[Tuko]

I built a rifle with a chrome lined 7:1 twist barrel and it won't cycle all the way.

It will eject the spent case but the bolt will not travel back far enough to pick up the next round in the mag.

I have a friend with a match barrel gun and he is having the same problem.

Do I need an adjustable gas block?

 

[sharky8js]

I don't know, I've never had the problem, but it sounds like the buffer spring is too strong?

Is there resistance (of some sort) when you manually pull the bolt back?

 

[Tuko]

Yea,


I thought that too.

I tried a buffer and spring from a colt that I know runs. No luck.

Thanks though.

 

[woodstock]

What kind of ammo are you shooting? Did you install the gas block and if so, how did you align it?

 

[Tuko]

Winchester 55 gr. ball.

I Aligned the gas block with the gas tube to the upper reciever.

 

[woodstock]

Why I ask is if the gas block is canted just slightly the hole in the gas block wont line up with the hole in the barrel and you won't get full gas pressure to cycle the bolt fully. If the gas block is flat on top, mount the upper in a vise (with a vise holding block of course) and then level the receiver across the top of the rail and then make sure the gas block is level as well. This should insure that the gas holes are lined up properly.

 

[BRL]

Winchester 55 gr. ball.

I Aligned the gas block with the gas tube to the upper reciever.

Why I ask is if the gas block is canted just slightly the hole in the gas block wont line up with the hole in the barrel and you won't get full gas pressure to cycle the bolt fully...

That would be the place I would double check as well.

 

[Tuko]

OK. I tried removing the block and tube. Double checked the tube to block alignmet and re-aligned the block. Still no luck. Fires one shot, ejects spent case and closes. I have to manually charge bolt again for the next shot.

There is also no unusual resistance or excessive drag when I charge it manually.

I'm starting to think that the 7:1 will not cycle 55gr ammo. Maybe the heavier bullet will cause more pressure.

I'll try some and see.

Now...where to find some.

 

[Carbine85]

The 1:7 twist doesn't have anything to do with the cycling of the action. 55 gr is light for that twist.
Check for gas problems or binding.
Check the gas rings in the bolt.
Check the gas key and make sure it is tight and staked.
Make sure the receiver extension (buffer tube) is tight.
Try different mags.
Make sure the gas tube is installed properly.
My son recently bought a gas block and we found that it was made wrong. When they drilled the gas tap they must have flipped it around and drilled the wrong end. Check the gas tap ports.

 

[usmc0341]

A couple of other issues that may cause this. All these questions tie together.

Do you have the LMT enhanced BCG?
Is your rifle chambered in 5.56mm?
Is the ammo you are shooting .223?

If you answered yes then what can be happening is .223 does not produce as much gas to reliably cycle the LMT BCG because the three holes drilled in the side of the BCG allow too much gas to escape before extraction and ejection which will cause short stroking.

I ran into this problem while shooting 55gr AE in my LMT, the problem disappeared when I switched to 5.56mm M855

* also I would try new mags. The mag springs may be weak and not feeding the round up quickly enough to be caught by the bolt on its way forward.