Correct Length for LR-308 Buffer Tube

[majordon]

What is the proper length for an LR-308 buffer tube? I am using a 7" mil-spec tube with DPMS 308 buffer and spring. My problem: I can't get the bolt to lock back; it appears that the the bolt doesn't trravel far enough to allow the bolt catch to activate.

 

Anyone have any opinions on this or as to what I might do to correct this?

 

Thanks from a new member. Majordon

[edgecrusher]

Do you have the correct buffer? Spring? We have all the info on the site. Welcome

[FaRKle!]

What is the proper length for an LR-308 buffer tube? I am using a 7" mil-spec tube with DPMS 308 buffer and spring. My problem: I can't get the bolt to lock back; it appears that the the bolt doesn't trravel far enough to allow the bolt catch to activate.

 

Anyone have any opinions on this or as to what I might do to correct this?

 

Thanks from a new member. Majordon

How long is your buffer? That combo should work (assuming it's explicitly an LR308 buffer and action spring).

[98Z5V]

What is the proper length for an LR-308 buffer tube? I am using a 7" mil-spec tube with DPMS 308 buffer and spring. My problem:

 

Bottom line: That should be the correct combination.  Pull your buffer and measure it's length, please report back.  Pull your spring, measure it's relaxed length, please report back.

 

EDIT - Where did you get these parts from (what vendor), and did you buy them as a kit?

Edited March 24, 2015 by 98Z5V

[majordon]

 The buffer is 2 17/32" long and weighs 3.7 oz. The buffer spring is 11 7/16" long. I am using Juggernaut Tactical billet upper and lower, with a CMMG lower parts kit.

[majordon]

All these parts were purchased through Buckeye Tactical, with the Juggernaut lower being drop shipped from Juggernaut.

[98Z5V]

The buffer is the correct length, but seems light as hell for a .308 AR application.  Need more weight, for sure.

 

Spring seems short, from available references.\

 

My bet on the lock-back problem - the CMMG lower parts kit, specifically, the bolt catch.

[FaRKle!]

The buffer is the correct length, but seems light as hell for a .308 AR application.  Need more weight, for sure.

Sounds like the standard DPMS LR308 carbine buffer weight to me. I agree it's sub-optimal though.

 

My bet on the lock-back problem - the CMMG lower parts kit, specifically, the bolt catch.

On that note, can you hand cycle the BCG far back enough to clear the bolt catch? It wasn't clear to me from your first post whether it wouldn't lock back due to firing, or hand cycling as well.

Edited March 24, 2015 by FaRKle!

[majordon]

Doesn't lock back either way. Have changed the CMMG bolt catch for an Armalite catch. Still won't lock back.

Had a lot of high hopes for this build. Have built three ar-15s in .223/5.56 and one in 300 BLK with no significant problems, other than finding the right loads for the 300 BLK.

[FaRKle!]

Let's partition out the spring.

Take the action spring off of the buffer, then insert the buffer into the receiver extension. Next take the BCG and push down on the hammer and slide it into the receiver extension till it hits the buffer and they bottom out inside the receiver extension. Can you push it far back enough for the bolt to clear the bolt catch (remember to place bolt in extended position)?

If not, then we need to see if your receiver extension is the right depth and how long your BCG is.

[majordon]

With the buffer spring removed, the buffer was slipped into the buffer tube. With the bolt extended, the BCG was pushed into the tube until encountering the buffer. The bolt cleared the bolt catch with about 1/4" to spare. The empty buffer tube is exactly 7" deep from the inside of the base to the lip of the tube at the front.

 

You mentioned the possible need for a heavier buffer. What would you recommend specifically?

[Dane Armory]

Look up heavybuffers.com it is a sponser look on a side colum on another page.

http://heavybuffers.com

The way you have describe your gun ; I would assume the receiver extension is too short.

Please count the coils on your spring to go with the 11 and 7/16 " length.

As stated above the Armalite bolt catch is bigger and taller it helps but may not be needed.

[survivalshop]

   Spring seems a little too long . As said, count the coils . Photo below is a DPMS carbine spring , 11 1/4 "

 

[FaRKle!]

With the buffer spring removed, the buffer was slipped into the buffer tube. With the bolt extended, the BCG was pushed into the tube until encountering the buffer. The bolt cleared the bolt catch with about 1/4" to spare. The empty buffer tube is exactly 7" deep from the inside of the base to the lip of the tube at the front.

Excellent, sounds like we've narrowed it down to the spring then.

