Short cycle problem

[Bbarsh]

I have a 16" midlength barrel from Dennys Guns. I am unsure of the gas port size. I checked everything and have found no issues with the bolt rings the carrier key or the gas tube. It has a standard carbine buffer and spring. It will cycle fine with 5.56 but short cycles with UMC 55 grain .223. Is there a lighter buffer or spring that I could try or is that not the answer? Thanks for any help, Brad

[Robb Jensen]

UMC (Remington) is notorious for being extremely weak.

[m4fun]

UMC is underpowered. If you checked everything else out, are you sure you dont have a rifle length buffer spring? Wolff or gunsprings.com sells a variety of different powered springs, but lets let other folks chime in.

[Eric]

What are you seeing in the short stroke? Does it fail to lock open with one round in the mag? We had one with no apparent issues on a visual inspection, but a new gas tube cured it.
I've actually had no issues with the UMC ammo. It runs fine in my agency M4's and various private ARs.

[Bbarsh]

What are you seeing in the short stroke? Does it fail to lock open with one round in the mag? We had one with no apparent issues on a visual inspection, but a new gas tube cured it.
I've actually had no issues with the UMC ammo. It runs fine in my agency M4's and various private ARs.
The bolt won't go back far enough to pick up the next round. Everything is new,it's the first time I had it out.

[Robb Jensen]

The bolt won't go back far enough to pick up the next round. Everything is new,it's the first time I had it out.

That's classic short stroke.

Measure buffer spring to make sure it's not too big.
Could be weak ammo.
Could be a worn gas tube.
Could be a lose carrier key.
Could be a misaligned gas block.

[Bbarsh]

That's classic short stroke.

Measure buffer spring to make sure it's not too big.
Could be weak ammo.
Could be a worn gas tube.
Could be a lose carrier key.
Could be a misaligned gas block.

Buffer spring free length is 10 13/16
Buffer weighs 2.9 oz
Ammo is UMC 55 gr .223
New gas tube
Carrier key tight
Bushmaster clamp on sight post I moved it around and no difference

I'm pretty sure its because the ammo is weak but it's cheap and easy to get so I would like to be able to figure out how to make it run. Is it possible?

[C4IGrant]

Buffer spring free length is 10 13/16
Buffer weighs 2.9 oz
Ammo is UMC 55 gr .223
New gas tube
Carrier key tight
Bushmaster clamp on sight post I moved it around and no difference

I'm pretty sure its because the ammo is weak but it's cheap and easy to get so I would like to be able to figure out how to make it run. Is it possible?

Clamp on front sight/GB? This could be your issue if it is not installed correctly.

Some GB's require that there be a space between the barrel shoulder and the block. Other's require that the block be pushed up against the shoulder. If is if in any way, it would cause your issue.


C4

[silvery37]

You should be able to get reduced power buffer springs. The only other place I know of to reduce weight would be to change out your bolt carrier. You could get something like the JP enterprise low mass carrier. Other than that I dont think there is much you can do without permanently changing the weapon like enlarging gas port.

I would also make sure the rifle is very clean. If it is the weak ammo that is causing the problem a dirty chamber could make the difference in it cycling.