AR 308 DPMS Rifle Bolt, Buffer, Buffer Spring Info Needed

[Load Master]

I'm looking for information about the bolt, buffer, buffer spring and how they interact with each other, tuning for proper bolt cycling.

Info I'm looking for:

1. What is the mass of a typical AR 308 DPMS bolt assembly? my bolt is 17.5 ozs
2. What happens if the buffer mass is increased? Will this decrease or increase bolt travel?
3. What happens if the buffer spring tension is increased?

From what data I could find, my build should have a 39 coil spring with uncompressed length of 12.75" and a buffer mass of 5.3 oz. My buffer weighs in at only 2.8 oz, my spring meets the requirement.

I'm having issues with the bolt not fully cycling. The spent case will eject but the bolt will not travel far enough to pickup the next round. The round being used is giving 2,660 fps. The gun has a total of 26 rounds through it.

Thanks...

 

[DoubleMag]

''The gun has a total of 26 rounds through it.''

I presume new factory rifle? I would contact the manufacturer like, NewYork quick vs. posting for an answer (??)

 

[Load Master]

''The gun has a total of 26 rounds through it.''

I presume new factory rifle? I would contact the manufacturer like, NewYork quick vs. posting for an answer (??)

No sir, this is a custom build. All on me. It is a learning tool. I'm getting quite the lesson. I think I have the "why" figured out. I have the wrong buffer spring. I'll test this out once I have the spring issue resolved.

I am still interested to learn how the buffer weight plays into the system. With removing one of the three masses in the buffer, it dramatically increased the amount of felt recoil. I was surprised by this. I will make the spring change and see how this affects operation and will play with the buffer mass until I zero in on the best combination. There are tungsten replacement weights available that I might try.

 

[dh1633pm]

I made my own weights by taking the hollow spacers and filling them with lead until the proper weight.

 

[Load Master]

I made my own weights by taking the hollow spacers and filling them with lead until the proper weight.

Is this in a 308 AR? If so, do you mind telling me what your total buffer weight ended up being? Any idea on your bolt assemble weight?

 

[dh1633pm]

I had a DPMS. I played with the buffer spring and the weight in the buffer. It wasn't too hard to get it just where I wanted to go. I had more of a problem when I had a 300 BLK AR. It did exactly as your AR did. That's when I swapped the spring and removed some weight. I took the hollow piece from the buffer tube and added lead by placing the tube on an anvil and dripping lead into the inside. A little adjustment here and there and I got it just the way I wanted it.

 

[Load Master]

I had a DPMS. I played with the buffer spring and the weight in the buffer. It wasn't too hard to get it just where I wanted to go. I had more of a problem when I had a 300 BLK AR. It did exactly as your AR did. That's when I swapped the spring and removed some weight. I took the hollow piece from the buffer tube and added lead by placing the tube on an anvil and dripping lead into the inside. A little adjustment here and there and I got it just the way I wanted it.

Well, I ended up cutting the buffer spring down and reinstalling the removed weight into the buffer. I'll test it out tomorrow. I'm hoping proper cycling with a minimum load round.

 

[dh1633pm]

Keep us up to data Load Master. Your information added to what we already know helps fill in blank spots and makes us all better at what we do.

 

[dh1633pm]

The hollow tubes within the buffer weigh can weigh up to about 400 grains when filled with lead. You can vary that amount. I think I put 200 grains of lead in each. My buffer had two. This was with a DPMS with an A2 style buffer.

 

[cougar1717]

No sir, this is a custom build. All on me. It is a learning tool. I'm getting quite the lesson. I think I have the "why" figured out. I have the wrong buffer spring. I'll test this out once I have the spring issue resolved.

I am still interested to learn how the buffer weight plays into the system. With removing one of the three masses in the buffer, it dramatically increased the amount of felt recoil. I was surprised by this. I will make the spring change and see how this affects operation and will play with the buffer mass until I zero in on the best combination. There are tungsten replacement weights available that I might try.

Question for you LM: You mentioned that the bolt doesn't fully cycle. You mentioned three buffer weights in the buffer. Are you using an AR-15 carbine buffer in a LR308 pattern carbine? Those buffers are different. The overall length of the LR308 carbine buffer is shorter to compensate for difference in the length of the bcg. A LR308 carbine buffer only has two weights and they are tungsten. An AR-15 buffer has three weights and are usually steel, tungsten, or a combination. I suppose there are some specialty LR308 buffer tubes that are longer to accommodate the extra length of an AR-15 buffer, but you did not describe the buffer tube length.

 

[12Bravo20]

It sounds more like a gas problem more than a buffer problem. It sounds like your rifle is under-gassed and is short stroking..

 

[dh1633pm]

If the spring is too heavy for the gas to overcome or to move the weight of the buffer then it could be any including under-gassed? Would that not be true?

 

[popper]

Run it really wet to start. Carbine or rifle? Use the proper setup. Shoot some before you tinker. If it won't run with WWB ammo it's a gas problem.

 

[Load Master]

Here is the latest. It is cycling almost perfectly with a low velocity load. 168gr bullet pushing 2575fps I had a couple times the bolt wouldn't lock open on the last round. The recoil is mild.

The fix was cutting 7 coils from the buffer spring. The buffer tube is carbine length and the spring was for rifle length. I'm going to run it as is for now and see how it does after I have more rounds through it. I have a total of 46 rounds fired. Of the 46 rounds 20 of them were a little hotter load at 2660fps. It ran through these with no issues.

For under $800 I am quite happy with it. I started collecting the parts last September, buying what I could when I saw something on sale. Once I have a few more rounds through it I'll focus on some better optics and move to the 200 yard range for some more development. I'm having a lot of fun getting it sorted out. This is my first venture into .308 Win.