If your receiver extension has a hole in the back, you can also confirm that when the spring is also installed the buffer isn't bottoming out by bringing the BCG as far back as it can go with the charging handle and then feeling for the rear of the buffer with a thin tool (paperclip or allen wrench work) that fits into the hole.

 

You mentioned the possible need for a heavier buffer. What would you recommend specifically?

What barrel and gas port location do you have? We can recommend the right buffer with that info. Heavybuffers also sells replacement .308AR carbine action springs.

[majordon]

I am using a16" barrel with a mid-length gas tube.

Would you recommend trimming a oil or two from the spring to get to 11 1/4"?

[majordon]

A "coil" or two

[98Z5V]

Excellent, sounds like we've narrowed it down to the spring then.

 

Thought I mentioned that previously.

 

Spring seems short, from available references.\

 

[Dane Armory]

Guns are like cars : I would NOT cut a coil on either ! It ruins the rebound. Buy the correct spring.

[FaRKle!]

I am using a16" barrel with a mid-length gas tube.

The CAR-10 buffer should work for you. If you feel your rifle is really overgassed though, you might want to try the CAR-10 XH.

Since we know the spring is the root cause, might as well grab one of Heavy Buffers' .308 carbine buffer springs too.

 

Would you recommend trimming a oil or two from the spring to get to 11 1/4"?

Guns are like cars : I would NOT cut a coil on either ! It ruins the rebound. Buy the correct spring.

I wouldn't recommend trimming the spring. You don't even know if you actually have an LR308 spring or an AR-15 one. Trimming springs isn't totally forbidden though. Heavy Buffers' LR308 carbine springs are Armalite AR-10 springs that are trimmed.

 

 

Thought I mentioned that previously.

 

Yup, you suspected it was the spring, however the spring would have to be too long to prevent the bcg from travelling rearward enough, not too short ;).

Edited March 24, 2015 by FaRKle!

[majordon]

Thanks for everyone's input on this. It appears that I need a heavier buffer, but I am still a bit confused about the spring. Is 11 1/4 " or 11 1/2" the correct length? The spring I am using was purchased as a 308-specific spring, and is considerably stiffer than the springs I use in my Ar-15s, and is 11 1/2" long.

 

Referring to an earlier post about checking the rear of the buffer tube to see if the buffer bottoms out: should the buffer bottom out?

[FaRKle!]

Thanks for everyone's input on this. It appears that I need a heavier buffer, but I am still a bit confused about the spring. Is 11 1/4 " or 11 1/2" the correct length? The spring I am using was purchased as a 308-specific spring, and is considerably stiffer than the springs I use in my Ar-15s, and is 11 1/2" long.

 

Referring to an earlier post about checking the rear of the buffer tube to see if the buffer bottoms out: should the buffer bottom out?

As others have said before, count the coils. The LR308 carbine spring should have ~29 coils IIRC.

The buffer should bottom out on the end of the receiver extension.

[majordon]

My spring has 29 coils. Haven't had a chance to test the bottoming out of the buffer yet.

[survivalshop]

   Not sure how you are measuring , but as said I have two 308AR DPMS Carbines & an extra spring & Buffer , measurements are ;

 

 Spring # 1-   11 1/4 " long 

 Spring # 2-   11 1/8 " long

 Buffer # 1-    2.497 " long ( Measured with Digital Caliper )

 Buffer  # 2-   2.495 " long ( Measured with Digital Caliper )

 Buffer # 1, WT. -  3.325 oz.( weighed with Pact digital Scale )

 Buffer # 2, WT. -  3.312 oz.( weighed with Pact digital scale )

  I didn't measure the third Spring/Buffer because I didn't think there would be significant differences with it & the others , but will if you want .All are DPMS components .

 

  Small differences can add up when Tolerances are close , as you probably know .

  I make springs for Firearms from spring stock , when the springs are no longer available ,(  happens a lot )  I cut them & fit & function are trial & error with nothing to go by except a out of spec spring that you know is not working , so cutting a spring is not a big deal .

 

 I would make sure that the Action components are not binding with in the Upper Receiver & its Extension . I would also check for Hammer to Disconnector interference , you will see a mark on the Disconnector from the Hammer hitting it .  

Edited March 25, 2015 by survivalshop

[majordon]

Thanks for the input. I will take a look at those points. Still havevn't had a chance to check the bottoming out of the buffer. Might require trimming a coil from the spring.

[Dane Armory]

Been thinking about this : could you take off the Buttstock then take a picture of the receiver extension (buffer tube). With and with out the BCG in. This will answer my question.