SBR cycling problems.
 

I am having issues with my SBR 300 whisper. 10.5 inch barrel.

the pistol length gas port is opened up to 100 thousandths, the buffer is without internal weights. the adjustable gas block is opened almost to the last thread.

I am shooting factory hornady 110 grain rounds

but also I have hand loaded 110 VMAX with 20.2 grains of H-110

150 grain bullets with 20.2 grains of AA1680

and 220 bullets with 15.5 G of AA1680

the gun will only cycle 50% of the time with the 110 and 220 bullets.

and 95% of the time with the 150 G bullets.

none of the loads will lock back the Bolt carrier on last round. works a little better with the suppressor on, but still no lock back.

what should I be looking for to solve my problems?

Your gas block is on crooked or the port is in the wrong place.

yeah, that's been checked, but I'll check again.

Does the gas tube go deeply enough into the gas key on the carrier? Is the fit good enough within the key?

If not, the rifle will short stroke or cycle inconsistently.

For a 10" the gas port is usually .125".Maybe .110 will work but i go the .125"The long barrels are .093" (16").That is for subsonic speeds.

I thought the gas tube was 100 thou, so I didn't think opening up the port larger than the tube would do any good.

and to Mstarling, how would I be able to see if the tube is into the gas key deep enough?

would a simple measurement be OK?

it is a factory part so I didn't even think that was an issue.:confused:

But he’s not cycling supers with the block open and zero weights in the buffer. If the port is lined up there would have to be something weird going on. Could be there are no gas rings, wrong tube length, possibly the carrier and buffer spring binding. I’m a little concerned that the gun could be damaged with the hot loads and no weights. On the lock back, I had a problem with a receiver that was not relief cut properly and would not allow the bolt release to engage the bolt correctly. You could lock it back manually and bump lightly the gun and the bolt would slam closed. On tube length, take the upper off and turn it upside down. There will be a semicircle cut into the receiver that is meant for the carrier key to rotate into when the gun goes into battery. The gas tube should stop at about the 50% point (or a bit more) of the relief cut.

Just to clarify, are you using a rifle or carbine length receiver extension tube? Are you using the correct standard spring? What was your logic for reducing the buffer assembly weight? Have you tried it with the standard weight buffer assembly? It seems to me that a reduced weight buffer could be the cause of the symptoms you're describing

The function of the buffer is to supply additional energy to compress the action spring and to increase the time period of the spring -mass pair so that the bolt can complete all of it's functions of extraction, re-cocking the hammer, ejection, and moving past the next cartridge or locking the bolt open BEFORE the spring and buffer start their return pushing on the bolt. Only after the bolt is fully open should force be applied to the bolt, picking up a new cartridge, chambering it, and locking the bolt. It takes more gas energy to fully cycle the bolt with a light buffer than with a correct one because the spring is trying to close the bolt long before the bolt is fully open. An excessively heavy buffer won't work well either as the spring will fully compress, waste energy, give slow harsh cycling, and eventually cause damage.

I'm not saying that the only possible problem but it's the only thing I can think of which explains all of the symptoms you've described.

Recusance,

tp555 has probably built more of these things that work than anyone else. He REALLY does know of where he speaks!

Mike

thanks to all of you for all the input. so many things to address.

the original gun was a 5.56 SBR and everything was working fine. all I did was put on a model 1 sales barrel. I had it cut down to 10.5 and the gas port moved to the pistol location. I bought a factory pistol length gas tube, yes it extends ~ better than 50% in the relief in the receiver.

I took the weights out of the buffer because I was originally shooting subsonics and having issues with it cycling then. I then readjusted the gas block but I admit I am not an expert on where it is located beyond the little experience I have. no change with supersonics.

it has a carbine receiver extension with carbine length buffer and correct spring.

I know it has gas rings and they are in good shape.

I will put a standard weight buffer in and try again next weekend.

I know there is a lot of great advice on this board that's why I'm asking.:grin:

I am really leaning toward the gas block being the culprit.

The way I align gas blocks is to plug the chamber and blow air into the muzzle. Just listen to the air flow coming out of the gas tube while moving the block to find the position with maximum airflow. The method is quick, reliable, and requires no special tools. I use a fired case for the chamber plug. The method is also a check for obstructions of the gas ports and gas tube at the same time without disassembling anything.

that's the method that I use. maybe my ears are not tuned. :twoguns:

I use a pencil and mark the center of the port on the barrel and the center of the port on the block. Then I line them up. The problem you run into is the port location. The distance from the shoulder of the barrel to the port and the distance from the back of the block to the center of the port on the block need to be checked. Either one could have been made on Monday or Friday. Blowing through the barrel will allow you to get maximum flow for the setup, but you may only have half of the barrel port exposed to the gas block port.