 

[12Bravo20]

That explains a lot. A rifle spring in a carbine buffer tube will cause you problems for sure (223 or 308). Watch the end of the buffer tube and also the receiver to make sure the gas key isn't making contact. I would try to find the correct carbine length buffer spring to be on the safe side. Glad that you got your rifle to run.

 

[Varminterror]

You have the wrong spring - 39 coils is a rifle spring, but you have a carbine buffer, so I'm assuming you also have a carbine extension too - hence, wrong spring. I'd venture it's a safe bet that you cannot MANUALLY withdraw the charging handle and make the action lock back, at least not easily if you can at all. 39 coils is rifle length, NOT appropriate for a carbine stock. Also sounds like you have a carbine buffer, not a rifle buffer, which is correct for your stock, but you may want to up to an H2 buffer to slow down your recoil impulse and improve your brass life.

You're comparing your carbine system against a rifle system, wrong spec sheet to compare against.

1. What is the mass of a typical AR 308 DPMS bolt assembly? my bolt is 17.5 ozs

Mine is 18.5oz, but that doesn't necessarily mean much. BCG mass isn't typically a cause for short stroking, and mine's an ounce heavier than yours and works fine. I also run H2 buffers, so I have about 3oz total weight more than you.

2. What happens if the buffer mass is increased? Will this decrease or increase bolt travel

It will not decrease the distance of bolt travel if your gas system is properly tuned. Are you able to withdraw the charging handle and BCG far enough to lock open manually? If not, then you have an overly long buffer or spring (i.e. rifle spring in a carbine stock -). If yes, then you don't have enough gas.

3. What happens if the buffer spring tension is increased?

The Bolt will close with more authority, and your recoil impulse will feel less aggressive. The spring effectively catches the BCG, it doesn't really offer much resistance to opening, so it CAN cause short stroking, but in a properly gassed rifle, it's exceedingly rare that any spring or buffer weight combination can cause a short stroke.

I'm having issues with the bolt not fully cycling. The spent case will eject but the bolt will not travel far enough to pickup the next round. The round being used is giving 2,660 fps. The gun has a total of 26 rounds through it.

As long as you don't have a rifle spring in a carbine stock, then my next thing to check is my gas block. If you don't have some physical dead stop or binding of the spring (too long of buffer, too long of spring, respectively), then it's all about gas flow. If you gas block isn't appropriately aligned, it will short stroke.

 

[Varminterror]

Just noticed you got it figured out - For $5, it's not really worth it to NOT get the right spring. A cut spring can work, but it doesn't really make sense to modify the wrong part instead of buying the proper part, when the part is so cheap.

 

[Load Master]

Just noticed you got it figured out - For $5, it's not really worth it to NOT get the right spring. A cut spring can work, but it doesn't really make sense to modify the wrong part instead of buying the proper part, when the part is so cheap.

Varminterror, Thanks for your input and taking the time to supply your bolt weight. I really wanted this for comparison. I also appreciate the concern. I'm a little different than most when it comes the figuring out moving parts. It was my job for over 35+ years before retiring. The primary reason for building this rifle, it is an educational piece for me. I will end up modifying the the entire buffer system once I make some dynamic measurements. I needed to have the rifle cycle. The spring and buffer are a starting point. You are correct. I have the rifle length spring. I blindly asked my local gun store for the buffer tube and spring, stock for my DMPS compatible 308 AR build. The person sold me a number of correct parts, but the spring wasn't one of them. I have some other springs and masses on order that I'll use later on.

Again, thanks for your input!

 

[Varminterror]

Don't get too caught up on matching exact operating specs. As an engineer myself, it's been pretty easy to understand the different operating conditions you can utilize for different desires - run everything hyper-light with an AGB throttled down to low flow, it'll cycle really fast and reliably, but will be hell on brass. Great for competition when you want a fast TTFS and fast splits, but not really advantageous for a guy hunting whitetails, or building an SPR. Run an extra weight BC and extra weight buffer and a stiff spring, then open the gas block, you can run a very, very low recoil system.

It's not a design point, it's a design spectrum. I have AR rifles set up to run super heavy, others set up to run hyperlight, and some in the middle. All of them run well for their application.

 

[Load Master]

Don't get too caught up on matching exact operating specs. As an engineer myself, it's been pretty easy to understand the different operating conditions you can utilize for different desires - run everything hyper-light with an AGB throttled down to low flow, it'll cycle really fast and reliably, but will be hell on brass. Great for competition when you want a fast TTFS and fast splits, but not really advantageous for a guy hunting whitetails, or building an SPR. Run an extra weight BC and extra weight buffer and a stiff spring, then open the gas block, you can run a very, very low recoil system.

It's not a design point, it's a design spectrum. I have AR rifles set up to run super heavy, others set up to run hyperlight, and some in the middle. All of them run well for their application.

Sound like fun. I have an Armalite A2 that I've made a number of changes too. I have a nice custom lower, part of a group buy through the Michigan Gun Owners with their logo on it. I'll build this into something. I'm thinking of doing a light weight ar just to do it.

Have you had any issues with the adjustment fowling to where you can no longer adjust it? My reason for asking, I've been told that it is a common issue. This is one of the reasons I've stayed away.

 

[Varminterror]

Not in any of mine. I use JP rifles gas blocks most of the time. Worst case, loosen 4 bolts and pull the block. This gives open access to the gas block port - if I can't get it dissolved that way, drive out the roll pin and have access to both sides.

Out of using AGB's on all of my personal AR's for ~25yrs, and some with thousands upon thousands of rounds, it's been a lot more common to have adjustment lock up because of red loctite installed by the user, not the fouling. MAYBE twice I recall I've needed to pull my gas block as described above.