Under gassed?

[Flyby]

I went to test some reloads this weekend. All did NOT go well. Here's the specs:

Rifle
Loki Weapon Systems 18" Stainless barrel and bolt
Rifle length gas tube
Spike's Tactical lower
Rock River National Match Trigger
Gemtech HVT 7.62 suppressor
About 60 rounds fired through it

Load
Test strings of 10, TAC 27.8 up to 28.2 gr with a 123 gr Hornady AMAX
Also had factory Hornady 123 gr AMAX


It is a new build, so it had Breakfree CLP liberally applied

Reloads
Reloads had FTF in 1 out of 3
Chronygraph average 2525 FPS
At FTF, It would eject the spent casing, but not load the next round. It would be sitting on a closed bolt, empty chamber.
I was distracted with the FTF, but all these loads had less than 1" MOA at 100 yds.
I had three rounds that were jammed into the lugs with two deep gashed in the brass.
Also switched the buffer and spring to a light 300 Whisper one for shooting 300 Blackout subsonic, no change


Factory Hornady ammo
Chronygraph average just over 2600
Fired fine with suppressor attached.
With suppressor removed, there was the same FTF as above, empty chamber with closed bolt.

I have a low profile gas block, I'll check it first. I don't know the diameter of the gas port, I don't have tools to measure it (what do you guys use?). I don't know the make of the gas block. The gas tube is a black YHM tube.

It sounds like a low gas situation to me. The factory loads were a little hotter than my reloads and worked as long as the suppressor was attached.
- I'll check the alignment of the gas block.
- I'll also check the gas block, unknown mfg
- Check gas key and staking
- The BCG seems to move freely, no abrasions or marks that I've seen
- Lower worked fine with a 300 Blackout upper I had with me (including subsonics).

Other thoughts?

[Drifter]

Sounds like short-stroking. Did you try more than one magazine?

If everything regarding the gas system and mags checks out okay, a lighter weight buffer and / or bolt carrier might be an easy fix if you have extra parts. I suggest enlarging the gas port only as a last resort.

An 18" with rifle-length gas leans towards reliability issues with light bullets and fast-burning powders, but it should run on Hornady factory loads.

[Flyby]

Sounds like short-stroking. Did you try more than one magazine?

If everything regarding the gas system and mags checks out okay, a lighter weight buffer and / or bolt carrier might be an easy fix if you have extra parts. I suggest enlarging the gas port only as a last resort.

An 18" with rifle-length gas leans towards reliability issues with light bullets and fast-burning powders, but it should run on Hornady factory loads.

I did have 2 mags, it functioned the same with each.
I did have a lighter buffer/sprint combo that I swapped out, it didn't help either.

I believe all my BCGs are mil spec.

[LRRPF52]

18" + RLGS with too small of a gas port will give you short-stroking. The fact that it ran with your suppressor attached says that the system is tuned for suppressor use, so if you end up opening the gas port, you might want an adjustable gas block. Also, with temps as warm as they have been, you also need to think about cold weather and how it will run then.

I always thought that an 18" RLGS made a lot of sense, and it does in 5.56 NATO, but Bill A. discoverer rather quickly that 18" Grendels with RLGS are finicky because of dwell time. 6.8 SPC is the same way. The 18" RLGS does seem to provide higher muzzle velocities though, and your chronograph readings support that suspicion I've had. Your factory Hornady average is 20fps faster than Hornady's reported velocity of 2580fps from a 24" barrel.

The cartridges that were jammed into the extension teeth are often caused by bolt over base FTFeed malf's. The carrier doesn't travel rearward enough, but the carrier face rather than the bolt, picks up a cartridge, then pinches the snot out of it between the bolt, the magazine, and the feed ramps.

Since you're getting accuracy and amazing muzzle velocity, I would think that opening the port will probably be the outcome of this trouble-shooting process. Think about the fact that a 20" Grendel with RLGS uses a .094" gas port diameter, and you know that the 18" RLGS system is going to have to be larger to get the same carrier thrust equivalent, although the dwell time will still be shorter.

For shooting pills with long bearing surfaces, your carbine is ideal, especially looking at the velocity you're getting. Let me know if you want to get rid of the barrel/bolt combo.

[Flyby]

I cleaned my rifle any inspected everything. Gas key, BCG, gas block all looked good. The gas port was open. I reset the gas block t make sure it had best flow possible. It was already there.

LRRPF52 you are right. FPS was higher than Ramshots manual by 100 fps. I believe Hornady now says 2620 on AMax, I was getting 2600+. I was so focused on FTF I missed the velocities.

