My build - my first - is just about complete.  I'm not running a can, at least not yet.  I'm putting together some mid weight ammo (around 125g), all supersonic. My question: does the buffer spring weight need to be specific to the ammo; would running subsonic ammunition with a can require a different spring weight?

Also, would there need to be any changes to the gas system between running super or subsonic ammunition, can or no can?

I know its not the answer you looking for, but I think its a try it and see.

I have a BCM 9.5" I think, and Larue and Noveske in 12" and 10.5" (or .3) and they all run fine suppressed or not, Supers or Sub. Though I'll admidt, I shoot way more subs than supers as I did not go .300 blk for Super capability.

But I have seen many others need to adjust their spring, buffer and in extreme cases the gas system.

In my experience, no. Not at all.

10.5" 300blk. I do run suppressed, but don't have to.

I didn't do anything differently than any other build I've done. Standard lo-pro gas block, standard BCG, and a Geissele Super42 with H2 buffer. Runs like a clock with supers, subs, suppressed, unsuppressed, or any combo thereof.

Don't overthink it. The cartridge is designed for what you're doing. Put it together and shoot it!

Also, hurry up with the can . 300blk is intended for supressed shooting

Depends.  Depends on the ammo, the gas port, and whatever buffer/spring.  Spring should be fine regardless.  When you add a can you effectively increase dwell time, which can result in gas coming back through the action.  Increasing buffer weight can slow the action, as can reducing the gas.  As said above, you will need to shoot it to find out, and respond accordingly.

Use a standard carbine spring and H2 buffer and you should be fine with super and sub.

I have a 10.5" PSA pistol w/pistol gas system and a standard carbine buffer (3.1oz) and spring. I have a can on-order and it's expected to arrive any day now, waiting on the ATF. In preparation for the can I loaded up 50 rounds of 220 gr subsonic ammo and ran some through it this morning. They ran ok but didn't always lock back the bolt on last round.  Lots of smoke. Accuracy at 50 yards was ok with open sights. Recoil was much less and more of a push.

Load:
220gr lead polymer coated (Blue Bullets)
7.2 gr Lil' gun
Win Small rifle magnum primers.

I've read Lil' Gun doesn't always reliably cycle in pistol platforms with sub loads so I'll try more powders as I can.

As far as supersonic loads, I run 110gr vmax, 125 gr spitzer and 147gr FMJ's without so much as a hiccup. I think your standard carbine spring and buffer will cycle all factory ammo. Check your ejection pattern and if it's at 3-4 o'clock you are good. If it's at 1 or 2 O'clock, use and H1 or and H2 buffer. You can buy tungsten weights on Amazon for $30 delivered and adding weight to the buffer is simple.  

diy 10.5" here, standard carbine lower with SBA3.  pistol gas, yhm clamp on gas block.  no issues at all.  zero.  eats anything I put through it.  subs, supers, all the big name ammo manufacturers, doesn't matter.  

not suppressed yet.  

this is what 300blk was designed to do.

I know many are going to disagree with me, but after having spent hundreds on magic springs, I've come to the conclusion that it's a bunch of BS. Just go with a good quality spring with a standard spring rate. Spring rate has almost no effect on actual lock time. Tune the rifle with buffer weight.  Start with a standard $15 buffer and order some tungsten weights from Amazon and you'll have everything you need. The only caveat is the chance that you get a barrel with a really oversized gas port. In that case, you'll need an adjustable gas block.

pretty sure lock time refers to trigger function and has nothing to do with cyclic rate or function of the bolt carrier group moving against the buffer spring.



the physics is that the spring rate determines how far the moving mass will move to the rear for a given amount of energy coming into the gas system.

the weight of the moving mass determines how fast it will move.  more mass, slower.  

down in the noise is how speed might have an impact on friction losses.

worked for me when fixing my first 300blk build (16" carbine gas) - perfect function but would not lock open on an empty mag.  changing lowers had no effect.  bought a $10 reduced rate spring, runs perfect now.  changing buffers had no effect.

Bud, your ignorance is showing and this is exactly how bad information turns into fact on the internet. I suggest you go and do some research on the effects of buffer weight and spring rate on lock time. If you don't know what lock time is, go look it up.

As for the light spring "fixing" your rifle, all you did was mask what appears to be a problem of under gassing. That light spring is gonna work fine in a clean rifle, but if you ever get out and really run it, I won't be surprised when your rifle starts failing to fully chamber a round due to it being dirty because there isn't enough mass and spring to drive the bolt all the way forward.

A properly gassed and buffered rifle shouldn't need anything other than a standard spring to function properly; although some do run a heavier spring to ensure reliability in adverse conditions.

