning a 7.5-8” 300 BLK

[GyroF-16]

I’m working on tuning a 300 BLK with a 7.5” PSA upper.

I started with a PSA lower w/ LAW folder and an Aero 8” upper. Initially, with the original carbine spring and buffer, recoil was harsh, and it felt like the bolt was slamming back. When I went to an Aero Precision H3 buffer (5.6 oz) (plus the weight of the LAW adapter) and a Springco “extra power” (Red) buffer spring, it all worked well, ejecting to 3-4 o’clock and locking back on empty magazines with 110 gr TAC-TX, as well as 147 gr FMJ.

Then I ordered a Surefire SOCOM 762MINI2 for my 300 BLK
Then I found a deal on an Aero stripped lower, and build it up with some nice Geissele bits.
Then I got a deal on a 7.5” PSA upper, complete with BCG and CH.

So I decided to assemble 2 x 300 BLK pistols from my components.
-One optimized for supersonic 110 Barnes TAC-TX for backpack carry (mostly in vehicles)
-One optimized for suppressed subsonic Hornady 190-grain Sub-X for indoor use in home defense
Bonus points if the “optimized for suppressed” gun also was good (not harsh) with supersonic Barnes.

In testing today, it turns out that the 7.5 PSA upper (when mounted on the PSA lower w\LAW & red spring & H3) is undergassed (or more accurately, over-sprung). It would cycle 147 gr FMJ and 110 gr Barnes, but wouldn’t lock back on an empty mag. And it wouldn’t even cycle 125 gr FMJ.

So I’m thinking that the 7.5” PSA upper will be the supersonic upper, and the 8” Aero upper (with more gas) will be the subsonic upper.
I’m open to suggestions as to whether I lower the spring or the buffer weight (or both, but that involves changing two variables at once) on the lower to get the “less gassed” 7.5” PSA working correctly.

Then I’m open to suggestions for what to use for spring & buffer for running the 8” “more gassed” Aero upper subsonic suppressed. In my ideal world, it would be reliable w/ subsonic unsuppressed, optimized for subsonic suppressed, and functional w/ supersonic (but probably a bit harsh). I’m guessing I’d start with a basic carbine spring & buffer. It will have a LAW folder (and associated additional reciprocating mass), too.

If I have the selection of more vs less gas for the uppers messed up, and using them in opposite roles somehow makes more sense, please set me straight.

Mentions below because you might be interested ( @FPS), you commented on the 7.5” BCM vs 8” Aero thread, or I just wanted to get your input ( @Trigger, @rob_s).

Attn: @FPS, @Flamingo, @@Doc_Glock, @vaspance, @Clusterfrack, @Idunnmobile, @Cool Breeze, @Totem Polar, @Trigger @rob_s

[Cool Breeze]

Sounds like a fun project.

Not sure specifically what your gun's issues are but SOLGW uses a "Blue Spring and H2 (can use a white hot as well)" for their 9'' 300 blk. Could be a good start?

[Clusterfrack]

@GyroF-16, here's my answer. But my approach to "tuning" may not fit your goals. I want my ARs to work under the widest range of conditions, with and without a suppressor. I do not want a worn spring or dirty gun to cause malfunctions. I don't care very much (actually not at all) about the feel of the recoil impulse, or exactly where the brass lands, as long as it's not extreme.

All of my 5.56 guns are relatively under gassed, and run great with full power 5.56 ammo suppressed and unsuppressed using H and H2 buffers and blue or standard springs. I don't shoot PMC Bronze or other underpowered ammo.

I was worried that 300BLK was going to be finicky and require a specific combination to work reliably. At least with my Palmetto 7.5" upper and a BCM BCG, that hasn't been the case. I was able to use a standard spring and an H3 for supersonics. Suppressed subs worked as well. But unsuppressed subs would not lock back a PMag on empty. Swapping in a C buffer solved that problem. I've now gone to a Strike short buffer system, and for some reason that works well with all types of ammo.

[FPS]

Thanks Gyro. I am hoping there won't be too many mods that will need to be done out of box but given my luck, probably. Following your journey with interest.

[mmc45414]

I like PSA stuff, but when I took my 7.5 upper apart to change some things the gas block was way out of alignment.

Not sure what you have but if it has a low profile gas block with set screws I would confirm alignment if you haven't. One of the reasons I took mine apart was to dimple it, so I am all good now.

And the more I type this, the more I am wondering about something from PSA being "under" gassed...

[rob_s]

@GyroF-16

I appreciate the vote of confidence in my possible knowledge of such things, but I haven’t touched a 300blk in…. A decade? I did get my hands on pre-production ammo both super- and sub-sonic, a very early upper (KAC, maybe? The rail was for sure…), a very early AAC can, etc all sent from rsilvers, and I did put somewhere in the neighborhood of 1k+ rounds through the combo, but at the end of the day back then it frankly led me to dub it the “300wtf” because after some initial excitement on my part I was left wondering “wtf is this thing good for?”

I don’t, generally, still feel that way about the caliber, but as evidenced by the lack of anything 300blk in my safe or storage (save, perhaps, some of the very early 300blk marked Pmags), I haven’t come up with a personal use case that makes the cost worthwhile.

