Overgassing?

[Tomahawk67]

Upon further inspection, I think my spring may be the issue.. Its 10.5 inches with 25 coils which is shorter than any dpms style 308 spring I can find and has fewer coils. That'd cause the carrier to crash into the front of the extension. I measured the buffer and tube as well, those look fine. 7" on the tube and 2.5" on the buffer. That spring has me scratching my head though. I might order an orange spring from sprinco and see what that does?

The length of the spring is inconsequential to the carrier contacting the reciever. The buffer length and buffer tube length are what controls the rearward movement of the carrier.

 

[campwilde]

I used a TUBB precession springs. Very little difference between running it suppressed and unsuppressed. It holds lock time a little longer with a couple extra pounds with out adding increased poundage when fully compressed.

 

[DirectDrive]

Upon further inspection, I think my spring may be the issue.. Its 10.5 inches with 25 coils which is shorter than any dpms style 308 spring I can find and has fewer coils. That'd cause the carrier to crash into the front of the extension. I measured the buffer and tube as well, those look fine. 7" on the tube and 2.5" on the buffer. That spring has me scratching my head though. I might order an orange spring from sprinco and see what that does?

That sounds like a standard carbine spring for an AR15
When you measured the tube, did you measure the entire length or did you "bump" the back of the receiver with your tape ?

Here are some DPMS part numbers :

DPMS LR-308 Carbine Buffer Spring - Midway #: 813595 DPMS #: 308-CS-10A
Relaxed Length - 11 3/8"
Compressed Length - 3 1/4"
28 coils (inclusive of both ends)
Spring rate - ≈10lbs/in (tested over 2" travel from rest on buffer retainer pin)

DPMS LR-308 Carbine Buffer - Midway #: 232006 DPMS #: 308-CS-10B
Length - 2.485"
Flange Diameter - 0.969"
Barrel Diameter - 0.686"
Weight - 3.808 oz.

 

[Grimm]

That sounds like a standard carbine spring for an AR15
When you measured the tube, did you measure the entire length or did you "bump" the back of the receiver with your tape ?

I measured the length of the tube itself from the back to the front of the threads. Like the numbers you just posted, I'm also seeing the springs coming in across the board an inch or so longer. That buffer length seemed long though, I've been seeing those right around the 2.5" mark which tells me mine is okay.

I don't understand how the length of the spring wouldn't have something to do with the bcg travel. the shorter the springs resting length, the shorter its compressed length allowing the bolt to go back further than it's supposed to. I could see if it was some high coil rate how that could compensate for it, but it's shorter in length and fewer in coils.

I'm going to try to figure out who put this thing together because I'm not paying for a new spring if they put the wrong one in it. It may only be $10 but its the principal! lol

 

[Tomahawk67]

The buffer length and buffer tube length determine the travel of the carrier, not the spring. I run 308 rifle length springs in my ar9's. The only way the spring could limit carrier travel is if it goes to coil bind.
You have to use the correct length of buffer with the corresponding buffer tube, wether it is ar10 style or dpms style.

 

[DirectDrive]

I don't understand how the length of the spring wouldn't have something to do with the bcg travel. the shorter the springs resting length, the shorter its compressed length allowing the bolt to go back further than it's supposed to. I could see if it was some high coil rate how that could compensate for it, but it's shorter in length and fewer in coils.

Tomahawk has it right, the tube length and buffer length determine maximum rearward travel of the BCG.
That is, unless the spring is too long for the tube and you get "coil bind" where the spring coils stack and become the limiting factor.

If you do have an AR15 carbine spring (I think you do), that could let your BCG fly rearward at faster rate than wanted.

I don't know why you're getting those two marks.
Do they track into the tube ?

 

[Tomahawk67]

Tomahawk has it right, the tube length and buffer length determine maximum rearward travel of the BCG.
That is, unless the spring is too long for the tube and you get "coil bind" where the spring coils stack and become the limiting factor.

If you do have an AR15 carbine spring (I think you do), that could let your BCG fly rearward at faster rate than wanted.

I don't know why you're getting those two marks.
Do they track into the tube ?

The marks are from the carrier going too far rearward from the buffer length being too short.

 

[Tomahawk67]

Because the carrier in a ar10/lr308 is longer than an ar15, you either need the longer ar10 buffer tube and the corresponding ar15 length buffer, OR you need the lr308 buffer tube (ar15 length) and the corresponding shorter buffer(lr308).

 

[Grimm]

Aero sells a 2.5" buffer for this rifle so that tells me this one is the right length considering its also 2.5". Could the bolt be coming back so fast its compressing the silicon tip against the back allowing it to make contact?

 

[Grimm]

Because the carrier in a ar10/lr308 is longer than an ar15, you either need the longer ar10 buffer tube and the corresponding ar15 length buffer, OR you need the lr308 buffer tube (ar15 length) and the corresponding shorter buffer(lr308).

Thats what I have. Ar15 tube with 308 short buffer

 

[Tomahawk67]

Take the buffer spring out, drop your buffer in the buffer tube then insert your carrier into the buffer tube and you will see that it probably hits the reciever.

 

[DirectDrive]

The marks are from the carrier going too far rearward from the buffer length being too short.

I thought he verified that he had a DPMS pattern buffer and DPMS pattern tube ?
Hard to track this and I'm too lazy to re-read the whole thing.

 

[Tomahawk67]

I thought he verified that he had a DPMS pattern buffer and DPMS pattern tube ?
Hard to track this and I'm too lazy to re-read the whole thing.

So what else would make the carrier contact the lower receiver?

 

[Grimm]

So what else would make the carrier contact the lower receiver?

Exactly why I'm so confused.. Everything except the spring seems to be as it should.

 

[Tomahawk67]

Exactly why I'm so confused.. Everything except the spring seems to be as it should.

Did you test fit the buffer and carrier into the tube without the spring?

 

[IronMonster]

So there is a real simple fix for this..

Simply drop a couple quarters in the buffer tube before you put the spring and buffer in.... Done.

Problem solved.

I went through this, damaged one of my receivers the same way. I bought slash's heavy buffers and springs for a couple of my guns which solved the problem but in the others I just put a couple quarters in the RE.... They run great

There is more too this, and the spring does matter... But I am not smart enough to tell you details about why... But I can tell you a quarter or two will fix it.

 

[Tomahawk67]

So there is a real simple fix for this..

Simply drop a couple quarters in the buffer tube before you put the spring and buffer in.... Done.

Problem solved.

I went through this, damaged one of my receivers the same way. I bought slash's heavy buffers and springs for a couple of my guns which solved the problem but in the others I just put a couple quarters in the RE.... They run great

There is more too this, and the spring does matter... But I am not smart enough to tell you details about why... But I can tell you a quarter or two will fix it.

You can also buy a harder bumper from KAK ind. But if it's hitting the bumper so hard to squish it then we are right back to overgassed condition. Slow down the bolt so it doesn't squish the buffer bumper.
Quarters will help the over travel like limiting a ar9 bolt to just beyond the bolt catch when used with AR-15 length buffer.

 

[Grimm]

Thanks @IronMonster, I knew id read you talling about a similar situation at some point. Im assuming a higher springrate/correct spring would help the overgas situation. I'll try the quarters out for the time being and see if that help. Any idea why this happens?

 

[campwilde]

If the balance is incorrect such as buffer weight, length, buffer tube and spring rate the gas flow still stays the same and over-gas is not real just a side effect of the wrong setup to begin with. Many workarounds that work well just a matter of desire to bring back to specs.

 

[Grimm]

I may just switch it over to an ar10 buffer system. It seems that extra 3/4" makes a big difference.