300aac build not cycling properly...Help

[mge_1 2]

Took the 300aac budget build upper to DVSC (Delare Valley Sports Center) this afternoon to do some test firing.  Fires straight but not cycling.  It's ejecting fine, but have to manually charge it every time.

• Was using FreedomMunition 147gr FMJ reman ammo.
• Maybe the gas tube and block moved out of place?
• I swapped the bolt carrier group from my MP15 Sport (known working bolt) with same results.

Any other suggestion would be much appreciated.

 

Thanks.

[MidwestPX 172]

Try different ammo.  300BLK can be picky about what will and won't cycle.  It was designed to be suppressed so the extra backpressure generated by the can is sometimes necessary to cycle certain setups.  Gas port sizing is also of critical importance.  I would contact the manufacturer of the barrel to find out what they recommend for ammo.  Sub vs. supersonic ammo is also a factor so it's best to find out directly from the OEM what they speced for the barrel.

 

Also, what buffer weight are you using?  You might want to go lighter.

 

It would also be worth checking to see if it's short stroking (I suspect it is).  To check for this, load one round in a mag and shoot.  If the bolt doesn't lock to the rear, it's short stroking and needs more gas or less mass to cycle.

[mge_1 2]

The bolt is not locking with the last round. Short stroking it is.

[Rob0115 1,105]

No lock back is common. I use and adjustable gas block and a JP adjustable buffer on my 300 builds. They behave differently with various ammos. It's a round that was built for suppression so a standard buffer and spring may some times be too much due to lack of pressure.

[mge_1 2]

Will try different ammo and what buffer weight would you suggest I buy? Thanks

[Rob0115 1,105]

Check and see what you have. Is there an H on it by any chance? What's the gas system length?

[MidwestPX 172]

Will try different ammo and what buffer weight would you suggest I buy? Thanks

I'd go as light as possible on the buffer and if you have an adjustable gas block, open it up completely.

[DargZ 5]

I went with a regular carbine buffer.  Anything more than that will probably not cycle the round.  It looks like you are pretty close to cycling the next round if it's ejecting.  You may just need a little bit more gas to cycle the system.

 

I also used a JP adjustable gas block but any adjustable gas block may help.  You may just need to provide more gas.

 

As for the gas system, it suggests carbine or pistol gas systems for the blackout.

[T Bill 649]

Fire a few more rounds and look for any signs of blow by around the gas block and barrel (Powder residue, etc.)  Otherwise I know some are opening up their gas ports on the barrels just a little bit till it cycles.  What bolt carrier are you using, could lighten it up there also. Make sure BCG cycles smoothly, check buffer spring.

[Displaced Texan 11,748]

OP, I have a carbine buffer I'm not using I'd be happy to donate to help you troubleshoot.

 

Let me know if you need it.

[MidwestPX 172]

You can always open up the buffer and remove a weight or two as well. 

[mge_1 2]

Was using my MP15 Sport's lower yesterday.  Not sure what is the weight of the buffer in that thing.  I don't see any marks anywhere.  After reading a few S&W boards, might be a standard buffer from factory which is the 3.0 oz correct?

 

Out of the 15 rounds per mag, I would say 3-4 rounds actually cycled ok.  The rest i'll have to manually charge.

 

Before I buy anything, I'll just use my other Spikes lower and see how it goes.  Thanks guys.

[mge_1 2]

I'd go as light as possible on the buffer and if you have an adjustable gas block, open it up completely.

I wish I had installed an adjustable gas block.  That bad boy is staying in there, already pin welded the brake lol.  Shucks!

[mge_1 2]

You can always open up the buffer and remove a weight or two as well. 

Hmm good idea...  Will try that next time.  Thanks.

[Rob0115 1,105]

That's one way to go but if you are going to be messing with different ammo I recommend an adjustable buffer for 300.  I have the JP which I like because it's absolutely silent.

[Kimber45 3]

Hey brother sorry to hear about your issues sounds like your running undergassed... as far as tbe Blackout goes Iam using a standard weight buffer ( Spikes ST-T1 amd a JP enhanced carbine spring ) the Blackout ia ment to be run with a standard mil spec BCG so I would leave that alone. Iam assuming your running unsuppressed/supersonic ammo through a 16 carbine gassed barrel that should be fine.first try a diffrent brand of ammo. Iam running a home brew that mimics the Remington 147 gr OTM and that works fine... if the ammo change dosent help I would slide down the gas block and check for obstructions in the gas block and the gas port.. I use some compressed air after a visual inspection with a good flashlight. There might be some machining crap stuck in their.. before you realign and assemble the gas block check your port size for unsuppressed/supersonic you should have a min. Of .099 dia.you can open it up to .104 without issues because you cant really over gas the 300... good luck I hope this helps.....

