Hey guys,

I have my pistol length, gas piston, suppressed 300 blk running.  It cycles and locks back on an empty mag.  One problem, on the first round from a new magazine the bolt will not slam home; won't even seat far enough to use the bolt push.  If I load the first bullet into the breach, close the bolt, insert a mag, runs every time and locks back on an empty mag.  I'm using a NEMO Arms .300 Blackout Custom Carbine Buffer Spring.  If I switch to a carbine buffer spring there isn't enough gas to overcome the spring and feed another round, and it won't lock back on an empty magazine.

Any tips or tricks?

Ok, minimal experience with ARs, I assembled two so far, both 16" carbine gas, one 5.56 and the other 300blk.  Zero experience suppressed.  

But I did spend a few years studying Physics.  

Failure to feed (where the issue is the forward motion of the bcg not some magazine lip/feed ramp/etc geometry) from a stationary, locked back bcg is not enough stored energy in the spring.  The compressed spring stores energy.  If the locked back bcg when released does not chamber a round, but after manually feeding one round into the chamber then dropping the bcg into battery it cycles reliably and locks back on an empty mag, by definition the bcg is cycling farther rearward and storing more energy in the spring than when in the locked back position.  

When you pull the charging handle until it stops, then release it, does a round chamber?  How far back is that position with respect to the bho position?

My completely inexperienced guess is that the rifle is a little overgassed for the spring you've got, which causes the bcg to cycle farther than the bolt hold open position.  So when the bho locks the bcg back, that small forward motion before it hits the bho represents enough lost energy in the spring to cause the bcg to not move forward with enough energy to chamber a round when the release is pressed.  I keep saying energy because as the spring pushes the bcg forward the stored potential energy in the spring turns into kinetic energy of everything that's moving forward (buffer, bcg, cartridge) and heat with the friction of all the various parts moving against each other and some of them not moving perfectly inline with the bore.  

I had the opposite problem, bcg forward on an empty chamber after the last round.  Perfect function otherwise.  The bcg wasn't moving far enough back for the bho to catch. I put a lower rate buffer spring in, fixed.  The spring still has enough stored energy at the bho position to chamber a round when released.  I didn't want to go through the "enlarge the gas port" routine to get more energy in the system.  

If you put a slightly higher rate buffer spring in and it doesn't cycle far enough back to lock open after the last round, figure out how to loosen up the action or get more gas into the system.

I hand cycled both mine >>100 times dry to wear in the components then cleaned it and the only issue was the 300blk failure to lock open after the last round with all but 2 different factory loads, and it still might loosen up after a couple thousand more rounds.  

I could be totally wrong.  But hey this is teh interwebz and everyone's an expert right?

Thanks and that helps.  Some more tinkering which is why I did this ;-)

Try short loading your magazine a few rounds.  Sometimes the magazine force causes enough drag that it won't want to cooperate.  

But, I would guess your spring is weak, or your rifle wants some lubrication.

What kind of a 300 ammo are you using? reloads or factory? Does it only do it on the first round. Check your casing on that round if its a reload.

I had your end end problem with my Syrac piston and a ballistic advantage 300 blackout barrel. My Piston needed more gas. It would operate fine with a JP silent capture spring but a normal carbine buffer and spring was a no go. After calling superlative arms (they made my piston for Syrac ordinance) they told me to drill out the gas port to .125" max. I did so and now my piston works fine with supers. Subs are still an issue without a suppressor. From all of this I have learned that 300 seems to work best with a DI setup.  What piston are you running?

Same results with different ammo.  I can drop the first round into the breach with no problem.  It's the buffer springs ability to overcome the friction of the magazine lips and feed ramp that is the problem.  I'm running the Adams Arms XLP adjustable.  Gas port is opened to 7/64" (0.109").  Supersonic works fine with the carbine buffer spring.  I'm stuck between too much spring with a standard carbine and not enough spring with NEMO 300 BLK spring.  I was going to polish the feed ramp but there isn't much of feed ramp to polish.  I may need to open up the gas port another hair.  Wish I could make my own spring.  I am so close. ;-)

Have you tried different mags? (GI, pmag, lancer)

Wait, so NOT a standard gas block?  You're outside my experience...

Yes, different mags but thanks.

Different BCG?