That said, I think the general advice for ARs about figuring out the ammo you want to use, the potential for a can that you want to use, and building in some ability to tweak settings, and being willing to put in the time to tweak settings, is good. Like any AR, the more variables you add in around ammo, suppressed, etc. the more potential for failure you add.

[GyroF-16]

I like PSA stuff, but when I took my 7.5 upper apart to change some things the gas block was way out of alignment.

Not sure what you have but if it has a low profile gas block with set screws I would confirm alignment if you haven't. One of the reasons I took mine apart was to dimple it, so I am all good now.

And the more I type this, the more I am wondering about something from PSA being "under" gassed...

Thanks for that suggestion. Yes, it is a low-profile block, and I guess I’ll have to check for set screws. I hadn’t planned to get “involved” enough in tuning to start fiddling with gas blocks, but if the block on my PSA upper isn’t installed properly, I guess I might need to get smart on how one checks alignment. *Sigh*

Guess I’ll do some Googling… if anyone here can point me to their favorite guide for gas block installation, it would be appreciated.

[mmc45414]

I guess I’ll have to check for set screws. ... I guess I might need to get smart on how one checks alignment.

Step one is probably just take the handguard off and peek at it. It was rotated on the barrel such that the top of the gas block wasn't facing up, something that would partially obscure the gas port.

As I mentioned, I circled the drain a bit on AR stuff when I was furloughed and quarantined, and decided IMO dimpling a barrel with just the rear most setscrew and using red Loctite makes a lotta sense. It eliminates the guesswork of various gas blocks and barrels, some gas blocks leave room between the shoulder for the metal that holds not free float handguards, some do not, but for sure always the port is going to be in alignment with the setscrew hole. And by indicating off of the gas hole to get a fore and aft location you also get up is up as a byproduct. It is something that can be done with a simple battery drill with the jig, and the jig is only $35. And the red Loctite removes with a butane torch if you ever want/need to. It is probably nearly as secure as a cross pin and in the scope of something a guy with simple tools can do. The cross pin is better, but that was when the FSB was part of the picture, and a lowpro gas block hidden under the handguard is not going to get pushed on. Here is one of the videos I watched in the process of forming my opinion:
https://youtu.be/igCwnb3Wokw

But if it is off you probably do not need to get into all those shenanigans, in my case it was far enough off to be in the Holy Shit, I Can't Believe This Thing Ran category and just loosening it and doing a better job of eyeballing it into place would probably work fine.

[GyroF-16]

Step one is probably just take the handguard off and peek at it. It was rotated on the barrel such that the top of the gas block wasn't facing up, something that would partially obscure the gas port.

As I mentioned, I circled the drain a bit on AR stuff when I was furloughed and quarantined, and decided IMO dimpling a barrel with just the rear most setscrew and using red Loctite makes a lotta sense. It eliminates the guesswork of various gas blocks and barrels, some gas blocks leave room between the shoulder for the metal that holds not free float handguards, some do not, but for sure always the port is going to be in alignment with the setscrew hole. And by indicating off of the gas hole to get a fore and aft location you also get up is up as a byproduct. It is something that can be done with a simple battery drill with the jig, and the jig is only $35. And the red Loctite removes with a butane torch if you ever want/need to. It is probably nearly as secure as a cross pin and in the scope of something a guy with simple tools can do. The cross pin is better, but that was when the FSB was part of the picture, and a lowpro gas block hidden under the handguard is not going to get pushed on. Here is one of the videos I watched in the process of forming my opinion:
https://youtu.be/igCwnb3Wokw

But if it is off you probably do not need to get into all those shenanigans, in my case it was far enough off to be in the Holy Shit, I Can't Believe This Thing Ran category and just loosening it and doing a better job of eyeballing it into place would probably work fine.

Okay, I think I might be on to something…
I took the handoff and checked the gas block.
It seemed straight. But… the aft set-screw (the one directly opposite the gas port) was silly-loose, like I could easily turn it either direction with an Allen wrench.
So I loosened both screws and slid the block forward…



Doe it look to any else like there was plenty of gas leaking from the port and not making it into the gas tube?
There’s a ring around the port, and reddish residue fore and aft of the gas block on the top of the barrel and muzzle device, and even some on the outside aft surface of the gas block.

Am I correct in assuming that that much leaking gas would make the gun “undergassed””

I realigned the gas block and applied Blue LocTite (all I had on hand), then tightened the set screws down good and tight. (Oh, and the barrel was dimpled).
I’m hoping that another trip to the range will show me a different result.

But I’ll probably bring along a variable-mass buffer and a carbine spring, too.

[mmc45414]

Am I correct in assuming that that much leaking gas would make the gun “undergassed””
(Oh, and the barrel was dimpled)
I’m hoping that another trip to the range will show me a different result.

Maybe.... My speculation would be that the barrel being dimpled (ETA: This is, IMO, a huge upgrade to the barrel) resolves much of the potential for this being a problem, but... if the screw was loose it probably means the person assembling it snugged it up when it wasn't directly over the dimple and after it shifted a little the screw became loose. Now that it has a few rounds through it the GB probably will rotate into position more readily. If you shoot it and it does what you want then maybe switch to red Loctite and party on Garth. I would leave the front screw snug while you remove and glue the rear, and then do the front after the rear is back in.

ETA: If the screw was loose it would also not be holding the GB against the barrel.