[Malsua 1,422]

It could need a heavier buffer.  This allows the pressure to build longer before the bolt starts to move.   

[Jon 264]

Late to the party, but have you verified that the gas port was properly drilled? I built a 300aac and had the same problem until I checked the barrel's gas port for an obstruction and found it had not been fully drilled. This resulted in obviously not enough gas for the gun to run. Once fixed it never malfunctioned.

[mge_1 2]

Hi folks...  Ended up picking a digital scale from Harbor Freight today and sure enough the buffer that I was using was on the heavy side at 4.65oz.  I will swap with lighter one as suggested above. Will also visually check gas block and port.

 

Will report in shortly or next range time happens.  Thanks!

[mge_1 2]

Finally got to take the 300blk out again to DVSC.  Using the same ammo, re-aligned gas block and swapped to a lighter buffer.  Everything works as it should.  Thank you all for the tips!

[T Bill 649]

Glad you found it.  Good Luck and Good Shooting.

[Kimber45 3]

Great glad it all worked out.... now GET OUT AND SHOOT.........

[Eric. 9]

It could need a heavier buffer. This allows the pressure to build longer before the bolt starts to move.

Umm, no. This is incorrect.

 

 

Eric

[Malsua 1,422]

Umm, no. This is incorrect.

 

 

Eric

 

Short Stroking can be caused by insufficient gas.  The way to increase the volume of gas is to have the bolt linger in battery longer.   How do you reduce the cyclic rate to increase gas volume and peak pressure?   While you're answering that question, ponder why machine guns use heavier buffers.

[Eric. 9]

It could need a heavier buffer.  This allows the pressure to build longer before the bolt starts to move.   

 

 

Short Stroking can be caused by insufficient gas.  The way to increase the volume of gas is to have the bolt linger in battery longer.   How do you reduce the cyclic rate to increase gas volume and peak pressure?   While you're answering that question, ponder why machine guns use heavier buffers.

The OP has issues with the bolt cycling.

 

First, you tell him he needs a heavier buffer, implying that the fix is delaying bolt movement.

 

Next (probably after reading up some) you now also say that its undergassed, trying to imply that a heavier bolt will increase gas pressure.  BTW, the gas pressure is the same, regardless of bolt weight, fyi.

 

At the end of the day, a heavier bolt has more mass.  As such, it takes more force to move it further (which is the problem here).  The fix is a lighter bolt or more gas to assist bolt movement.

 

Lastly, machine guns use heavier buffers to adjust the cyclic rate of fire/ bolt movement, just so you now know. It has zero bearing towardsnincreasing pressure.

 

 

Eric

[MidwestPX 172]

Short Stroking can be caused by insufficient gas.  The way to increase the volume of gas is to have the bolt linger in battery longer.   How do you reduce the cyclic rate to increase gas volume and peak pressure?   While you're answering that question, ponder why machine guns use heavier buffers.

With select-fire ARs, heavier buffers are used to reduce the cyclic rate by increasing reciprocating mass and preventing bolt bounce. I've lost track of how many guns I've fixed that were running H buffers and wouldn't run more than 2 or 3 round bursts due to bolt bounce. I'd swap in a heavier buffer and they'd run like sewing machines.

[El_Mal90 0]

I went with a regular carbine buffer.  Anything more than that will probably not cycle the round.  It looks like you are pretty close to cycling the next round if it's ejecting.  You may just need a little bit more gas to cycle the system.

 

I also used a JP adjustable gas block but any adjustable gas block may help.  You may just need to provide more gas.

 

As for the gas system, it suggests carbine or pistol gas systems for the blackout.

Would the JP adjustable gas block you're using be the low profile clamp on? If so, how is it? I was looking to pick one up.

[DargZ 5]

Would the JP adjustable gas block you're using be the low profile clamp on? If so, how is it? I was looking to pick one up.

 

It works well.  I have it on several of my builds.  The only issue is with some of the really skinny handguards (Noveske NSR), the gas block may get in the way of mounting stuff on the keymods as the bottom of the gas block does not provide enough clearance between the gas block and the keymod nut.  You will just need to place your keymod accessory further forward or backward.  Another option is get a thicker handguard.

 

One other issue I had was the adjustment screw with the Troy Alpha handguard.  There was no hole for me to put the allen key to adjust the gas block.  This was an easy fix to put a hole in the handguard to fit the allen key.  The Syrac Ordinance which JP purchased alleviates this issue by moving the adjustment to the front.

[El_Mal90 0]

Good to know. I looked at the dimensions and took the dimensions of my handguard (Strike Industries Mega Fins XL), and it should fit nicely. The foregrip I plan on using will be at the front of the handguard any way so that should work. 

 

Thanks for all of the information!