I'll pick up a set of drill bits with index and measure my current gas port.

Good to know about the bolt over base TFS. I think you nailed it with the short stroke. I saw similar things with subsonics in my 300 blackout right after I completed the build. A super light buffer helped until I got it broke in. I tried the same light buffer in the Grendel, it didn't help. That buffer and a light bcg way help, just don't have one handy.

I'll see the measurement of my port and go from there.

Thanks for all the help.

[LRRPF52]

There are Grendel owners who have been in your exact same shoes with an 18" RLGS, and they ended up opening the gas port. Guns ran fine after that.

I've been thinking about a 16" or 17" RLGS, but I know it will need a larger port than a 16" MLGS, which is what I have now. I could really learn to be fond of a carbine that shoots 123gr at 2600fps, because my 16" is still supersonic out to 1318yds at common conditions I shoot in, from a 2460fps mv.

[Drifter]

If you open the gas port, do it carefully. Use a method to prevent the drill bit from going too deep. Put a wooden dowel inside the barrel also. You don't want to harm the rifling. It will likely require very little port enlargement. While it will hopefully solve your undergassed issue, you might need an adjustable gas block to curb excessive gas. You might lose a little velocity since more gas will be bled off.

[Flyby]

Thanks for all the help guys. Next step is to see what the current gas port diameter is. Drifter, good points on the dowel. I had already been thinking about the rifling. I'm getting good accuracy, so I don't want to mess that up.

I like the 2600 fps mv with the 123 gr also. That puts me in a comfortable range for white tail. I know people use a lot less, but I believe with velocity and hunting, the more the better.

[Drifter]

I like the 2600 fps mv with the 123 gr also. That puts me in a comfortable range for white tail. I know people use a lot less, but I believe with velocity and hunting, the more the better.

I agree, and that's why I think the Grendel needs a good hunting bullet (SST, AccuBond, Interbond, etc) in the 107-108gr class. Perhaps good discussion for another thread...

[Flyby]

I measured the gas port with calipers, it was around .084. I bought a set of bits at 1/64th increments and used those to see what the largest bit was that would fit the gas port. Its was a .083. There's where I hit a problem. The gas port wasn't opened up all the way through. It looks like they used a depth limiting device that didn't round out the hole. I was down to a .067 bit before it would go all the way through. I didn't go with a larger port size, but used the .083 bit to round out the hole. It seemed to make a difference with the "blow" test.

Need a range test now.

[Flyby]

A quick follow up. I did have to drill the gas port to .093 and use a light buffer to get it to run. I don't remember the buffer weight, it is out of my 300 Blackout. The adjustable gas block is wide open.

I may run it like this a while and brake it in, then go one bit size larger and tune the gas if the regular buffer is too heavy.

[Drifter]

Thanks for the followup. There's been some recent discussion regarding optimum gas port size for an 18 with RLGS. It seems that .094 is standard for a 20, so slightly larger (maybe ~.096 ?) might be ideal for the 18.

It's no wonder you were having issues if your port was .083. Glad you're on the right track now.

[frank]

I ran some numbers and calculated the ideal gas port diameter for 18" RLGS to be 0.108". This also happens to match the gas port diameter of the MK12.

Of course numbers on paper sometimes don't always equate to real-world performance. Use this data at your own risk...

[Flyby]

I ran some numbers and calculated the ideal gas port diameter for 18" RLGS to be 0.108". This also happens to match the gas port diameter of the MK12.

Of course numbers on paper sometimes don't always equate to real-world performance. Use this data at your own risk...

That's interesting. Most drill bit sets are in 1/64th increments. So .094 = 3/32 and the next step up is .109 (7/64ths). I didn't want to make that jump yet, but I feel it is still a little low since it takes a super lite buffer to run. The next step is to go to .109 and tune down the gas with the adjustable block.

This is an all new build, so I may run it as is for a while and see if I can move back up to a standard buffer after it has several hundred rounds through it.

[explorecaves]

That's interesting. Most drill bit sets are in 1/64th increments. So .094 = 3/32 and the next step up is .109 (7/64ths). I didn't want to make that jump yet, but I feel it is still a little low since it takes a super lite buffer to run. The next step is to go to .109 and tune down the gas with the adjustable block.

This is an all new build, so I may run it as is for a while and see if I can move back up to a standard buffer after it has several hundred rounds through it.

Look for a wire gauge bit set. You should be able to get the increments needed with it.