Gunsmithing Grendel cycling issue

[BigAndy]

Hey fellas, anyone out there have suggestions for fixing a 6.5 Grendel AR that doesn't completely reset the trigger each time? After firing a round, the buffer sounds like it is being slammed pretty hard. I am not worried about the slightly higher recoil (than say a 5.56), but would like to have a more reliable rifle. My question is, should I go with a heavier buffer and/or stiffer buffer spring, or do you guys think something else might be acting up? I am running a rifle length gas system, DPMS slab-sided upper (For once a forward assist might help, as the bolt doesn't always go completely into battery), Black Hole Weaponry medium contour 20" barrel, Rguns bolt and LPK. I did have Bill Springfield lighten up the trigger and hammer, and installed JP springs throughout the lower, if that is pertinent. Let me know what you guys think.
Thanks,
-BigAndy

 

[bigbear_98]

I am just finishing my Grendel build. I plan to go shoot it today. The biggest thing I kept reading was that they could easily be over gassed. I read it enough that it made me put an adjustable gas block on mine.

That being said, can you tell us more about what buffer tube and buffer you are using?

 

[BigAndy]

Mil-spec AR buffer and spring, but thinking about a Spike's or BufferTechnologies set-up, though they are a bit more pricey

 

[Hoser]

I bet the Springfield trigger job is the main problem...

 

[bigbear_98]

I bet the Springfield trigger job is the main problem...

I would bet so too. Try installing a different one and see if it continues. Pm me. I have a mil spec you can stick in it to try if you don't have one to try.

 

[BigAndy]

You guys may be onto something, as I can feel a little slap as I release the trigger after follow-through. I do really love the trigger; better than some pricey ones I have tried. My only question would be, why does the BCG not go completely into battery? In other words, why would a lightened trigger caused increased resistance during cycling?
Thanks again.

 

[bigbear_98]

I guess I missed the part of it not going into battery. Does the trigger not reset only when the bolt isn't in battery? If so the trigger may be reset but the round doesn't get fully chambered and when you pull the trigger the hammer is still pushed down by the bolt carrier.

There are a few threads over at 65grendel forums about reliability. One of them mentions rounding the ejector a little to help with feeding. Feed issues seem to be a common problem with larger cartridges in the ar15.

Can you give more info on the gun? Understand, I just shot mine for the first time yesterday, so I am no expert or do I claim to be.

Just an FYI, I put an adjustable gas block on my 65. I have a 20" rifle length barrel with a carbine buffer tube and buffer. The gun reliably cycled and locked back with only 1/4 turn open. Of course I am running a supressor , but I would say this cartridge is way over gassed.

 

[338RUM runner]

What size of gas port dia. are you guys running

 

[bigbear_98]

What size of gas port dia. are you guys running

I don't know. I bought the barrel straight from Krieger. It started as a 26" and they cut it down to 20. Everything I can find on the topic says it should be around .093" for the grendel. It doesn't say on my receipt so I would have to take the gas block off and measure it. I don't really want to.

 

[bigbear_98]

Have you looked at your bolt to make sure it isn't broken?

 

[BigAndy]

Bolt is fine. I think I might try the easy way of increasing resistance, via heavier buffer and stiffer spring. The rifle is still pretty new; I built it 120 rounds ago. I just had to fix the 5.56, so not really looking to take this one apart unless necessary.

 

[toygunner]

Sounds like its under gassed. You really need to check the port size. A heavier buffer and or spring will only make the problem worse. I would venture to say that you need about an .080-.083 port.

 

[Red_SC]

My only question would be, why does the BCG not go completely into battery? In other words, why would a lightened trigger caused increased resistance during cycling?
Thanks again.

If it's overgassed as you suspect, you may have bolt bounce; it's going back into battery, but bouncing out. That's the purpose of the loose weights in the buffer- as the spring pushes the buffer and BCG forward during cycling, inertia pushes the buffer weights to the rear. As the action slams shut it tries to bounce back, but the weights slam forward a split second later to counteract the bounce. Try a heavier buffer.

 

[Red_SC]

Here's a video, the buffer weights have been removed to enhance bolt bounce. There is still some bounce with standard buffers, how much depends on the ammo, port location and size, buffer weight, spring weight, and more.

 

[kvwpwr]

Does the trigger reset normally when you cycle by hand?