Great…saves me some “learning curve” time
Brand new 300 blk I put together ( I had a spare lower sitting around, next thing I know, it turned into a rifle! :D).
A5, standard buffer
BCM upper receiver
BCM bcg
16" AR stoner SS barrel
Vltor SS lowpro gas block
SS Battlecomp
Geissele SSA
Old doublestar stripped lower I had collecting dust
Troy Alpha 15"
Magpul FDE Ceracote
It would eject beautiful, but would not lock back on last round. I swapped in another spare lower I have with a standard carbine buffer setup to see how that worked. Same issue. No bolt lockback.
I had 2 spare standard A5 buffers ( I use the A5 on my 556 & 6.8), so I took them apart and swapped 1 steel weight with 1 tungsten weight. This gave me a lighter than standard A5 buffer.
It works great! Bolt locks back everytime, cycles beautifully, ejects the brass into a neat little pile. 
I now see Vltor makes a lower weight A5 buffer, I do not know what the weight difference is between it and what I did with parts on hand. Hell, probably the same as what I did.
This is what worked for me and my setup. 
The slightly dented case mouths is 100% normal in my experience. I use a A5 5.4oz buffer with my 16" S&W and did with my AAC 16" and it does it with my 9" AAC as well.
Trying to get some updated info on this.
I’m running a 10" noveske barrel on a SR15 lower and it will cycle but not lock the bolt open when suppressed, Unsuppressed to will not pick up a new round. I believe the buffer is standard carbine weight. Everything appears to be correct on the upper, would the A5 system help? I would like it to function reliably suppressed. All ammo is 220gr remington or 225gr hornady reloads at approx 1000fps with 1680.
FWIW, factory Noveske 300BLK rifles come with pistol gas and H2 buffer.
16" 300 AAC BLACKOUT Stainless Barrel
[…]
–Pistol length gas system
[…]
–H2 Buffer
Here is the blurb from their web site which notes the only caveat is running SUBsonics UNsuppressed.
The Noveske 300 AAC BLACKOUT is designed to run optimally firing both subsonic suppressed and supersonic unsuppressed. This barrel is tuned to lock back on the last round using subsonic 220 and 208 grain ammunition with a sound suppressor. The firearm may not lock back with subsonic 220 and 208 grain ammunition without a sound suppressor.
-john
I’m trying to figure out how a heavier buffer would get the bolt to lock back.
Your setup pistol gas?
-john
Yeah. It’s the standard noveske setup.
Note their blurb goes on to say:
The barrel is tuned to operate correctly with the back pressure provided by the AAC 762-SDN-6 using an H2 buffer, or a Surefire 762K suppressor and an H buffer. In the development of this barrel, we found evidence that the suppressor may require a symmetrical baffle stack as found in the AAC Cyclone K, 762-SDN6, and Surefire 762K suppressor. Suppressors which cause off axis turbulence, such as those with K type baffles, may cause baffle strikes and/or point of impact shift, and may damage your suppressor.
So it sounds like there are some tuning issues depending on the back pressure produced by your can.
-john
I’m using the Surefire mini 762. I’m not sure where that one fits in. I believe the Sr 15 uses a carbine buffer, so I don’t think I can go lighter. I might try using it with the A5 and trying different buffer weights in that. Otherwise, I’m at a loss.
You might also ask over at http://www.300blktalk.com/forum/.
-john
I was searching over there, but no luck so far. I may have to post up.
To recap here
3.0 oz CAR buffer short stroked.
4.56 oz A5H1 buffer worked beautifully.
5.33 oz A5 buffer short stroked.
16" CAR gassed 300 BlK
It appears there is more to cycling than just buffer weight.
This is not unexpected.
There are several competing forces at work during cycling.
The gas system delivers an overall impulse or total momentum to the BCG, regardless of how heavy it is.
A heavier mass will have a lower energy, while a lighter mass will have a higher energy, given the same momentum.
It takes energy to compress a spring, so in general, given two buffers with the same momentum, the lighter buffer will compress the spring more than a heavy buffer.
The work required to extract a case changes with time.
In general, the longer you wait, the easier it is.
I believe there is also a “snap action” effect that occurs when the case is cooled, contracted and “done”.
Extraction work is lowest just after this point in time.
Whatever total energy the BCG has must be split up to extract the case, recock the hammer, compress the action spring and fight friction.
If one of those tasks takes too much energy, there may not be enough left over to complete the others.
My take in this 300 BLK scenario, is the barrel/gas port/cartridge had overall gas drive that was a little low.
Too low to fully cycle the heavy A5.
Due to the CAR gas system that starts the cycle early, the lightweight CAR buffer was trying to extract too early, using us too much energy to extract the case.
This did not leave enough energy to compress the spring.
The mid weight A5H1 with biasing spring held the action closed long enough for the case to be “done” and was able to extract is using very little energy.
It had enough energy left over to fully compress the spring and cycle.
I’ll try swapping some different weights and see what happens. I might get luck and get this thing running like a top with a buffer switch.
A guy at my local range was running the A5 system with a 10.5" barrel that he made at work, he was also running a Gemtech Sandstorm I believe and that thing was running like a f’in champ. I believe he said he was using a standard H buffer, he said the H3 was slowing the gun up a lot. You could try opening up the gas port if you can’t find any resolutions with various buffer weights.