It would take some pretty weird fluid dynamics to get maximum flow with "only half of the barrel port exposed to the gas block port. If they're different sized being exactly concentric isn't critical as long as the smaller hole isn't blocked at all. The larger hole will always be blocked some. I turn the bock side to side and slide it end to end till i hear an equal change then split the difference each way.

Diagnosing a rifle though snips of symptoms on an Internet forum isn't nearly as easy has having the rifle in hand to inspect and shoot.. I saw one AR-15 at friend's gunshop which had similar symptoms to this one. The problem only became apparent when the gunsmith attempted to remove the gas tube from the receiver and it wouldn't come out. The reason was fairly obvious and the owner of the rifle admitted that he'd checked the torque of the barrel nut He had not removed the gas tube first.

I don’t think you’re getting what I’m saying. My ear can’t calculate CFM by sound. Yes someone may be able to rock the gas block back and forth until you hear the maximum flow, but unless you measured the ports you’re guessing. If the center of the barrel port is 0.300 from the shoulder and the block port is 0.310 from the shoulder you may get flow, but how much? Ports get drilled in the wrong place. I’ve bought used blocks that were full of copper. They had flow, but caused the gun to short stroke. If the ports are in the right place your method works just fine. It’s a “Trust but verify” kind of thing. :tongue:

Gas port is .295" from the edge.That is the spec.I have seen a gas block very nicely machined pretty and all that was .250 from the edge.Customer supplied it.I do machine a dimple exactly 180 degrees from the gas port for the screw on my barrels.That makes it easy for block alignment.The gas hole going up in the block is usually .125" so you have some slack.

that does sound like the ticket. I'll "improve" my gas port to .125.

thanks gentlemen.

Check to see if your carrier key is bent and double check to see if the carrier key screws are not backing out.

checked, staked properly, all is good.

I currently have a registered M6A2 LWRC SBR with a 10.5 556 upper. I have an AAC M4-2000, I got a chance to actually go shooting this weekend and I brought a long a 10" 9mm Upper to try out. The 9mm fired fine for about 5 rounds and then I started getting dual feeds but I knew it could be problematic and well didn't give it much thought.
I put the 556 upper back on and fired a couple mags with the canon no problems. Took the can off just to go louder because I could and no one was there to complain about the sound. The gun would fire one or two rounds before I had to manually cycle it would extract and eject but wouldn't strip another off the top of the mag. I tried a different mag even though I was using pmags. A buddy put the mag in his 16" AR no problem. I got really worried and stuck the can back on wham bam 25 rounds down range no problem. Has anyone seen this before? Does it take the can to build up the sufficient back pressure? My buddy ex cop and military says he has heard of this but never seen it we checked everything in the field. The gun has less than 300 rounds through it but I have always ran the can because my neighbors fuss and call the cops every time I shoot even though they have all been told I am legal. I just stopped shooting at my place because the local government started talking about a possible statute about shooting in the county. I didn't want to screw it all up for everyone else.
Sorry so long and thanks for any help or suggestions.

Cans add back pressure.Sometimes too much pressure.Ask the 308 crowd or dpms sass crowd.They buy lots of my gas blocks.In your case not enough pressure to cycle without the can.Check gas port hole size.On shorties 1/8" is std.

ok, now I have a different but related question.

in my 5.56 AR15's if I load up a subsonic 5.56 round I know it will not cycle the next round.

If I load up a subsonic 300 Whisper round the same thing happens.

why is the Whisper round supposed to work but the 5.56 round will not?

is it only a function of a heavier bullet?:confused:

A 220 grain bullet requires four times the energy to drive to 1000 fps as a 55 grain bullet. Caliber doesn't matter much. It also carries four times the energy when it leaves the muzzle. E=1/2M*V^2 Note also that it only takes 1/9 the energy to drive any bullet to 1000 fps that it takes to drive it to 3000 fps which is why it's very difficult to make a gun which will shoot both sub and supersonic bullets without modifications. AR-15's were never designed to shoot multi-energy cartridges. . Not like a bolt action!.

So yes it's the heavier bullet mostly. You really have to look at the pressure curve vs bullet position in the barrel using an appropriate powder. Burn rate of the powder can also make a big difference in the gas impulse available at the gas port. Quickload computer simulations can give insight on the avaialble impulse energy, though it's not a direct output of the program.

perhaps I should look to a different powder.

currently I am using H110. for both subs and supers

where should I look for a little better pressures as a starting place?