About to throw this thing out the window

[DIESEL758]

Also iam getting a slight dent on the spent rounds due to the deflector. Should the little black o-ring be on the spring? Mine does not have one but I do have one to put on if this would help.

[leopard6.5]

DIESEL758: A lot of us just put a piece of velcro, the fuzzy side, on the deflector to eliminate the denting.

I also have to say I'm really dissappointed your rifle is working now. I'd been hanging around outside your window waiting for you to throw it out! Oh well.
Just kidding!

It's really nice to see it working for you after all this. Now you're just down to the little problems like dented brass.

Good luck and great shooting.

Lee

[DIESEL758]

Thanks Lee I will try the Velcro. So that was you hanging around my house!!!!!!

Thanks for the help

Jason

[bobke]

Diesel-
Do a search for 'ejector spring mod'-simple ejector spring length reduction, that along with the Velcro, will virtually eliminate the dented case syndrome. I've done it to all of my AR's without any downside. However, measure twice-cut once. Too short does not work. Caliper is your friend.

[WillD]

Don't you love it when a plan comes together?

[CZfour52]

I had the same problem with my 18" barrel rifle length gas from BHW, send my upper to them for testing for bolt not closing, they fixed that when i got it back it would not cycle my loads tried everything, even buying factory ammo to test, still did not work so i kept reading and my fix, so i don't bore you guys was opening the gas port to something like in the .900's.

[Dos Perros]

I hope no one minds me posting my issue in this thread, as I'm experiencing the same issue as the OP. However, I do believe the root cause to be different, so this thread might serve as a good reference for FTF issues.

Okay, here goes.

Lilja 470 barrel, rifle length gas system
DPMS gas tube
Precision Reflex low profile & adjustable gas block
M16 Nickel boron BCG, unknown manufacturer
Maxim Firearms bolt as recommended by Lilja - I added an o-ring to the extractor spring as the original design wasn't strong enough to hold the rim during ejection; ejects cleanly now
Brownell's kit for Magpul MOE rifle stock, buffer tube, buffer, and spring
ASC (I think that's the name?) magazine

So I'm just going to cut right to the chase about what I believe the issue is, and I'm hoping you guys can confirm that this sounds right and offer a solution.

I believe the gas port hole on the barrel is not correctly lining up with hole on the gas block causing short strokes. The brass is ejected cleanly, but the BCG isn't traveling far enough to the rear to stay open on the last round.

According to this drawing, Lilja dimensions the gas port hole at 0.300" off the surface that the gas blocks rests up against. I have found some drawings, the following for example, that dimensions the gas block side at 0.277" off that same datum.



When I measure the gas block I have, I get a dimension around 0.240", although admittedly it's a pretty dirty measurement. So, the hole is around 0.060" out of alignment, which is reducing the amount of gas getting back to the BCG, causing my short stroke issue. Does this sound right? If so, what's the solution? Maybe a out-of-spec block? Should I shim it? Maybe just stand the gas block off 0.060" and see if that works? Any additional thoughts? I do have plenty of pictures if anyone wants them I can post. Thanks guys.

[Drifter]

Maybe just stand the gas block off 0.060" and see if that works?

That should be easy enough to try, and might solve your problem.

[rasp65]

Some gas blocks are designed to work with stock hand guards and their caps. So if you put the gas block tight against the shoulder on the barrel you will cover part of the gas port.

[LRRPF52]

It's up to the assembler to align the gas block, as they rarely mate up to the shoulder before the journal. As mentioned above, most shoulders are cut to allow installation of the handguard cap and a front sight base/gas block per the original design, not a free-float.

The short story is to measure your port distance from the shoulder, measure your port distance inside the gas block from the rear edge, and align them center-to-center. That responsibility rests solely on the person assembling the system.

[Dos Perros]

It's up to the assembler to align the gas block, as they rarely mate up to the shoulder before the journal. As mentioned above, most shoulders are cut to allow installation of the handguard cap and a front sight base/gas block per the original design, not a free-float.

The short story is to measure your port distance from the shoulder, measure your port distance inside the gas block from the rear edge, and align them center-to-center. That responsibility rests solely on the person assembling the system.

Understood. For some reason that detail always escaped me whenever I had read instructions or watched videos.

I eye-balled it and stood the block off and now the rifle cycles flawlessly (nice little groups, too). Precision Reflex was very prompt to return my email and even offered to send me some shims for free. Great customer service and I'll be buying their blocks again in the future.

[pinzgauer]

Many of the gas blocks I've seen had a hole in the bottom to allow you to rotate the block 180 around the barrel and line up the port. Then measure or shim the space between the shoulder to get correct spacing. Then rotate back and secure.

And others have had a hole in the top, plugged with an allen screw, etc.

But you get the idea! There are multiple ways to measure this and align to the gas port.

[Jakal]

Take a spare hand-guard cap and cut in half. Slide the gas block onto the barrel and then place the modified hand-guard cap in between the gas block and the shoulder. This should put you spot on. But measure as stated in the post above, just to be sure. Keep this hand-guard cap as a jig for future gas block installations.