SBR cycling problems.

[Recusance]

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?

[Alleycat]

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

[Recusance]

Originally Posted by Alleycat

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

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

[mstarling]

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.

[tp555]

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.

[Recusance]

Originally Posted by tp555

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.

[Alleycat]

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.

[LouBoyd]

Originally Posted by Recusance

...what should I be looking for to solve my problems?

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.

[mstarling]

Recusance,

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

Mike

[Recusance]

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.

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