My 300's do fine with or without suppressors with both supers and subs, 16", 9.5" and 7.5"  .308 is a different story, where I had issues with buffers and gas was a 10" .308, it took a bit of time and trials to get it running right

Ummmm…you might be confusing Lock Time with Dwell Time in your lecture.

“Lock time is defined as the amount of total elapsed time between actuating the firing mechanism and the firing of the projectile. It is broken down into two distinct phases: trigger actuation to sear release and sear release to firing pin impact.”

Vs.

“DWELL TIME is the total amount of time the GAS pressure from a fired cartridge acts on an AR-15’s operating system.”

Broski, your ignorance is showing and this is exactly how bad information turns into fact on the internet. I suggest you go and research some definitions. If you don't know what they are, go look them up.

Ok, I had no idea that I was dealing with people that didn't have a clue.

I understand that there is a reference to lock time as it applies to triggers.

I also understand that there is a dwell time  that refers to the amount of time that there is positive pressure acting on the bolt which is largely determine by the amount of barrel between the gas port and the end of the barrel.

When I used the abbreviated term "lock time," within the context which it was used, I assumed that I was speaking to people that had a clue. When I said lock time, I was referring to the locked bolt dwell time. This is the amount of time that the bolt stays locked, after a round is fired before the extraction process begins.

Assuming a fixed amount of gas being introduced into the system, the only way to control lock time is with buffer weight. A heavier buffer will allow more time for chamber pressure to drop before the bolt starts to unlock. Too much lock time and the bolt won't cycle properly. Too little lock time and you'll see things like accelerated bolt and receiver wear and stuck cases, especially when running steel cased ammo.

Testing that is done to achieve a proper ejection pattern (which is supposed to an indication of proper gassing/lock time) assumes a standard spring and an ejector and extractor in good working order. If you have a rifle that is spitting brass at 1:30 or 2:00 (over gassed) and you throw in a super heavy spring to bring the ejection pattern to 3:30 or 4:00, it doesn't mean you fixed the over gassing problem. All it means is that you've masked it by slowing the bolt down during cycling. Over springing the action has little to no effect on actual lock time.

If you had to put a very light buffer and spring in your rifle to get it to cycle, you're likely dealing with an under gassing situation. That would have been better remedied with the use of a drill bit.  Of course, if you're one of those guys that just goes to the range and does 2 R 2's and then runs home and cleans your rifle, you may never having a problem.

Now back to my original point ... to the OP. Get a quality milspec spring and use buffers to tune your rifle unless you have a reason to go heavier. I do know guys that over spring their AR and run over gassed for the sake of reliability at the expense of parts wear. Just remember that what goes back will also come back forward and can be a detriment to follow up shots.

You’re kind of an arrogant douche, not going to lie.

Not saying you’re wrong, because you’re not.  But your pompous attitude and condescending tone is why forums like this are dying.

I suppose I could have come off like that, but it seems that arrogance met arrogance in this instance.  Had you considered for one second that there might be something that you don't know, you may not have triggered the response.

I have to imagine that the OP is working from a financial pool that is not infinite and would like to avoid needlessly spending money based on bad advise.

Forums like this are dying because they are full of misinformation, much of which is coming from the "ARs are like Legos" crowd. Just because you can change an alternator, it doesn't mean you know engines.

I'm out of this one. Good luck to the OP.

I'm pretty sure I know enough to say that "lock time" is not the same thing as what we're talking about.

the rifle in question with the slightly lower spring rate will never be run hard - I listened to the wrong people 10 years or so ago and it's carbine gas not pistol gas.  it's a range toy.  the other 300blk has pistol gas and runs great with standard carbine buffer and spring.

and I thought I was interacting with someone who could read and understand English and could have a civil discussion without ad hominem noise.  

never said lighter buffer.  said reduced rate buffer spring.  and it's not "very light" it's 10% reduced, whatever JP sells that's one step less than standard carbine springs.  and it cost me all of $10 including shipping.

and 60 seconds to change a buffer spring is way easier than completely disassembling an upper to enlarge a gas port.  which is what I was about to do when I realized I had actually fought this kind of battle before (spring rate, preload, stroke, force) when setting up my roadrace motorcycle suspension.  and taken a few Physics classes in college where this exact discussion and labs happened.  so I have a perfectly functional, second AR I ever assembled, 10 year old rifle I will never part with.  

you were and are the one using an incorrect term to describe how an AR operates.  not me.

Lock time is the time between when the trigger is activated until the firing pin strikes the primer. Like you said, look it up.